A&A 372, 516-526 (2001)
DOI: 10.1051/0004-6361:20010502
B. Y. Welsh 1 - D. M. Sfeir 1 - S. Sallmen 1 - R. Lallement 2
1 - Experimental Astrophysics Group, Space Sciences Laboratory, UC Berkeley, Berkeley, CA 94720, USA
2 - Service d'Aéronomie du CNRS, 91371 Verrières-le-Buisson, France
Received 14 February 2001 / Accepted 2 April 2001
Abstract
We present Far Ultraviolet Spectroscopic
Explorer ()
observations of high-velocity
gas (
= +65 kms-1) seen towards the star HD 47240
which lies just behind the Monoceros Loop Supernova
Remnant at a distance of
1800 pc. This high-velocity
absorption feature is detected in the far ultraviolet lines
of O I, Ar I, N I, C I, Fe II and P II, in addition to being detected
at visible wavelengths in Na I and Ca II and at near ultraviolet
wavelengths in Mg II, Mg I, S II, O I, Si II, C II*, Al II and Fe II.
High-velocity interstellar gas
has not been detected in the high-ionization (high-temperature)
species of O VI, C IV
and Si IV.
Gas phase abundances relative
to that of sulphur for this high
velocity feature have been derived.
The refractory elements of Fe, Si and Al are all less depleted than
that normally found for cold
disk gas in the interstellar medium, with
a pattern of relative abundance more similar to
that of warm interstellar disk gas.
However, the elements of
N, O, and Ar show an opposite pattern of relative depletion
in which their
apparent elemental deficiency may be attributed to ionization
effects, as also found for high-velocity gas associated with
the Vela SNR by Jenkins et al. (1998).
The lack of detection of high-ionization gas
at high velocity
suggests that the Monoceros Loop remnant is more evolved than
other remnants such as the Vela SNR or Cygnus Loop, and that an age
of 30000-150000 years seems appropriate.
Key words: ISM: supernova remnants - ISM: abundances
The NASA Far Ultraviolet Spectroscopic Explorer ()
was launched
on June 24th 1999,
and its instrumentation is ideally suited to probe
interstellar plasma existing in
a wide range of different physical conditions using the many
astrophysically important absorption lines present throughout the 912 to
1187 Å wavelength region (Moos et al. 2000). A particularly
interesting region for
FUSE to study highly disturbed interstellar
gas is the Monoceros Loop Supernova Remnant (SNR). This is a
well evolved remnant of age
105 years, is some 100 pc in diameter
and lies at a distance of
1.6 kpc (Odegard 1986).
The study of SNRs is important for understanding
both the energy and ionization balance of the interstellar medium,
since
supernova explosions and strong stellar winds from OB stars are the
dominant sources of high-temperature and high-velocity shocked gas in the
interstellar medium (ISM). Much of the energy released in a
supernova explosion is deposited in the kinetic energy of the ejecta
which subsequently interacts with the ambient
interstellar gas.
The emission characteristics of the gas associated
with the Monoceros Loop SNR have been
well studied at radio (Graham et al. 1982),
X-ray (Leahy et al. 1986),
-ray (Jaffe et al. 1997) and
visible (Fesen et al. 1985) wavelengths.
These and other studies indicate that
the Monoceros Loop is probably interacting with the
adjacent (and nearer) Rosette
Nebula (NGC 2244), which contains a cluster of several ionizing
early O-type stars (NGC 2264) known to be losing appreciable amounts
of mass to the ambient ISM (Kuchar
Bania 1993). Remarkably
for this well-known region, only one study of the Monoceros Loop/Rosette Nebula complex has been
undertaken using interstellar absorption lines.
Wallerstein
Jacobsen (1976) (hereafter WJ) have observed 25
stars in the direction of this SNR region using measurements
of the interstellar Ca II K-line (3393 Å) and Na I
D-lines (5890 Å) taken at a spectral resolution of
12 kms-1. These observations revealed
high-velocity absorption features
at
kms-1 towards the
star HD 47240 and another feature at
kms-1towards HD 47359, both of which have been associated with the
expansion of the nebular gas.
