A&A 397, 693-710 (2003)
DOI: 10.1051/0004-6361:20021545
C. J. Davis 1 - E. Whelan 2 - T. P. Ray 2 - A. Chrysostomou3
1 - Joint Astronomy Centre, 660 North A'ohoku Place,
University Park, Hilo, Hawaii 96720, USA
2 -
Dublin Institute for Advanced Studies, School of
Cosmic Physics, 5 Merrion Square, Dublin 2, Ireland
3 -
Department of Physical Sciences, University of Hertfordshire,
Hatfield, Herts AL10 9AB, UK
Received 27 August 2002 / Accepted 22 October 2002
Abstract
Near-IR echelle spectra in [FeII] 1.644 m emission trace Forbidden
Emission Line (FEL) regions towards seven Class I HH energy sources
(SVS 13, B5-IRS1, IRAS 04239+2436, L1551-IRS5, HH 34-IRS,
HH 72-IRS and HH 379-IRS) and three classical T Tauri stars
(AS 353A, DG Tau and RW Aur). The parameters of these FEL regions
are compared to the characteristics of the Molecular Hydrogen Emission
Line (MHEL) regions recently discovered towards the same outflow
sources (Davis et al. 2001 - Paper I). The [FeII] and H2 lines
both trace emission from the base of a large-scale collimated outflow,
although they clearly trace different flow components. We find that
the [FeII] is associated with higher-velocity gas than the H2, and
that the [FeII] emission peaks further away from the embedded source
in each system. This is probably because the [FeII] is more closely
associated with HH-type shocks in the inner, on-axis jet regions,
while the H2 may be excited along the boundary between the jet and
the near-stationary, dense ambient medium that envelopes the
protostar. Indeed, there is spatial and kinematic evidence that
[FeII] and the more typically-used optical emission lines, the red
[SII] doublet, do trace almost the same shock-excited regions in HH
jets and FEL regions alike.
Key words: ISM: jets and outflows - stars: pre-main-sequence - ISM: Herbig-Haro objects
Comparitively little is known about Herbig-Haro (HH) jets and molecular outflows close to their protostellar driving sources. Optical forbidden-line emission from the base of HH jets from classical T Tauri stars (CTTS) has been studied with high-resolution imaging (HST and ground-based AO) and spectroscopic techniques (e.g. Ray et al. 1996; Bacciotti et al. 2000; Dougados et al. 2000; Hirth et al. 1994, 1997; Takami et al. 2001; Woitas et al. 2002), yet only recently have observations yielded information on the same regions in molecular outflows from more deeply embedded protostars (Davis et al. 2001 - hereafter referred to as Paper I; Davis et al. 2002). The recent discovery of "Molecular Hydrogen Emission Line (MHEL)'' regions in Class I young stellar objects (YSOs) is of considerable interest, since the survival of H2 in the inner jet regions constrains models of HH jet acceleration, collimation and evolution within a few hundred AU of these very young outflow sources.
The H2 echelle observations presented in Paper I reveal complex, low and high-velocity molecular line emission almost coincident with each Class I source. The characteristics of these MHEL regions are much like those of the "Forbidden Emission Line (FEL)'' regions observed in CTTSs (e.g. Hirth et al. 1997), even though the optical lines and near-IR H2 lines trace very different excitation conditions. In both MHELs and FELs: i) multiple (low and high) velocity components are observed; ii) the emission regions are generally offset from the source, along the blue-shifted flow axis, by a few tens to a few hundred AU; and iii) the higher-velocity gas is always slightly further offset than the slower gas. Indeed, even though molecular hydrogen is dissociated under extreme excitation conditions, radial H2 velocities approaching 50-150 km s-1are observed in some MHEL regions.
To further investigate the relationship between MHEL and FEL regions,
we now present spectroscopic observations made in [FeII] at
1.644 m. The same Class I sources observed in H2 at 2.122
m (apart from GGD 27), as well as three CTTSs (Class II sources) known
to possess optical FEL regions and HH jets, have been observed in
[FeII] emission. [FeII] is a useful tracer of
intermediate/high-excitation shocks in HH objects and molecular
outflows. Much like the optical forbidden lines from, e.g. [OI],
[SII], [NII], etc., the near-IR [FeII] lines can be used to complement
H2 observations (e.g. Reipurth et al. 2000). In outflows the
shock-excited H2 emission derives from dense (104 cm-3),
warm (
2000 K) molecular gas, while the metastable transitions of
species like [FeII] dominate the cooling at similar densities
though higher temperatures, of the order of 104 K (Hollenbach &
McKee 1989). Hamann et al. (1994) have used H-band [FeII] lines to
probe FEL regions in a handful of T Tauri stars. The same [FeII]
lines should therefore be a good tracer of the FEL regions
associated with Class I YSOs, where optical observations will be
hampered by extinction.
Source | RA | Dec | 1Slit | 2Exp. |
(2000.0) | (2000.0) | PA | Time | |
(min) | ||||
SVS 13 |
03 29 03.7 | +31 16 02 | 123![]() |
40 |
B5-IRS1 | 03 47 41.6 | +32 51 46 | 73![]() |
33 |
IRAS 04239+2436 | 04 26 56.4 | +24 43 36 | 359![]() |
40 |
L1551-IRS5 | 04 31 33.6 | +18 08 11 | 66![]() |
33 |
HH 34-IRS | 05 35 29.8 | -06 26 58 | 167![]() |
33 |
HH 72-IRS | 07 20 08.4 | -24 02 23 | 90![]() |
40 |
HH 379-IRS | 21 45 08.2 | +47 33 07 | 82![]() |
40 |
DG Tau | 04 27 04.7 | +26 06 17 | 46![]() |
40 |
RW Aur | 05 07 49.6 | +30 24 05 | 125![]() |
33 |
AS 353A | 19 20 31.0 | +11 01 55 | 106![]() |
40 |
1
Slit position angle (east of north).
2 Total on-source exposure time. 3 A PA of 45 ![]() This new angle was measured from the HST images of Reipurth et al. (2000). |
Echelle spectra were obtained at the U.K. Infrared Telescope (UKIRT) on 1-3 November 2001 UT using the facility 1-5 m spectrometer CGS 4 (Mountain et al. 1990). Observations in the
transition of [FeII] at
m (Johansson 1978) were acquired towards seven Class I
sources and three Class II (T Tauri) stars (listed in Table 1).
CGS 4 is equipped with a 256
256 pixel InSb array; the pixel
scale at 1.64
m measures 0
41
0
88 (0
41
in the dispersion direction). A 2-pixel-wide slit was used, resulting
in a velocity scale of
8.0 km s-1 per pixel. The
instrumental profile in the dispersion direction, measured from
Gaussian fits to arc lines, was 19.0(
1.0) km s-1. For each target
the spectrometer slit was orientated along the outflow or HH jet axis,
as defined by published, large-scale images of each system (the same
slit position angles as in Paper I were used, except for
IRAS 04239+2436 where a revised angle was employed based on newly
published data).
A sequence, comprising one sky followed by three object exposures was
repeated a number of times for each source to build up
signal-to-noise, the sky position being typically 30
-60
away from the source (in a direction orthogonal to the flow
axis). Each spectral image was bias subtracted and
flat-fielded. Sky-subtracted object frames were then co-added into
reduced "groups'' (one group frame per target). To
wavelength-calibrate these group images, argon and krypton arc lamp
spectra were observed just prior to observing each source (and each
standard star). The combined lamp spectral images yielded six lines,
spread across the dispersion axis, that could be used for accurate
wavelength calibration. The argon and krypton lines at
1.644107
m and 1.647035
m were observed directly (in the
33rd order) while lines at 1.694521
m and 1.599386
m (argon) and
1.670137
m and 1.694043
m (krypton) were detected from adjacent
(32nd or 34th) orders. The rest wavelength of [FeII] also lies close
to the bright OH sky line, OH(5,3)R1(2) at 1.64421
m (Maihara et al. 1993); the separation between the [FeII] and OH lines is only 39 km s-1, or about 5 pixels. This sky-line did not subtract out
perfectly in some cases, although because the line is narrow and
extended along the full length of the slit (along whole columns), it
was easily distinguished from the [FeII] emission associated with each
outflow. In these cases, the line was fitted and removed from the
spectral image. Before doing this, however, we used it to check the
wavelength and subsequent velocity calibration (described below).