Although the gross kinematical properties of the Monoceros Loop/Rosette
Nebula region are well documented, the detailed interactions
of the associated complex
ionized, neutral
and molecular gas structures interaction are still much debated. For
example, although WJ detected gas cloud
components with 5 different velocity values
towards the 25 stars that they observed,
only
two of the velocities of these components were common to those
found by H I 21 cm observations.
Furthermore,
both of the high-velocity cloud components observed by WJ
in the Na I and Ca II lines towards HD 47240 and HD 47359 have
anomalous Na I/Ca II ratios < 1.0, which is consistent with an
enhancement
of interstellar Ca due to the destruction of ambient interstellar
dust grains by the passage of
high-velocity shocks (Siluk
Silk 1976).
Observations of stars associated with the
Vela SNR by Danks
Sembach (1995) have shown similarly
low values of the Na I/Ca II ratio for interstellar gas clouds with
velocities up to
60 km s-1.
UV observations of the same remnant have
revealed that
the gas at high velocity exhibits higher than normal ionization and
has unusually low abundances of N I and O I (Jenkins et al. 1998),
whereas the depletion of Al, Si and Fe in high velocity interstellar gas clouds
is substantially less (Jenkins et al. 1984).
As part of a long-term program using
the
satellite in which we
hope to observe the absorption
characteristics of the disturbed interstellar medium
towards 4 early-type stars in the direction of Monoceros
Loop SNR,
we currently present preliminary observations of high
velocity interstellar gas absorption seen towards
the B1Ib star HD 47240 (Mv = 6.2,
E(B-V) = 0.31)
which lies just behind the south central region of the SNR
at a distance of
1800 pc.
These far ultraviolet data have been supplemented
with archival near ultraviolet and newly
presented high resolution visible absorption line data
taken towards this star
to further understand the physical
and chemical nature of the
high-velocity interstellar gas clouds associated
with this expanding supernova remnant.
The spectral integrations accumulated with the instrument were obtained
in the histogram integration mode on March 15th, 2000.
The observations were performed with the star
centered in the
arcsec aperture of the
LiF1 channel of the
spectrograph, but
due to observational difficulties
no data were obtained in the
SiC spectral channels (which
cover the short wavelength end of the bandpass).
Thus, the far UV data presented
in this paper consists of eleven separate, 500 s long
integrations that cover the 990-1187 Å wavelength region,
and all data have been individually processed using version 1.8.3 of the
science data reduction (CALFUSE) pipeline. This processing of the
detector raw histogram data nominally accounts for
geometric image distortions, background level subtraction,
image drift due to instrument thermal
problems, detector deadtime,
wavelength and flux calibration (Sahnow et al. 2000).
Each of the 11 processed spectra were co-added and averaged using standard
IDL data reduction routines. The data contained in the LiF1a and
LiF1b detector segments were deemed to be of the best quality
and these were subsequently used in the following spectral
analysis.
(However, we note that the LiF2a and LiF2b spectra were used
to check for consistency in all the line detections that we
shall now report.)
For the purposes of this preliminary paper we have restricted
our analysis to the two wavelength regions shown in Figs. 1 and 2 that cover the 1030-1050 Å and 1130-1155 Å ranges respectively.
![]() |
Figure 1: HD 47240 - Average FUSE LiF1a wavelength corrected spectrum. |
Open with DEXTER |
![]() |
Figure 2: HD 47240 - Average FUSE LiF1b wavelength corrected spectrum. |
Open with DEXTER |
The zero point of the wavelength scale
was determined with reference to the many H2molecular lines that were detected in both spectral regions.
We have assumed that these lines occur at the same velocity
as that of the visible CH molecular line at
4232.5 Å (i.e. V = + 3 kms-1),
as observed
at a spectral resolution of 5 kms-1 by Sfeir (1999).
For the convenience of comparison with radio data, all velocities
in this paper are in the local standard of rest (LSR) frame.
The resultant
spectra typically have a S/N ratio >15:1 and
a velocity resolution of
13 kms-1 as determined
from absorption line-profile fitting of the weak interstellar
lines of H2 at 1041.2 Å and 1047.6 Å .
In addition, we have supplemented the
far UV data with high
spectral resolution observations of the Na I (5890 Å) and
Ca II (3933 Å) interstellar
lines taken at the Lick Observatory
and with ultraviolet observations of HD 47240 extracted from the on-line
data archive (Rodriguez-Pascual et al. 1999)
that have a spectral resolution
of
15 kms-1, which is comparable
to that of the
data. Further
details of the visible and
data
gained towards HD 47240
and other stars in the Monoceros Loop SNR
have been presented in the Ph.D. Thesis of Sfeir (1999).
Inspection of the
absorption
spectra shown in Figs. 1 and 2 reveal significant detections of the important interstellar absorption
line of O VI (1031.9 Å), as well as detections of the ground state
resonance lines of interstellar O I (1039.2 Å), C I (1139.8 Å),
Ar I (1048.2 Å), N I (1134.9 Å), Fe II (1143.23
1144.9 Å)
and
P II (1152.8 Å).
It is immediately apparent from these line profiles that
many of the strongest absorption lines are accompanied by a well resolved
high-velocity (HV) component at
+65 kms-1. We
identify this as
the high velocity component observed by WJ in
both the Na I and Ca II interstellar lines
at
= +69 kms-1 towards HD 47240.
Unfortunately
we do not uniquely detect the
kms-1
component (that was seen in the visible Na I lines towards HD 47359 by WJ
and in Ca II towards HD 47240 by Sfeir 1999) in
our present
spectra.
This is presumably due to blending of this component with
the strong central line-of-sight absorption at
= +3 kms-1that is not resolved by the
instrument.
However, we note that the asymmetry in
the blue-wing of the weak Fe II 1143.23 Å line is most probably
due to an absorption component at
- 30 kms-1.
Additionally, we also detect many of the lines associated with the
Lyman (B-X) and Werner (C-X) bands of the H2 molecule in
these
spectra (some of which are also
accompanied by an HV component at +65 kms-1) whose
detailed analysis will be
deferred to a future paper.
For each of the atomic lines that are accompanied by a detection
of the HV component we have
fitted their local
stellar continua with a multi-order polynomial to
produce a residual intensity profile.
The placement and shape of a stellar continuum was guided by
those of the many stellar absorption lines observed throughout
the
spectrum.
The interstellar absorption profiles were
then fit with one or more
absorption components (i.e. "clouds'') using
line oscillator strengths listed by Morton (2001).
This fitting procedure is
discussed in detail by Sfeir et al. (1999), such
that each theoretical absorption
profile is described by a gaussian velocity dispersion parameter,
,
a cloud component LSR velocity, V, and a cloud column
density, N. These best-fit values of V,
and N for all
the detected interstellar lines are also listed (together
with their respective errors and equivalent width values) in
Table 1.
Line |
![]() |
![]() |
b | N | S/N |
(mÅ) | (kms-1) | (1012 at cm-2) | |||
C I 1139..... | 28.2 | 61.2 (0.6) | 9.6 (0.9) | 182 (34) | 19 |
N I 1134.4..... | 43.2 | 71.8 (0.4) | 9.4 (0.6) | 175 (31) | 16 |
N I 1135........ | 49.4 | 73.1 (0.4) | 10.4 (0.6) | 137 (18) | 16 |
O I 1039........ | 47.5 | 68.4 (2.3) | 11.7 (3.1) | 724 (42) | 15 |
O VI 1032..... | 74.9 | 2.3 (0.8) | 17.3 (1.1) | 82 (14) | 11 |
(u) | <9 | HV | <7 | ||
P II 1153...... | 8.8 | 63.9 (1.1) | 7.9 (1.7) | 4 (1) | 20 |
Ar I 1048..... | 20.9 | 66.4 (0.6) | 8.5 (1) | 9 (2) | 16 |
Fe II 1143..... | 10.9 | 65.4 (1.4) | 9.5 (2.1) | 55 (12) | 17 |
Fe II 1145..... | 53.5 | 66.6 (0.5) | 14 (0.7) | 58 (7) | 16 |
(u)- Upper limits on the HV component. |
In Fig. 3 we show the FUV residual intensity profiles
that possess an HV feature
together with their respective best-fit profile models.