Routines available through Starlink were used to identify arc lines and wavelength-calibrate each spectral image, row-by-row. In this way we could also correct for curvature along the slit (spatial) axis in each image. The arc spectral images were first "self-calibrated'' so that we could check for variations in the absolute wavelength calibration along the slit axis (along columns); these variations were found to be small, of the order of 5 km s-1 towards the edges of the array, and a factor of 2 better within the central 30% of the array where the [FeII] emission was observed. Instrument flexure over the duration of the observations could, however, introduce additional uncertainties in the absolute velocity calibration, i.e. by shifting the individual frames with respect to the wavelength reference used to calibrate the reduced group spectral image. By comparing the positions of sky lines in a number of raw frames we found that this effect was also small; indeed, the narrowness of the [FeII] lines and/or residual sky lines in the combined group data for each source (as compared to the instrumental profile width, which is measured from just one frame) confirms this finding. We therefore conclude that the overall velocity calibration is accurate to better than 6 km s-1, while perceived velocity shifts between adjacent spectra observed along the same slit will be more accurate, to within 2-3 km s-1.
Finally, bright G- or K-type giant or main-sequence stars were
observed with the same instrumental configuration prior to each
outflow source. Narrow telluric absorption features were evident at
1.6429 m and 1.6455
m in these data, lines which are displaced
by
200 km s-1 and
220 km s-1 from the rest wavelength of the
target [FeII] line. Some of the standard star spectra also possessed
photospheric absorption features (from permitted FeI or other species)
at 1.6427
m, 1.6437
m, 1.6444
m and 1.6454
m. These
absorption lines were narrow (
25 km s-1), so we were able to
"patch'' across them by interpolating between the continuum on either
side of the line, before the standard star spectra were used to flux
calibrate the extracted 1D source spectra.
Our goal was to use [FeII] to trace FEL regions in the Class I YSOs
where MHELs had already been observed. We therefore observed each
target from Paper I (except GGD 27) using the same slit position
angle (PA) and slit width, so that we could directly compare the
H2 and [FeII] characteristics. In [FeII] we also observed three
T Tauri stars. One of these, AS 353A, was observed in H2 in Paper I: the other two CTTSs have known FEL regions from published optical
observations. With these CTTS data we can compare the optical and
near-IR characteristics of the FELs, as traced in [SII] and [FeII]
respectively. Note that shock models and observations of HH objects
do indicate that [FeII] and [SII] trace similar post-shock regions
(Hollenbach & McKee 1989; Hamann et al. 1994), even though the [FeII]
1.64 m line has a higher critical density for collisional
excitation than the [SII]
lines
(
cm-3 as compared to
cm-3 for
[SII]). The electron temperature in an [SII]-bright region is
expected to be
13 000 K, while the Fe+ ionisation fraction is
predicted to peak at temperatures of approximately 14 000 K (Hamann
1994); these temperature limits are not strongly dependent on the
density (they were calculated for
cm-3).
[FeII] position-velocity (P-V) diagrams for the seven Class I YSOs are shown in Fig. 1; the data obtained for the T Tauri stars are presented in Fig. 2. Where the FEL emission is obscured by strong continuum emission, we also show continuum-subtracted P-Vdiagrams, in Fig. 3. In these plots the continuum emission on either side of the line emission has been fitted, row by row, with a 3rd order polynomial. The fits are then subtracted to leave only the line emission associated with each target.
From the P-V diagrams we also measure the relative positions of the
FEL components with respect to the source continuum position. This is
done by fitting Gaussians to profiles taken perpendicular to the
dispersion direction. For each column in a given P-V diagram (i.e. at
each velocity) a measurement of the position of the line emission or
the continuum emission is made (line emission positions are obtained
from the continuum-subtracted P-V diagrams). These are then plotted
against velocity in Fig. 4. Because our spatial resolution along the
slit is relatively poor with respect to the seeing, for error bars on
the line emission positions (the boxes in Fig. 4) we use the Full Width
Half Maximum (FWHM) of the Gaussian fits divided by
,
where N is the number of points across the profile (
/0.88
). The same technique was used in Paper I, where
offsets of the MHEL components were presented (although note that the
seeing was much better during the H2 observations). The position
of the source continuum is known much more accurately and is indicated
in each plot by a polynomial fit to the source continuum positions.
The scatter in these continuum points about the fit is an indication
of the accuracy with which the continuum position is known; this
scatter is of course directly related to the strength of the continuum
emission.
Spectra, representing in all cases the sum of 3 rows
(equivalent to an on-source area of
)
are shown inset in Figs. 1 and 2 for select positions in each
region. We only present spectra for regions where complex or
interesting line profiles were observed.
Finally, a comparison between these new [FeII] data and the H2observations discussed in Paper I is made in Sect. 7 and Figs. 6 and 7. First, however, we describe the [FeII] results pertaining to each source separately.
![]() |
Figure 1:
[FeII] 1.644 ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
The HH 7-11 outflow in NGC 1333 ( pc) has been observed
extensively at near-IR wavelengths (e.g. Garden et al. 1990; Stapelfeldt et al. 1991;
Carr 1993; Gredel 1996; Chrysostomou et al. 2000; Khanzadyan et al. 2002). The HH objects themselves reside along the edges of a
southeastern, blue-shifted cavity that is bounded by a fast-moving
molecular shell (Bachiller & Cernicharo 1990; Bachiller et al. 2000).
The probable source of the flow, SVS 13, is known to have undergone
an outburst in recent years (Eislöffel et al. 1991) and, from Paper I, is clearly associated with complex H2 line emission. Recently,
SVS 13 was found to be a close-binary (separation
0.3
),
with an orbital period of
1700 yrs (Anglada et al. 2000).
In Fig. 1a we show a P-V plot of the region near the source. [FeII]
line emission was not detected from the HH objects that were covered
by the spectrograph slit (the slit extended 44
to the SE, and
42
to the NW; it did not reach HH 7). However, [FeII] was
detected towards SVS 13. An extracted spectrum is shown inset in
Fig. 1a. The profile is double peaked and blue-shifted with respect to
the systemic LSR velocity of +8 km s-1 (Bachiller &
Cernicharo 1990), which is marked with a dashed line in the P-Vdiagram. The stronger of the two [FeII] components peaks at
km s-1; the weaker component peaks at somewhere
in the range -20 to -50 km s-1. Both components appear to be quite
broad; the FWHM of the stronger, higher-velocity component (HVC)
measures 73(
10) km s-1; deeper spectra are needed to establish the
width of the weaker, marginally-detected, low-velocity component
(LVC), although a two-component Gaussian fit to our data suggests a
similar FWHM.
In Fig. 3a we show a continuum-subtracted P-V diagram which shows the
FEL region more clearly. By employing the same spectro-astrometric
technique used in Paper I (and described above) we measure the
position of the brighter [FeII] HVC with respect to the source
continuum position (Fig. 4). We find that the HVC is coincident with
the source, to within a conservative measurement error of 0.5
,
or
110 AU. Likewise, the
H2 emission towards SVS 13 was found to be offset by less than
0.4
from the source (see Paper I).
The embedded YSO B5-IRS 1 ( pc) drives an extensive
east-west molecular outflow (Yu et al. 1999) that
excites two distant groups of HH objects, HH 366E and 366W. Much of
the emission from HH 366 is situated well beyond the range of our
spectrograph slit (Bally et al. 1996). There is, however,
H2 emission within 30
of the source in both lobes (Yu et al.
1999). The optical knot HH 366 E5 in the eastern, blue-shifted flow
lobe lies along our spectrograph slit. Although high velocity H2emission was detected from this object (Paper I), we did not detect it
in [FeII].
H2 and [FeII] emission are detected from the source position,
though in both cases the emission is very weak (see Sect. 7 for a
comparison). Overall, the [FeII] emission appears to be blue-shifted
(with respect to the systemic velocity of 10 km s-1; Yu et al.
1999) to LSR velocities over a broad range from
km s-1 to
km s-1.