![]() |
Figure 3: FUSE spectra absorption profile fitting for HD 47240; results are given in residual intensity. Solid bars indicate typical error sizes to the continuum level fits. |
Open with DEXTER |
Line |
![]() |
![]() |
b | N | S/N |
(mÅ) | (kms-1) | (1012 at cm-2) | |||
C II* 1336...... | 70 | 59.2 (0.9) | 12.1 (1.8) | 171 (25) | 28 |
C IV 1548(u)....... | <15 | HV | <3 | ||
O I 1302........ | 80 | 66.1 (0.8) | 11.6 (1.9) | >350(s) | 20 |
Mg II 3p 2796/2803 | 460/386 | 57.7 (1.5) | 11.9 (2.7) | >110(s) | 17 |
Mg I 2852........ | 107 | 66.7 (2) | 5.7 (6) | 1.6 (5) | 14 |
Al II 1670....... | 80 | 67.0 (0.8) | 12.1 (1.7) | 4.2 (1) | 32 |
Al III 1855(u)..... | <12 | HV | <50 | ||
Si II 1304..... | 99 | 66.1 (1.2) | 13 (2.4) | 150 (14) | 18 |
Si IV 1394(u)....... | <24 | HV | <3 | 19 | |
S II 1253.... | 37 | 65.2 (1.7) | 9.9 (2.3) | 290 (90) | 23 |
Fe II 1608........ | 65 | 72 (0.9) | 8.3 (2.3) | 70 (43) | 20 |
(u)- Upper limits on the HV component. | |||||
(s)- Saturated lines. |
In Figs. 4 and 5 we also show the high resolution absorption
profiles of the Na I and Ca II lines towards HD 47240 (from
Sfeir 1999) and
in Fig. 6 we show the near ultraviolet absorption
profiles of the Al II (1670 Å), Si II (1304 Å),
S II (1253 Å), Fe II (1608 Å), C II* (1336 Å),
Mg I (2852 Å) and
Mg II (2800 Å doublet) lines as recorded by .
![]() |
Figure 4: Na I absorption profile fitting for HD 47240. Data taken from Sfeir (1999). |
Open with DEXTER |
![]() |
Figure 5: Ca II absorption profile fitting for HD 47240. Data taken from Sfeir (1999). |
Open with DEXTER |
![]() |
Figure 6: IUE absorption profile fitting for HD 47240; results are given in residual intensity. Solid bars indicate typical error sizes to the continuum level fits. |
Open with DEXTER |
Our high resolution ground-based observations
of the Ca II line towards HD 47240
indicate that this HV component is actually a
blend of (at least) two
absorbing clouds at
+60
and
+ 71 kms-1.
However, the lower resolution
and
observations were unable to
resolve these two clouds and it was found that
the best fit to the HV component in all the UV data was obtained using
a one-cloud absorption model centered at
kms-1.
Line |
![]() |
![]() |
S/N |
![]() |
b | N |
(mÅ) | (mÅ) | (kms-1) | (1011 at cm-2) | |||
Na I........ | 601.6 | 475.9 | 150 | -16 (0.1) | 5.6 (0.1) | 8.8 (0.0) |
2.4 (0) | 2.5 (0.1) | 3934 (1256) | ||||
12.1 (0.1) | 4.4 (0.1) | 15.2 (0.6) | ||||
59.6 (1.4) | 1.1 (3.9) | 0.5 (6.6) | ||||
70.6 (0) | 1.9 (0.1) | 10.9 (1.1) | ||||
Line |
![]() |
![]() |
S/N |
![]() |
b | N |
(mÅ) | (mÅ) | (kms-1) | (1011 at cm-2) | |||
Ca II........ | 356.2 | 224.7 | 120 | -37.1 (a) (1.4) | 1.6 (2.4) | 0.5 (1.3) |
-16.4 (0.1) | 9.2 (0.2) | 13.5 (0.1) | ||||
7.6 (0) | 7.3 (0) | 50.1 (0.1) | ||||
62 (0.3) | 1.9 (0.7) | 1.7 (1.3) | ||||
74 (0.1) | 2.1 (0.3) | 8.3 (2.8) | ||||
(a)- The line is not present in Na I, but faint in Ca II. |
In this section we discuss the main physical and chemical aspects of the HV
interstellar gas component observed in the
far UV absorption line data, which is supplemented by
both
and visible line data.
We note that the doppler
values derived
from the high resolution observations of both Na I
and Ca II for the two resolved HV components are
<2.1 kms-1, indicating a gas temperature
<8000 K.
Although we have derived
values
in fitting the UV line data, their usefulness in
determining reliable gas temperatures is
limited due to the dominant contribution of the
instrumental resolution of both
and
spectrographs in
fitting the blended components in these spectral lines.
Hence,
for the HV component observed with
,
the average
value is found to be
10 kms-1, implying
a typical gas temperature of <90000 K.
In contrast to the low
ionization interstellar lines, the high ionization
absorption line profile of O VI shown in Fig. 7 is quite
different in
appearance.
![]() |
Figure 7:
FUSE spectra O VI absorption profile fitting for HD 47240; results are
given in residual intensity. Solid bars on the continuum level indicate
typical error size to the continuum level fit.
The absorption line at ![]() |
Open with DEXTER |
Clearly, as more
spectra of stars in the Monoceros
Loop region become
available to us, the spatial extent and range of
ionization of the various absorbing components present
throughout this evolved remnant will become clearer. For example,
if the HV feature detected towards HD 47240 is ubiquitously
detected in the FUSE absorption spectra
of the other three target stars towards the Monoceros Loop, then
this would argue strongly
in favor of it being associated with a dense SNR shell arising from
radiative cooling during the Sedov-Taylor phase.
Such observations will be
of particular importance in answering the outstanding question
as to why there is (low-level) soft X-ray emission from the Monoceros
Loop SNR but (as yet) no detectable O VI absorption/emission.
Supernovae explosions can significantly alter the elemental abundances
of the ambient interstellar gas through both the dissemination of
the newly nucleosynthesized material and through the sputtering
and evaporation of interstellar dust grains by the incident SNR shock
front. WJ reported an anomalous N(Na I)/N(Ca II) ratio of 0.7 for the
HV absorption component towards HD 47240 which is typical for
high velocity gas in the general ISM (Siluk
Silk 1976). Since
sodium depletion is thought to be approximately constant in the
diffuse ISM (Jura 1976), then the low Na I/Ca II ratio can be
attributed to the removal of adsorbed calcium atoms from dust
grain surfaces by interstellar shock waves.
However, our higher resolution visible observations reveal the HV
component to be split into (at least) two components whose
Na I/Ca II ratios are 0.29 (V = + 60 kms-1) and 1.3 (V = + 70 kms-1)
respectively. This variation in Na I/Ca II column density
ratio between the two cloud components can be easily
seen in Figs. 4 and 5, and suggests that the gas in the inner (slower)
regions of the SNR
may well have been more affected by the SNR blast wave.
Element | Mon Loop | Vela SNR | Cold ISM Gas | Warm ISM Gas |
HVC Depletion | HVC Depletion(1) | Depletion(2) | Depletion(2) | |
S | 0 | 0 | 0 | 0 |
Mg | N/A | N/A | -1.24 to -1.56 | -0.73 to -0.9 |
Si | -0.57 (0.17) | -0.5 | -1.31 | -0.35 to -0.51 |
Fe | -0.96 (0.21) | -0.55 | -2.09 to -2.27 | -1.19 to -1.24 |
N | -0.98 (0.21) | -1.72 | -0.07 | -0.2 |
O | -1.21 (0.16) | -1.14 | -0.39 | -0.4 |
P | -0.17 (0.24) | -0.58 | -0.5 | -0.23 |
Al | -1.05 (0.24) | -1.2 | -2.4 | -1.1 |
Ar | -0.77 (0.24) | N/A | -0.48 | N/A |
(1)- Jenkins et al. (1984), Jenkins et al. (1998). | ||||
(2)- Savage & Sembach (1996), Welty et al. (1999). |
Assuming that solar abundances reflect
the present element abundances in the
interstellar medium, then the depletion factor ()
of a given element in the gas phase can be defined as
.