![]() |
Figure 1:
Continued. In HH 34-IRS and HH 72-IRS the contours measure
3, 5, 10, 20 and 50![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
The low-luminosity Class I YSO IRAS 04239+2436 in Taurus
( pc) drives a highly-collimated, knotty,
10
-long [FeII] jet that has recently been observed in detail with NICMOS
on HST (Reipurth et al. 2000). However, in these imaging data, within
1
of the source the [FeII] jet is obscured by the continuum
emission from the star. The jet also probably drives a group of
extensive optical HH bow shocks (HH 300 A-C; Reipurth et al. 1997) found about 30
to the southwest, as well as a
more compact conical HH feature, HH 300 D, about 40
to the
northeast of the source.
The source itself, which is a binary (separation 0.3
;
Reipurth et al. 2000), is notable for having a rich near-IR
spectrum (Green & Lada 1996). We detect both H2 (Paper I) and
[FeII] emission from the source continuum position (see Sect. 7). In
the extracted [FeII] spectrum in Fig. 1c emission from both lobes of
the flow is detected. The two jet lobes must therefore be traced to
within an arcsecond of the star. Moreover, extinction does little to
impede our detection of the jet and counterjet in [FeII] near the
source. This suggests that the flow axis must be orientated close to
the plane of the sky, and that the accreting, circumstellar disk must
have a thickness that is less than our 3-pixel-wide extracted region
(
2.7
,
or <380 AU).
The blue- and red-shifted [FeII] line-emission components in Fig. 1c
are shifted to about the same radial absolute velocity with respect to
the systemic rest velocity (8 km s-1); the peaks occur at LSR
velocities of
km s-1 and
km s-1 respectively.
The blue peak is also probably associated with a weaker, though much
broader velocity component that extends from
km s-1 to
km s-1. The red counterjet peak is, on
the other hand, extremely narrow (we measure a FWHM of 24(
) km s-1); this line is only marginally broader than the instrumental profile.
For the [FeII] observations we used a revised slit PA of 59
(an angle of 45
was used for the H2 spectroscopy in Paper I),
which is better aligned with the jet axis. In [FeII] we trace the
entire
10
-long blue-shifted jet lobe, as well as
spatially-extended red-shifted emission from the counter-jet. These
jet components are most clearly seen in the continuum-subtracted P-Vplot in Fig. 3b. The [FeII] line emission is strongest at an offset
of 4
(
0.5
)
along the northeastern, blue-shifted
jet lobe. This emission peak probably corresponds to the bright
[FeII] knots J3-J5 in the HST images of Reipurth et al. (2000). In
Fig. 4 we again plot the positions of these emission peaks with
respect to the source continuum position. The 2
-5
offsets in the blue lobe reflect the fact that the apparent [FeII]
intensity increases with distance from the source. Extinction near
the source is probably not the cause of this increase, since the
red-shifted counterjet emission peaks closer to the source in
Fig. 4b, at offsets of only
1
.
Instead, the [FeII]
emission must be enhanced along the jet axis, probably in discrete
HH-type shocks. We also note in Fig. 4b that the higher-velocity
blue-shifted [FeII] emission appears closer to the source. In other
words, we see an apparent decrease in offset with velocity.
Finally, in addition to [FeII] emission from the source and inner jet
(FEL) region, we also detect [FeII] at a distance of about 38
to the northeast of the source. This emission is presumably associated
with HH 300D (Reipurth et al. 1997). The [FeII] peak shown in Fig. 1c
is blueshifted to an LSR velocity of -167(
) km s-1, though the
emission line is also extremely narrow (FWHM
km s-1); this
suggests that the [FeII] is associated with high-velocity jet
material, though with a relatively low-velocity shock within the flow.
L 1551-IRS5 is an archetypal bipolar molecular outflow that can be
observed at relatively high spatial resolution because of its close
proximity to the earth (
pc). The flow consists of a
striking blue-shifted, wind-swept cavity that extends over 10
(0.5 pc) to the southwest of IRS 5 (Stocke et al. 1988;
Moriarty-Schieven & Snell 1988). The cavity is associated with an
array of optical and near-IR shock features (e.g. Mundt & Fried 1983;
Davis et al. 1995). The source itself (which is a binary system) appears to drive two small-scale jets (Fridlund & Liseau 1998;
Hartigan et al. 2000; Itoh et al. 2000; Davis et al. 2002). However,
these jet-like features may instead represent the edges of a single
collimated flow (Mundt et al. 1991; Pyo et al. 2002). Indeed,
the new images of Pyo et al. represent perhaps the strongest evidence
yet that the two "jets'' are simply the limb-brightened edges of a
small, ovoidal cavity, that is bounded at its leading edge by a bright
bow, which they label PHK 3.
In H2 we detected complex velocity structure along the L 1551-IRS5
jet (Paper I). Likewise, in [FeII], multi-component profiles are
again observed, though in detail the H2 and [FeII] emissions are
very different. As we shall discuss further in Sect. 7, the LVC
observed in H2 is not detected in [FeII]; in the latter, only an
HVC and what we shall refer to as an "extremely-high-velocity
component'' (EHVC) is observed. The HVC and EHVC in Fig. 1d peak at
-120() km s-1 and -285(
) km s-1 respectively (both are
strongly blueshifted with respect to the systemic velocity of 6 km s-1;
Moriarty-Schieven & Snell 1988). Both components are also quite
broad; a two-component Gaussian fit yields FWHM widths of 113(
) km s-1 and 125(
) km s-1 for the HVC and EHVC. The overall
FWZI range in radial velocities observed in [FeII] is also roughly
equal to those seen in [SII] (Hartigan et al. 2000).
In Figs. 3 and 4 we again show a continuum-subtracted P-V diagram and a
plot of the positions of the line-emisson peaks with respect to the
source continuum. Spatially, the HVC peak is closer to the source
continuum position than the EHVC; the EHVC is offset by about 2
-3
to the southwest, about twice as far as the HVC along the
blue-shifted flow axis. However, although the EHVC peak is
offset further downwind, overall the HVC emission extends almost
twice as far along the flow. We also note once again an apparent
decrease in offset with velocity in Fig. 4c, when considering the
EHVC separately. Very similar results were recently reported by
Pyo et al. (2002).
Further downwind, we detect compact [FeII] peaks at offsets of 12
and
23
.
In velocity space both peaks are
narrow; for the brighter feature we measure a FWHM of 40(
) km s-1.
This bright peak is blue-shifted to an intermediate velocity of -140(
) km s-1. Both features correspond to discrete knots along the jet axis.
The feature at 12
coincides with the optical/[FeII] bow shock
PHK 3 (labeled knot D by Fridlund & Liseau 1998).
The HH 34 outflow in L 1641 ( pc) is part of a
parsec-scale "superjet'' that includes HH 33, 40 and 85 to the
north, and HH 86-88 to the south (Devine et al. 1997a). The
collimated, knotty, HH 34 jet itself has been the subject of intense
scrutiny at optical and near-IR wavelengths. Within 22
of the
IRS source, the jet remains highly collimated, the optical emission
being confined to within a width of less than 1
(Ray et al. 1996; Reipurth et al. 2000). The flow is orientated at an angle of
23
to the plane of the sky, and in optical emission lines
radial and tangential jet-knot velocities of
km s-1 and
150-300 km s-1 have been recorded (Bührke et al. 1988;
Heathcote & Reipurth 1992; Eislöffel & Mundt 1992).
Like L 1551-IRS5, the HH 34 jet near the source is observed in both
H2 and [FeII] emission (the latter was first detected in the images
of Stapelfeldt et al. 1991), though there are again differences in the
details (discussed in Sect. 7). The [FeII] flux distribution closely
follows that seen in [SII] (Ray et al. 1996), being much weaker within
the first 10
of the source. The [FeII] profiles along the
jet, at offsets between 10
and 20
,
comprise a
blue-shifted peak at
km s-1 plus an extended
red-wing. These broad lines appear almost double-peaked between knots
E and I (Fig. 1e), although the [FeII] profiles furthest from the
source (towards knot J) converge to a single, slightly less
blue-shifted radial velocity of about -100 km s-1. The [FeII] profiles
are very similar to those seen in [SII], in terms of the velocity at
the emission peak, the overall range of radial velocities observed,
and the general "inverted-V'' shape of the emission profile in the
P-V plot between knots E and J (Bührke et al. 1988; Heathcote
& Reipurth 1992).