Element abundances may be thus
determined if the column density of hydrogen, N(H), is known for each cloud
component along the line-of-sight to HD 47240. Unfortunately
such values are, as yet,
unavailable from Ly
or H I radio 21 cm studies.
Thus, assuming that sulphur (S) is
undepleted in the interstellar gas (as is normally
found for lines-of-sight with relatively low reddening),
and that it has a solar abundance relative to hydrogen, then
the (equivalent) hydrogen
column densities for each
absorption component
may be determined. This also assumes that
the S II lines (around 1255 Å) are the dominant
ionization state of interstellar sulphur in line-of-sight towards the
Monoceros Loop.
The HV component at V = +65 kms-1 was clearly detected
in the SII (1253 Å) line
of the
spectrum
of HD 47240. It was also seen
in the SII 1259 Å line profile, but was contaminated by the nearby
strong interstellar line of Si II (1260 Å). We failed to detect
an HV component in the weaker SII line at 1251 Å line.
Using the derived
column density, log N(S) = 14.46 cm-2, we
calculate an equivalent value of log
cm-2 for
the HV component. Using this value of N(H I) we derive other element
abundances under the assumption
that the observed ionization stages for each element are the dominant
ones in the ISM. The resultant element depletion patterns (relative
to that of S) for the HV
component are shown in Table 4. For
comparison purposes we also show the patterns of element
depletion for the (younger) Vela SNR (Jenkins et al. 1984, 1998)
and the depletion patterns associated with both
warm and cool interstellar disk gas
as listed by Savage & Sembach (1996) and
Welty et al. (1999).
In regions
of the interstellar medium that have been highly
disturbed one may expect that O, N and S will be relatively undepleted,
and that Mg,
Si and Fe will be depleted onto refractory grains that are
gradually returned to the gas phase by sputtering and by grain-grain
collisions. We note that for both the Monoceros Loop
HV component and the Vela HV gas that the refractory elements
of Fe, Si and Al are less depleted than that found
for the cold ISM disk gas,
suggesting that grain sputtering has infact returned these elements
back to the ambient SNR interstellar gas. These levels of Fe, Si
and Al depletion
in both of the Vela and Monoceros Loop SNR HV gas
clouds are more consistent with that found
in the warm disk gas of the interstellar medium, which is generally
characterized by gas with temperatures of 8000 K.
In contrast, we find that the elements of N, O and Ar
are more depleted in
both the Vela and Monoceros Loop
HV gas than that generally found for both the cold and warm
ISM.
Jenkins et al. (1998) have argued that the
strong deficiencies of both N and O
in HV gas components
can probably be attributed to ionization effects in which the
observed lines of N I, Ar I and O I are not the dominant
states for these elements in the HV gas.
However, we note from Bohigas (1983) that a gradual
dilution of nitrogen-rich SNR gas filaments is expected as a SNR
ages and the supernova ejecta are enriched with swept-up elements
from the surrounding ISM.
Finally, we note the relatively small depletion
of phosphorus in the Monoceros Loop
HV gas which is similar to that
found for the warm phase of the ISM disk gas.
This pattern of near solar-system abundance for P (and Zn)
has also been observed towards the disk stars
Columbae and
Oph
(Howk et al. 1999).
For a SN explosion energy of
erg occuring
in a uniform medium of ambient density, 1 cm-3,
the relationship between
the SNR shell velocity (V)
and that the remnant age, t (in units of 105 years)
is given by Chevalier (1974) as:
V = 66.5
t-0.69 kms-1.