Towards the continuum source position the [FeII] profile comprises a
sharp peak at -95() km s-1 and a broad red wing that extends
back almost to the systemic rest velocity of
8 km s-1 (Chernin &
Masson 1995). The bulk of the [FeII] emission towards HH 34-IRS is
therefore blue-shifted to velocities that are almost as high as those
seen along the jet.
In Paper I we found that the H2 emission towards HH 34-IRS is coincident with the source continuum position (unlike the other YSOs observed, where the H2 towards the source is usually offset along the blue-shifted flow lobe by at least a few tenths on an arcsecond). Unfortunately, we detected only very weak continuum emission from HH 34-IRS in our H-band observations, so we cannot measure (or set accurate upper limits on) the offset between the [FeII] line emission and the continuum position.
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Figure 2:
Same as Fig. 1, except that in AS 353A the contours measure
5, 10, 20, 50, 100, 150, 200, 400, 800![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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The distant, intermediate-mass YSO HH 72-IRS ( pc) in
L 1660 drives an east-west bipolar molecular outflow (Schwartz et al. 1988; Reipurth & Graham 1988). No collimated HH jet is observed
from this YSO; optical HH emission knots are only observed near the
eastern end of the flow, where it exits the dense core that harbours
the powering source. However, additional shock features are detected
closer to the source in H2 emission (Davis et al. 1997, 2002).
From the partial overlap between the CO flow lobes, this outflow is
probably orientated at 30
-60
to the plane of the sky.
Unpublished [FeII] images obtained by one of us (CJD) reveal emission
only from the optical HH 72 bow shock (the knots labelled A, B and C
by Reipurth & Graham 1988). Our spectrograph slit, which was aligned
along the small-scale H2 jet axis that extends a few arcseconds to
the east of the IRS source (Davis et al. 2002), passed just to the
north of these [FeII] features, so they do not appear in the P-V plot
in Fig. 1f. However, [FeII] emission was detected towards the source
continuum position. The [FeII] profile in Fig. 1f is very similar to
the profile seen towards HH 34-IRS, even though it is much weaker and
is associated with a more distant and more massive YSO. The [FeII]
emission from HH 72-IRS peaks at
km s-1, while a broad red wing extends to near-zero radial velocities (with
respect to the systemic rest velocity of +20 km s-1; Schwartz et al. 1988).
As with HH 34-IRS, we are not able to accurately measure the offset between the line emission and the source continuum because the latter is too weak in the H-band.
The HH 379 outflow is situated near the molecular cloud 093.5-04.3 in
Cygnus (Dobashi et al. 1994) at a distance of 0.9 kpc.
HH 379-IRS may be associated with a nearby compact optical nebula
(Devine et al. 1997b). In Paper I we assumed that this
conical nebula harbours the HH energy source and therefore positioned our
spectrograph slit through the nebula and the HH object; we have done the
same here with our [FeII] observations.
No continuum emission was detected from the outflow source in the [FeII] data. However, a distinct [FeII] line-emission peak is observed towards the nominal source position (Fig. 1g). The emission from HH 379-IRS is morphologically and kinematically very similar in [FeII] and H2. Both H2 and [FeII] peaks in the P-V diagrams appear to be elongated at a position angle that implies a blue-shifted flow towards the west of the source and a red-shifted flow to the east. However, note that HH 379-IRS is somewhat unique amongst the Class I YSOs observed here, since it is the only source with [FeII] that peaks near the systemic rest velocity (discussed further in Sect. 7).
Again, we do not measure the offset between the line emission and the IRS source position because of the lack of continuum emission at these shorter wavelengths.
![]() |
Figure 3:
[FeII] P-V diagrams of five of the outflows. Only emission within
approximately 15
![]() ![]() ![]() |
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![]() |
Figure 4: Offsets of the [FeII] emission peaks from the stellar continuum centroid. The boxes (with error bars) represent the [FeII] positions, measured in each column (at each velocity bin) from the P-V diagrams. The points at high (blue- and red-shifted) velocities are measures of the continuum position. The dashed line in each plot is a polynomial fit to these continuum points; these lines show the source continuum position. |
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DG Tau and RW Aur were not part of our original H2 survey (Paper I), although AS 353A was observed in the earlier study. We discuss these three sources separately below.
The classical T Tauri star DG Tau (
140 pc) drives a
well-collimated HH jet (HH 158) that extends over about 10
to
the southwest (Mundt et al. 1987; Lavalley et al. 1997;
Eislöffel & Mundt 1997). The jet itself has recently been observed
at high spatial resolution, with HST by Bacciotti et al. (2000, 2002)
and with adaptive optics (AO) by Dougados et al. (2000). These data
show the jet structure within a few arcseconds of the source, and even
point to possible precession and/or jet-rotation on arcsecond scales.
Here we adopt the same slit PA as was used by Bacciotti et al. (1998) in their HST STIS observations.
Hamann et al. (1994) report the detection of blue-shifted [FeII]
1.644 m emission from DG Tau, though they did not observe along the
jet axis. Takami et al. (2002) show [FeII] 1.257
m spectra which
likewise exhibit high, blue-shifted velocities, though again their
spectrograph slit was not aligned with the jet axis. Here we detect
[FeII] predominantly from the DG Tau jet (although we do not
detect a counter-jet). In these data, within 1
-2
of
the source the [FeII] peaks at
km s-1 (this
spectrum is labelled "jet'' in Fig. 2a) while further downwind (offset
-11.4
), towards the bow-shock-like knot labelled knot C by
Eislöffel & Mundt (1997), the [FeII] peaks at
km s-1. Note that for DG Tau the systemic LSR velocity is
6.0 km s-1 (Kitamura et al. 1996) and that the weak features
at -460 km s-1 in the two spectra in Fig. 2a represent the same artefact,
namely an imperfectly-subtracted sky line.
The decrease in velocity between the source and bow shock C in the
DG Tau jet is continuous along the weaker [FeII] emission observed
between these two features. The [FeII] is broad near the source (FWHM
km s-1) though the profile narrows further
downwind. The [FeII] radial velocities compare very closely with
published [SII] observations (Mundt et al. 1987; Bacciotti et al. 2000).
In Fig. 3d we show a continuum-subtracted P-V diagram which more clearly shows the [FeII] emission near DG Tau. In Fig. 4d we plot the positions of these [FeII] emission peaks with respect to the source continuum. As with some of the Class I sources, we see that the offset of the FEL emission decreases with increasing blue-shifted velocity.
High-resolution optical observations of the DG Tau jet, in
[SII] and [NII], reveal a compact knot at a projected distance of
about 0.6
-0.8
from the source and a broader bow shock
feature at a distance of 3
-4
(labelled A1 by
Bacciotti et al. 2000). Based on the offsets recorded in Fig. 4d we
associate the [FeII] emission with this bow shock. Excitation in a
bow shock would certainly explain the broad emission-line profile
shown in Fig. 2a. The FWZI of the profile points to a high shock
velocity, of the order of 200 km s-1. If this bow shock is an "internal
working surface'', associated with faster jet material catching up
with slower jet gas, then the velocity difference between these two
outflow episodes must be very high.
Within 1
of DG Tau we do not detect any [FeII] emission. The
knots in Fig. 3d (labelled "noise'') are due to Shot noise
associated with the bright source continuum emission; further along
the slit, where the data are read-noise limited, noise levels are
lower so faint [FeII] emission can be detected. Thus, any similarly weak
[FeII] associated with the [SII] knot observed within 1
of the
source (knot A2; Bacciotti et al. 2000) could easily be lost in this
noise.
RW Aur is a complex multiple star system located in Taurus-Aurigae
( pc). The HH 229 jet is associated with the brightest
stellar component "A'', a classical T Tauri star which may itself be
a spectroscopic binary (Gahm et al. 1999). The fainter components
"B'' and "C'' form a close binary system (separation 0.12
)
that is situated about 1.5
away. The lobes of the bipolar jet
from RW Aur-A (hereafter referred to as simply RW Aur) are visible
over a considerable distance; Mundt & Eislöffel (1998) report a jet
length of
145
.
In optical forbidden lines, radial
velocities of 100-200 km s-1 have been observed along the central
20
of the jet (Hirth et al. 1994; Bacciotti et al.
1996). Recently, this region of the jet has been observed at high
spatial resolution, using AO imaging (Dougados et al. 2000) and HST
STIS spectroscopy (Woitas et al. 2002). Woitas et al. estimate an
inclination angle for the jet of <37
to the plane of the sky.
In [FeII] we detect emission from both lobes of the bipolar jet
(Figs. 2b and 3e). Again, the emission peaks are offset along the jet
axis, by 0.5
-1.0
with respect to the source continuum
centroid (Fig. 4e), with the red-shifted [FeII] emission peaking
closest to the source. AO and HST STIS observations of RW Aur show
that, in the optical, both lobes of the jet are knotty, though well
collimated (FWHM < 0.6
;
Dougados et al. 2000; Woitas et al. 2002). Woitas et al. find that the [SII] flux along the
blue-shifted jet lobe is rather evenly distributed within
2
of RW Aur; in [FeII] the emission is also extended
over this same region (Fig. 3e). In the red-shifted counterjet,
however, the flow appears more knotty on subarcsecond scales in the
[SII] data. We do not have the spatial resolution to resolve these
knots in [FeII], though we do see emission along the length of
this [SII] counterjet. We also note that, as in [SII], the red jet
lobe is more extended at higher velocities (between
km s-1 and 160 km s-1) than it is at lower velocities (
50-100 km s-1; see Fig. 3e). Moreover, in
Fig. 4e there is some evidence that the [FeII] peak is slightly
further offset at higher (red-shifted) velocities.
From the [FeII] profiles in Fig. 2b we measure peak radial LSR
velocities of -175() km s-1 and +150(
) km s-1 for the jet
and counterjet features (the systemic LSR velocity is
6 km s-1;
Ungerechts & Thaddeus 1987).
On similar spatial scales, Hirth et al. (1994) measure [SII]
radial velocities for the southeastern jet
and northwestern counterjet of -190 km s-1 and +100 km s-1 respectively
(note that we use the same slit PA). The [FeII] line profiles in
Fig. 2b are narrow, however: Gaussian fits yield line widths at FWHM of
50 km s-1 for both lobes in RW Aur. We do not detect any
[FeII] emission at low radial velocities, in either jet lobe (the
[SII] emission is weaker, though still observed, at these low radial
velocities). In other words, no LVC is detected in [FeII] in either
lobe. This current lack of an LVC in FEL emission was also noted by
Woitas et al. (2002) in their optical [SII] and [OI] observations.
These authors have suggested that the LVC may well be variable on a
timescale of a few years.
The highly blue- and red-shifted radial velocities evident in the [FeII] spectra indicate that the emission must be associated with fast-moving gas along the RW Aur flow axis. However, as with some of the other sources discussed above, the narrow line widths point to low shock velocities. Higher shock velocities are predicted for DG Tau (discussed above), where the [FeII] profiles are much wider (note that in HST images the bow shock feature A1 in DG Tau is laterally more extended than the knots in DG Tau; i.e. it has more extended wings).
The classical T Tauri star AS 353A ( pc, Mundt et al. 1983; Eislöffel et al. 1990) drives an
obliquely-viewed bipolar HH flow, known as HH 32 (Hartigan et al. 1986; Davis et al. 1996; Curiel et al. 1997).
Optical and near-IR images and spectroscopy of the leading, redshifted
HH 32 bow shock are convincingly modelled if the flow is inclined at
an angle of
60
to the plane of the sky (Solf et al. 1986; Hartigan et al. 1987; Davis et al. 1996).
As was the case in Paper I, two stars were detected along the single slit position observed in the AS 353A region, which we again label 1 and 2 in Fig. 2c (AS 353A itself, the apparent source of the bipolar HH 32 outflow, is referred to as star 1; note, however, that star 2 is not AS 353B). H2 emission was detected from star 2, and not from star 1; we did not detect [FeII] from either.
We do detect spatially-compact [FeII] from the leading edge of the
HH 32 bow shock. As expected, the [FeII] profile is red-shifted to
very high radial velocities; the double-peaked profile in Fig. 2c
comprises components at
km s-1 and
km s-1 while, overall, the emission extends over
km s-1(the
systemic LSR rest velocity is at
8 km s-1; Edwards & Snell
1982). The [FeII] profile is again quite similar to its optical
counterpart in [SII], where double-peaked lines with components at
km s-1 and
km s-1 have been reported (Hartigan et al. 1987). However, in the [SII] spectra discussed by Hartigan et al. the low-velocity component is much stronger than the high-velocity
peak; in our [FeII] data, we see the opposite behaviour. This is
because the optical profile is summed across the whole HH 32 clumpy
bow shock region, while our single slit passes through only the centre
of the bow (it largely bypasses the limb-brightened bow shock wings,
seen clearly and labelled knots B and C in high-resolution optical
images; e.g. Curiel et al. 1997). Observing more of the emission from
the bow wings would certainly explain the "enhanced'' lower-velocity
component in the [SII] profile. The double-peaked [FeII] profile in
Fig. 2c, and the low-velocity H2 reported in Paper I, do therefore
comply with previously published bow shock model fits to kinematic
studies of HH 32 (e.g. Hartigan et al. 1987; Davis et al. 1996).
The Br12 HI recombination line (
1.641168
m)
was included within the wavelength coverage of our echelle
observations. The difference between the Br12 and [FeII] rest
wavelengths is -0.00283
m, which is equivalent to a velocity shift
of 516 km s-1. Br12 was detected in two Class I sources and in all
three T Tauri stars. These data are presented in Fig. 5, with
the rest wavelength of Br12 set to
km s-1.
In SVS 13 the Br12 spectrum is broad and possibly slightly asymmetric
(Fig. 5a). Gaussian fitting yields a FWHM of
km s-1 and a
peak velocity of
km s-1, although the actual intensity peak
is shifted to about -20 km s-1. A similarly broad, low-velocity
line is observed towards the only other Class I YSO that we
detected in Br12 emission, IRAS 04239+2436 (Fig. 5b). Fits yield a
FWHM of
km s-1 and a peak velocity of
km s-1 for
this source.
Br12 emission was detected towards all three T Tauri stars (Figs. 5c-e), where the integrated line luminosities were at least
5
stronger than they were for the Class I YSOs. In DG Tau,
the Br12 profile is blended with blue-shifted [FeII] emission, which
in the rest frame of the HI line appears at high (>250 km s-1) radial
velocities (indicated in Fig. 5c). The Br12 is, nevertheless, again
very broad and centred near the systemic rest velocity. The RW Aur
Br12 profile in Fig. 5d is markedly asymmetric and strongly
blue-shifted. The line consists of a central peak at
km s-1 and broad blue and redshifted line wings. The profile extends over
about 600 km s-1 FWZI. In AS 353A we observe very strong Br12
emission. The profile towards this source is notable for being
extremely symmetric and Gaussian in shape, with no sign of enhanced
line-wing emission, nor blue or red-shifted absorption features.
Overall, the width of the AS 353A profile (FWHM) measures
km s-1; the line also peaks at a very low radial velocity of
km s-1.
HI observations of many of our targets were also presented in Paper I,
where the K-band Br
emission line was observed. The same slit
positions and slit angles were used for the Br
data as were
used for Br12/[FeII] and H2. Br
was detected towards four
Class I YSOs; SVS 13, IRAS 04239+2436, HH 34-IRS and GGD 27(1), as
well as towards the only T Tauri star studied in Paper I, AS 353A.
Here we detected Br12 in the same Class I sources, except for
HH 34-IRS and GGD27 (the latter was not observed at 1.64
m). Our Br12
non-detection for HH 34-IRS was not unexpected since the Br
line was very weak in this source and, for the other Class I sources,
the Br12 emission was 120-160
weaker than Br
.