Under the simple (and as yet unsupported) assumption
that the
HV component we have detected towards
HD 47240 is representative of a SNR shell
with a resultant nebular expansion velocity of
50 kms-1 (Davies et al. 1978), then
we obtain an age for the Monoceros Loop
SNR of
years, in agreement with previous estimates for
this remnant
by Graham et al. (1982).
However, two important caveats must be taken into account in
this age determination.
The first is that the ambient interstellar density that the SNR
is expanding into is unknown, and is probably inhomogenous
by large magnitudes. Secondly,
we note that Leahy et al. (1986) have derived a far younger
age of
years for the Monoceros Loop SNR.
This younger age was derived by matching a model
with the shock radius, density (
= 0.001 atom cm-3) and
temperature of the X-ray emitting gas.
Thus, although at present our
observations cannot provide a definite
age for the Monoceros Loop remnant it would appear that
an age of 30000 to 150000 years seems appropriate.
This age range could be consistent
with the intepretation that the HV feature we have observed in both the
and
data represents that of an evolved
shocked SNR shell with no associated
high-ionization (high-temperature) absorption.
As stated previously, such high-ionization features have thus far
only been detected in
the ultraviolet towards SNRs with an age <15000 years (Cygnus Loop
and Vela SNR).
We have obtained a far ultraviolet spectrum of the star HD 47240
covering the wavelength range 990-1187 Å using
the NASA
satellite. Using these new FUV data supplemented
by visible and
archival
absorption spectra taken towards this star, we have
detected a high-velocity absorption component at
= +65 kms-1 whose origin can be associated with the
highly disturbed SNR gas.
This high-velocity feature is formed
only in lines with an ionization potential range up to 23.3 eV.
High spectral resolution visible observations of this feature
reveal a more complex velocity structure (with at least two
velocity components at
= +60 and +71 kms-1),
suggesting that it is composed of several ionized and neutral
gas shells expanding at slightly different velocities.
This high-velocity feature has not been detected in any of the high-ionization absorption lines of O VI, C IV or Si IV. Similar non-detections of these high ionization lines have been reported for HV gas associated with the Shajn 147 SNR (which is of a similar age to the Monoceros Loop), whereas both detections and non-detections of HV high ions have been reported towards the (younger) Vela SNR which is expanding at velocities >100 kms-1. Thus, until more lines-of-sight are sampled towards the Monoceros Loop SNR this present lack of HV high ion line detectability cannot be attributed solely to the low (V< 80 kms-1) shock velocity and evolved nature of the expanding remnant gas.
We have derived elemental abundances (relative to that of S) for gas associated with the HV component. A pattern in which the refractory elements of Fe, Si and Al are all less depleted (i.e. more abundant) than in the cold, interstellar disk gas has been found. This pattern of overabundance of refractory elements found for both the Monoceros Loop and Vela SNRs is consistent with the levels of depletion found in warm disk gas, and is probably due to grain sputtering processes. In contrast N, O, and Ar are all more depleted in the Vela and Monoceros Loop SNR HV gas than in both the cold and warm interstellar disk gas. This apparent element deficiency can be attributed to ionization effects of the SNR gas. The element phosphorus is essentially undepleted in the high-velocity gas, in agreement with observations of interstellar gas in the galactic disk.
Finally, we believe that an age of 30000 to 150000 years for the Monoceros Loop SNR is appropriate. Such an evolved remnant appears to possess absorption properties more like those of the Shajn 147 SNR as opposed to those of the far younger (age <15000 years) Cygnus and Vela SNRs.
Acknowledgements
We wish to thank the NASA
Operations and Science Center at the Johns Hopkins University which is operated for NASA under contract NAS5-32985. Particular thanks go to Dr. Bill Blair, Dr. Robin Shelton and Dr. Ed Jenkins for very useful discussions and important suggestions that have greatly improved this paper. DS, SS and BYW all acknowledge funding through the NASA
science support contract with the Experimental Astrophysics Group at the Space Sciences Laboratory, UC Berkeley.