For
AS 353A, the ratio is lower, Br
,
because of the
reduced extinction to this source. Indeed, extinction is probably the
main cause of the differences in line ratios observed. For
K and
cm-3 a ratio of 5 is expected
(Hummer & Storey 1987); this ratio is observed in dense shock regions
and PDRs like Orion Peak-1 and Hubble 12 (Everett et al.
1995; Luhman & Rieke 1996). Assuming similar excitation conditions,
the Br
/Br12 ratio may therefore be used to roughly estimate
the extinction to the HI region. The difference in extinction at
1.6
m and 2.2
m may be written as:
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(1) |
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Figure 5: Br12 spectra towards the source continuum positions in five regions (the sum of three adjacent rows is again plotted). Br12 was not detected in any of the other YSOs. Stationary [FeII] emission would be offset by +512 km s-1 in these spectra. |
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In Paper I we identified the Br
emission with hot gas in the inner
disk and accretion flow, rather than with outflow material, because
the Br
emission was found to be spatially coincident with each
source and confined to the source position (i.e. the emission was not
extended along the outflow axis, unlike the H2 and [FeII]). The
Br
profiles were also found to be very broad, typically
200 km s-1 FWHM, symmetric in shape, and slightly blue-shifted,
typically by 10-30 km s-1. Such profiles could be due to a combination
of keplerian rotation in the inner regions of a circumstellar disk
(within 0.1-0.01 AU of a 1
star) and magnetospheric accretion
(Hartmann et al. 1994; Muzerrolle et al. 1998), although accretion
models do tend to produce red-shifted absorption features, which were
not observed in Paper I and are statistically rare in optical and
near-IR surveys of HI emission from T Tauri stars (e.g. Reipurth et al. 1996; Folha & Emerson 2000). In any case, the Br12
emission in the five YSOs observed here appears to have a similar
origin, since the profiles in Fig. 5 are all broad and they peak at
low radial velocities. From Gaussian fits to cuts made perpendicular
to the dispersion axes in our P-V plots, we also find that the Br12
peaks are coincident with the stellar continuum centroids (to an
accuracy of <1
), and that the emission is not extended
along the slit/outflow axes.
The one possible exception to the characteristics described above is
RW Aur. Here the Br12 profile is blue-shifted and clearly
asymmetric. The velocity shift and the extensive blue wing evident in
Fig. 5d could be explained in terms of emission from an outflow,
although we do not see clear evidence in our data that the Br12
emission is extended along the jet axis. The permitted HI Brackett
lines are not usually detected in outflows on large, arcsecond
(>100 AU) scales. However, HI may be excited at the very base of
some CTTS jets. In the optical, Takami et al. (2001) have measured
spatial offsets - on AU scales - in the high-velocity wings of their
H
spectra of the CTTS RU Lupi. This suggests that the
emission could, at least in part, be excited in the flow.
Consequently, although the excitation conditions necessary for
Brackett line emission are typically not met in the extended outflow
lobes and HH objects, they may be met in a jet within a few AU of the
central source. Higher-resolution, near-IR spectro-astrometric
observations of embedded YSOs, similar to those acquired by Takami et al. (2002) for DG Tau, are urgently needed to study these regions in
more detail.
Finally, we briefly consider why we do not detect Br12 (nor indeed
Br
in Paper I) towards all of the YSOs observed. Although
extinction and, to a lesser extent, differing excitation conditions
may play a role, the lack of Br
and Br12 emission towards the
majority of the Class I sources may be due to the fact that the
embedded source, and therefore the line emission region, is not
observed directly. If this is the case, then the continuum emission
we detect in the K-band and, particularly, the H-band may be
nebulosity associated with the YSO that is slightly offset from the
true source position, rather than photospheric emission from the
protostar. On the other hand, if HI emission is observed (as is
the case for two of the most interesting Class I YSOs, SVS 13 and
HH 34-IRS), then we may be confident that we are observing the source
directly, and that the offsets reported above (and in Paper I) between
the MHEL and FEL regions and the continuum centroids are indeed
between the emission line regions and the central stars in each case.
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Figure 6: A comparison of the [FeII] and H2 spectra extracted from the source-continuum position in each target (the H2 and [FeII] spectra represent the same 3-row-wide area on each source). H2 spectra are drawn with full lines; [FeII] data with thick, dotted lines. The spectra have been multiplied by the values shown to the right of each plot (H2 multiplication factor is unbracketted; [FeII] factor is bracketted). A constant has also been added to each plot so that all data can be included in the same figure. |
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Having described the [FeII] and H2 observations in Sect. 4 above and in Paper I, we now compare the two data sets for each source in some detail. To aid with this comparison, we plot in Fig. 6 H2 and [FeII] spectra on the same plot axes. In both cases the spectra represent the sum of three adjacent rows centred on the source continuum position, so they effectively cover the same area on each YSO. We assume that any slight spatial shift in the absolute position of the slit with respect to the source, caused by a combination of orientation and the change in extinction between the H and K-bands, is insignificant.
Line peak velocities and line widths derived from the spectra in
Fig. 6 are listed in Table 2. In most cases these are measured from
multi-component Gaussian fits to each profile, although for B 5-IRS1,
L 1551-IRS 5 and HH 34-IRS the parameters were measured "by eye''
from the spectra, since these lines are clearly non-Gaussian in shape.
The errors on the individual velocities are probably dominated by
systematic effects (as described in Sect. 2) rather than by errors in
the fitting; errors in
and VFWHM are therefore
of the order of <10 km s-1.
In L 1551-IRS5 and HH 34, where complex line emission spectra were detected along the jet axes, in both H2 and [FeII], we also show (in Fig. 7) P-V diagrams plotted side-by-side, and discuss these jets further in Sect. 7.2.
Towards SVS 13, the [FeII] and H2 profiles in Fig. 6 are both
complex and double-peaked. The H2 components peak at lower radial
LSR velocities,
km s-1 (LVC) and
km s-1 (intermediate-velocity component, or IVC), as compared to
km s-1 (LVC) and
km s-1 (HVC) in [FeII]. The most
significant difference in these data is the fact that, in H2, the
intermediate-velocity component (IVC) is weaker than the LVC, while in
[FeII] the opposite is the case - the HVC dominates. Also, overall
the [FeII] profile is about twice as broad as the H2 profile. It
therefore seems likely that, although both low- and
intermediate/high-velocity components are observed in H2 and
[FeII], the latter is nevertheless a better tracer of the highest flow
velocities at the base of the jet.
H2 and [FeII] emission profiles observed towards B5-IRS 1 are also
shown in Fig. 6. The H2 profile is mildly asymmetric,
exhibiting a weak blue-shifted wing that extends out to about
-40 km s-1. The line profile is centred at
km s-1,
close to the systemic velocity of
10 km s-1, though it extends
over almost 100 km s-1 FWZI. The
[FeII] profile may be double-peaked, though our detection is marginal.
Overall, the [FeII] emission appears to be blue-shifted with respect
to the H2.
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Figure 7:
A comparison of the [FeII] and H2 P-V diagrams for L 1551-IRS 5
and HH 34-IRS. In each plot the contours increase in multiples of
the 1![]() |
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Towards IRAS 04239+2436 only a single LVC is observed in H2; the
profile peaks within a few km s-1 of the systemic velocity. By
comparison, [FeII] traces much higher radial velocities, in both the
blue and red jet lobes (Table 2). Note, however, that for the [FeII]
observations we used a different slit PA of 59
(an angle of
45
was used for the H2 spectroscopy in Paper I), which is
better aligned with the jet axis.
Towards the H-band continuum position of L 1551-IRS 5 a
multi-component [FeII] spectrum is observed (Fig. 6); at least two
components are identified, a bright HVC at
km s-1 and an EHVC
at -285 km s-1. In H2, a single LVC is observed, peaking at
approximately -7 km s-1, though there is a blue-shifted "bump'' at
km s-1 superimposed onto this, otherwise Gaussian, line
profile.
Near HH 34-IRS the [FeII] is strongly blue-shifted, while the much narrower H2 profile peaks at a relatively low velocity (Table 2). The H2 peak velocity is considerably lower towards the source position than it is along the jet, while in [FeII] the emission towards HH 34-IRS is almost as high as it is along the jet (described further below).
The [FeII] emission from HH 72-IRS peaks at
km s-1; a broad red wing extends to near-zero radial velocities. By
comparison, very complex H2 line emission is observed towards
HH 72-IRS. The H2 profile comprises at least three velocity
peaks, an LVC at +13 km s-1, an IVC at -40 km s-1 and an HVC -130 km s-1;
these are superimposed on to a blue wing that extends to a velocity of
km s-1. The [FeII] peak appears to be associated only with
the most blue-shifted component, the HVC.
Lastly, towards HH 379-IRS, the H2 and [FeII] profiles peak at almost the same, low, radial velocity (Table 2). This is in stark contrast to the other sources in Fig. 6; in all other sources the [FeII] towards the central YSO is strongly blue-shifted. These low H2 and [FeII] radial velocities in HH 379-IRS could simply be due to the orientation of the flow with respect to the line of sight (which is not well known), if the flow lies in the plane of the sky. The [FeII] may still trace the higher-velocity jet component. Notably, the [FeII] line is about twice as broad as the H2 profile, as would be expected in such a scenario (Table 2).
To summarise then, within a distance along each outflow axis of less than an arcsecond (i.e. within approximately 140 AU-1500 AU of the outflow source, depending on the distance to the target), for all Class I sources observed, the [FeII] emission is accelerated to much higher radial velocities than the H2 emission (see also Table 2).
Complex H2 and [FeII] line emission is also observed along the inner jet regions in L 1551-IRS 5 and HH 34. In Fig. 7 we show these P-V diagrams together for ease of comparison.
In the L 1551-IRS 5 system we detect H2 line emission in both the
blue jet and (weakly) in the red-shifted counterjet. The counterjet is
not detected in [FeII], presumably because of increased extinction at
these shorter wavelengths. In the southwestern blue lobe (negative
offsets in Fig. 7) the H2 velocities are generally much lower than
the [FeII] velocities. A bright H2 feature is observed at an
offset of -6
with a radial velocity of -55 km s-1 which has no
obvious compact [FeII] counterpart (although diffuse [FeII] is
detected in this region with
km s-1), and
double-peaked H2 is observed towards knot PHK 3 (as compared to
the single, narrow [FeII] component). For PHK 3, the combined [FeII]
and H2 observations can be understood in terms of a geometrical bow
shock model, if the H2 is excited in the oblique bow wings, with
the [FeII] produced in the high-velocity/high-excitation bow shock cap
(e.g. Hartigan et al. 1987; Tedds et al. 1999).
Along the HH 34-IRS jet axis (positive offsets in Fig. 7) the H2and [FeII] emission features peak at very similar blue-shifted
velocities, even though the H2 and [FeII] emissions are clearly
excited in different regions of the flow. H2 is observed just
ahead of knot L and between the source and the first optically-bright
HH knot in the jet (knot E), while the [FeII] is brightest between
these two regions, at offsets of 10
-25
.
The [FeII]
profiles along the jet clearly comprise a blue-shifted peak (
km s-1) plus an extended red-wing. At higher spectral
resolution these profiles would probably appear double-peaked between
knots E and I, though the [FeII] profiles furthest from the source
(towards knot J and K) converge to a single, slightly less
blue-shifted radial velocity of about -100 km s-1. By comparison, the
H2 profiles along the jet are narrower, single-peaked and centred
at a radial velocity of
km s-1. Collectively, these [FeII]
and H2 characteristics can, once again, be understood in terms of
excitation in unresolved bow shocks. We identify the [FeII] emission
features with the optical knots E-K (Bührke et al. 1988; Ray et al. 1996). These features are spatially resolved in the HST images of
Ray et al. (1996). The double-peaked [FeII] profile at offsets of
18
-20
probably derives from the bow-shock shaped knot
I, which in [SII] is one of the broadest and brightest knots in the
inner jet region. The [FeII] line widths (and indeed the presence of
[FeII] emission in the HH34 jet), suggests excitation in J-type shocks
with velocities of the order of 100 km s-1. Because each bow is running
into fast-moving pre-shock gas, the H2 profiles may also be
strongly blue-shifted (because the gas is non-stationary) although the
lines will be much narrower, since the molecular gas must be excited
(rather than dissociated) in the oblique bow shock wings, where
incident shock velocities will be low.
Source/ | [FeII] | [FeII] | [FeII] | H2 | H2 | H2 | ||
velocity |
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Component | (km s-1) | (km s-1) | (degrees) | (km s-1) | (km s-1) | (degrees) | (km s-1) | (degrees) |
SVS 13 | 8 | ![]() |
||||||
LVC | -35 | ![]() |
100 | -20 | 34 | 86 | ||
IVC | -89 | ![]() |
32 | |||||
HVC | -133 | 70 | 42 | |||||
B5-IRS16 | 10 |
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LVC | 4 | 35 | 105 | |||||
HVC | - | ![]() |
- | |||||
IRAS 04239+2436 | 8 |
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LVC | 7 | 35 | 174 | |||||
HVC | -125 | ![]() |
24 | |||||
L1551-IRS56 | 6 |
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LVC | -7 | 28 | 113 | |||||
HVC | -120 | 111 | 64 | |||||
EVC | -285 | 124 | 34 | |||||
HH 34-IRS6 | 8 | ![]() |
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LVC | 1 | 16 | 81 | |||||
HVC | -95 | ![]() |
23 | |||||
HH 72-IRS | 20 | ![]() |
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LVC | 13 | 24 | 135 | |||||
IVC | -40 | 39 | 49 | |||||
HVC | -130 | ![]() |
24 | -129 | 19 | 10 | ||
HH 379-IRS | 4 | - | ||||||
LVC | 0 | 53 | - | 4 | 15 | - |
Source | 1d | ![]() |
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3 M
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3 M
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(pc) | (mags) | (W m-2) | (W m-2) | (![]() |
(![]() |
(![]() |
(![]() |
|
SVS 13 | 220 | 30 |
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B5-IRS1 | 350 | 10 |
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IRAS 04239+2436 | 140 | 30 | ||||||
blue-jet |
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red-jet |
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- |
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- |
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- | ||
L1551-IRS5 | 140 | 18 |
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HH 34-IRS | 450 | 5 |
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HH 72-IRS | 1500 | - |
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HH 379-IRS | 900 | - |
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The [FeII] observations reported in this paper have kinematic and spatial characteristics that are very similar to [SII] observations of jets from YSOs, in the outflows from the Class I YSOs and the CTTS jets. We may be certain, therefore, that [FeII] is a powerful tracer of FEL regions towards HH energy sources, particularly at high velocities and amongst the more deeply-embedded near-IR Class I sources.
We have compared in detail the [FeII] data with the H2 observations from Paper I. We find that the H2 traces a low-velocity molecular flow component (which is notably absent from the jet driven by the only T Tauri star observed in Paper I, AS 353A) while the [FeII] traces intermediate and high-velocity gas (from both Class I YSOs and CTTSs).
Towards the Class I outflow sources themselves, the [FeII] profiles are usually very wide and strongly blue-shifted, to radial velocities comparable with the rest of the jet further downstream, while the H2 emission towards the source is only slightly blue-shifted, by a few tens of km s-1, even though higher-velocity H2 is often detected further downstream (see e.g. HH 34 and L 1551-IRS 5 in Fig. 7). Also, the [FeII] emission peaks are spatially further offset along the jet axes than the H2 peaks (compare Fig. 4 above with the same graphs in Fig. 4 of Paper I). It therefore seems likely that the [FeII] emission is more closely tied to emission knots and shock fronts along the extended jet axes, while the H2 is predominantly excited closer to the outflow source in each system, where ambient gas densities rise sharply. The slow-moving H2 probably derives from the boundary between the jet and the stationary ambient gas.
In Table 2 we compare the line peak velocities and the line widths of
the individual velocity components observed in H2 and [FeII]. From
these data we crudely estimate an opening angle, ,
for each
velocity component, from the velocity of the emission peak and the
component width (we assume that the velocity dispersion,
). These data are
corrected for outflow inclination angle (where known) and the systemic
velocity. The estimated angles are at best only upper limits, since
each line component width will be broadened by dynamical processes
other than the lateral expansion of the jet (e.g. turbulence,
rotation, etc.). Nevertheless, the higher-velocity jet components,
seen predominantly in [FeII], are probably more highly collimated than
the lower-velocity molecular components seen (in all cases) in H2,
even though the [FeII] profiles are in all cases broader.
In Table 3 we then list integrated line intensities measured from each
extracted H2 and [FeII] spectrum in Fig. 6. From these we are
able to estimate a mass outflow rate and momentum for the flow
components seen in H2 and [FeII] respectively. M
and
are taken directly from Paper I. M
and
are derived in essentially
the same way. The observed line intensity,
,
is
related to the column density of Fe+ ions in the upper state of the
1.64
m transition,
,
by
,
where
is measured in W m-2 ster-1,
is the radiative decay rate of the 1.64
m transition (
s-1; Nussbaumer & Storey 1988) and
is the frequency of the transition. The total column
density of hydrogen atoms and ions,
,
is then given by
,
where
and
are the
fraction of Fe+ ions in the upper state and the fraction of Fe that
is singly ionised. (Fe/H) is the Fe-to-H abundance ratio; we adopt
a solar ratio of
(Grevesse & Ander 1989). Hamann
et al. (1994) have calculated values of
over a range of electron
temperatures and densities; they find that
is sensitive only to
.
For
cm-3 (close to the critical density
for excitation of the observed transition) a value of
is
predicted, which we use here. Hamann (1994) also predict
under similar conditions.
The mass outflow rates and momenta in Table 3 are calculated for the
areas encompassed by the 3-row-wide extracted spectra displayed in
Fig. 6. In the H2 and [FeII] observations this angular area is
the same, though because of the different distances to each region the
actual area differs from source to source. In calculating M
we assume that the emission is extended along the jet axis, i.e.
,
where
(
)
is a dynamical time scale for the observed section of the jet. If the
emission is unresolved along the jet,
will be overestimated
and M will be underestimated, although because we have already
established that the emission is offset and/or extended along the axis
in each system (in H2 and [FeII]), this is probably not a dominant
source of uncertainty.
The largest sources of error in M and MV in Table 3 are
probably; (1) the value of
used to correct the observed
intensities,
and
,
for extinction, (2)
the inclination angle of the flow, and (3) the velocity adopted for
the [FeII] and H2 flow components. The values of
used
(references are listed in Paper I) are estimated usually from
molecular column densities measured from (sub)mm observations made at
low angular resolution, or optical line ratios. Clearly, because
neither the submm nor the optical emission derives from the same
region as the near-IR emission very near to each source, the values of
used in our analysis will be uncertain, and probably
underestimated (because of beam dilution in the submm, or because the
optical emission is detected only from lower-extinction regions
further out). Low-resolution H-band spectroscopy would be useful to
measure the extinction from the ratio of the [FeII] lines at
1.644
m and 1.257
m (e.g. Reipurth et al. 2000). Then the
extinction towards the [FeII] region - probably the largest source of
error in Table 3 - would be measured directly. In a few sources
is not know at all; for HH 72-IRS and HH 379-IRS we do not correct
the parameters in Table 3 for extinction, so these are listed as lower
limits. An
of
25-40 would result in an increase of the
order of 102-103 in M
and
,
and 10-40 in M
and
.
(Note, however,
that a very high value of Av would probably render the [FeII]
emission unobservable!)
The inclination angle of the jet in most cases is known to within
10%, so this is probably a secondary source of uncertainty.
The choice of flow velocity used to calculate M and MV is,
however, somewhat arbitrary. We use the radial velocity of the
emission peak,
,
since we assume that the broad line
widths observed (particularly in [FeII]) are largely due to lateral
expansion of the flow, turbulence in a shear layer between the jet and
the stationary ambient medium, and thermal motions in the post-shock gas.
An uncertainty in
of 10-50 km s-1 could nevertheless
result in an additional error in M and MV of a factor of
2-3. Overall then, we estimate an approximate error of the order of a
factor of 10-100 on the mass loss rates and momenta listed in Table 3.
Even given the uncertainties listed above, the mass-loss rates and
momenta in Table 3 are fairly typical of HH jets from low-mass YSOs,
where e.g. the average mass loss rates vary from
to
yr-1 (Bacciotti & Eislöffel 1999).
In HH 34, for example, which is the most well defined jet in [SII],
[FeII] and H2 emission, a mass-loss rate of 2-
yr-1 has been measured from optical
emission-line studies (Heathcote & Reipurth 1992; Bacciotti &
Eislöffel 1999); in [FeII] we derive a mass loss rate at the base of
the jet of 7
yr-1, which is only a factor
of 3-5 lower. And in L 1551-IRS5, a momentum of
km s-1 has been predicted from recent HST observations
(Fridlund & Liseau 1998); in [FeII] we estimate
km s-1. The mass loss rates and momenta in
Table 3 are therefore not unreasonable.
It is also worth noting that, although the absolute errors on M and MV for the H2 and [FeII] flow components may be large,
the relative errors between these two parameters will be much
smaller, because the same extinction is used for each data set.
Consequently, if the [FeII] traces a collimated jet component, then
the above analysis suggests that the momentum in this jet is equal to
or greater than the momentum in the more poorly-collimated, molecular
outflow which we trace in H2 in each system. Moreover, given the
higher velocities associated with the [FeII] jet component, the
momentum supply rate, MV, of each jet should be sufficient to
drive the H2 flow. In other words, the [FeII] jet will supply
enough momentum per unit time to entrain and accelerate the molecular
flow seen in H2. There is even some evidence that this process is
more efficient in the more deeply embedded outflows, like SVS 13 and
B5-IRS1 (which have no optical jet) than it is in the less-embedded
flows, like L 1551-IRS 5 and HH 34-IRS (both YSOs have well-known
optical jets observed close to the central engine), since in SVS 13
and B5-IRS1 the momentum in the [FeII] and H2 components are
roughly equal, while in L 1551-IRS 5 and HH 34
is an
order of magnitude greater than
.
Finally, we reiterate that there is also some evidence to suggest that the H2 component is more poorly collimated than the [FeII] jet, as one would expect if the former is entrained in a boundary layer between the jet and the ambient medium. It seems likely, therefore, that entrainment of molecular material is present in YSO jets even within a few hundred AU of the central driving source. Higher-resolution spectroscopy across the width of a few MHEL flows are clearly needed to investigate this possibility further. Moreover, X-wind and disk-wind models should also now strive to predict FEL and MHEL characteristics within the first 1000 AU of the central outflow source, since observational data at high spatial and spectral resolution are now forthcoming.
[FeII] long-slit echelle spectroscopy of seven Class I YSOs and three CTTSs is presented. We detect emission towards the H-band continuum positions of all seven Class I sources (Fig. 6) and along the extended jet lobes of three of these. [FeII] emission is also detected in the HH jets of the three CTTSs; in RW Aur and DG Tau this emission is traced to within a few arcseconds of the source.
From a comparison of the [FeII] observations with published [SII] observations and the H2 observations in a companion paper (Paper I) we arrive at the following conclusions:
We suggest that the [FeII] emission derives from the base of a collimated, high-velocity jet which entrains ambient molecular gas within a few hundred AU of each HH energy source. The entrained gas is observed in H2 emission. Our analysis indicates that the [FeII] jet may well have sufficient power to drive the H2 flow.
Lastly, Br12 emission was also detected towards the CTTSs and towards
two of the Class I YSOs. These HI emission profiles are very broad,
though they are single-peaked and relatively symmetric in shape. The
profiles peak at low blue-shifted velocities. The emission is also
confined to the source position, i.e. we see no distinct offset of the
emission peak along the jet axis, and we do not detect Br12 from the
extended jet regions. We therefore associate the emission with the
same high-excitation regions observed in Br
in Paper I, namely
the inner regions of the accretion disk, magnetospheric accretion
flows, and/or the first few AU of the jet.
Acknowledgements
We thank Watson Varricatt for his assistance at the telescope. The UKIRT is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council. The Starlink Software Collection is managed and distributed by the Starlink Project which is funded by PPARC.