Issue |
A&A
Volume 510, February 2010
|
|
---|---|---|
Article Number | A14 | |
Number of page(s) | 7 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200913278 | |
Published online | 29 January 2010 |
Circumstellar H I and CO around the carbon stars V1942 Sagittarii and V Coronae Borealis
Y. Libert1 - E. Gérard2 - C. Thum3 - J. M. Winters3 - L. D. Matthews4 - T. Le Bertre1
1 - LERMA, UMR 8112, Observatoire de Paris, 61 Av. de l'Observatoire,
75014 Paris, France
2 - GEPI, UMR 8111, Observatoire de Paris, 5 place J. Janssen, 92195
Meudon Cedex, France
3 - IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France
4 - MIT Haystack Observatory, Off Route 40, Westford, MA 01886, USA
Received 11 September 2009 / Accepted 20 October 2009
Abstract
Context. The majority of stars that leave the main
sequence are undergoing extensive mass loss, in particular during the
asymptotic giant branch (AGB) phase of evolution. Observations show
that the rate at which this phenomenon develops differs widely from
source to source, so that the time-integrated mass loss as a function
of the initial conditions (mass, metallicity, etc.) and of the stage of
evolution is presently not well understood.
Aims. We investigate the mass loss history of AGB
stars by observing the molecular and atomic emission of their
circumstellar envelopes.
Methods. We selected two stars that are on the
thermally pulsing phase of the AGB (TP-AGB) for which high quality data
in both the CO rotation lines and the atomic hydrogen line at
21 cm could be obtained.
Results. A carbon star of the irregular variability
type, V1942 Sgr, has a complex CO line profile that may
originate in a long-lived wind of rate 10-7
yr-1,
and from a young (
104 years)
fast outflow of rate
yr-1.
The intense H I emission
is indicative of a detached shell with 0.044
of hydrogen. This shell probably results from the slowing-down, by
surrounding matter, of the same long-lived wind observed in CO that has
been active for
years.
On the other hand, the carbon Mira V CrB is presently
undergoing mass loss at a rate of
yr-1,
but was not detected in the H I data.
The wind is mostly molecular and has been active for at most
years,
with an integrated mass loss of at most
.
Conclusions. Although both sources are carbon stars
on the TP-AGB, they appear to develop mass loss in very different
conditions, and a high rate of mass loss may not imply a high
integrated mass loss.
Key words: stars: AGB and post-AGB - stars: carbon - circumstellar matter - stars: individual: V1942 Sagittarii - stars: individual: V Coronae Borealis
1 Introduction
Low- to intermediate-mass stars, at the end of their main-sequence
evolution, become first hydrogen shell-burning red giants (RGB - red
giant branch - stars), then hydrogen and helium shell-burning red
giants (AGB - asymptotic giant branch - stars). In this second phase,
they may undergo mass loss at a very high rate (>10-8 yr-1),
sufficiently high to have a decisive effect upon their late evolution
(Olofsson 1999).
They are surrounded by expanding envelopes of gas and dust, which have
been extensively observed with radio molecular lines and infrared
continuum emission. These tracers are used to estimate mass-loss rates.
However, the estimates are somewhat ambiguous because the mass-loss
rate of a given source may vary on many different timescales. The
change in mass as a function of time due to mass loss is thus difficult
to evaluate, and to relate to stellar evolution models. Furthermore,
molecular lines probe an extent of the circumstellar shell (CS)
that is limited by photo-dissociation, and therefore provide
information mainly about the inner parts of CSs, and on the recent mass
loss.
To try to circumvent these difficulties, we have started a systematic program of observations of red giants in the line of atomic hydrogen at 21 cm. We have published some of our results in several recent papers, and the first reports for sizeable samples were presented by Gérard & Le Bertre (2006, hereafter GL2006) and Matthews & Reid (2007, hereafter MR2007). A major difficulty of this program is the confusion caused by the 21 cm emission from the interstellar medium (ISM) located along the same line-of-sight as the source of interest. This has a strong impact on the observations, which must be conducted using a specific approach, and on the data processing that aims to provide spectra corrected for ISM emission. Furthermore, since circumstellar matter is expected to be at some stage injected into the ISM, the confusion by the local ISM might actually be partly of stellar origin, i.e., caused by material ejected at an earlier stage of evolution.
In addition to observing the H I line at 21 cm systematically in a large number of sources with different properties, it is also important to choose objects for which the Galactic confusion is low and/or can be tracked easily, and therefore corrected accurately. The detailed study of these spectra should serve as a guide to exploit the data that are obtained in more difficult situations.
Here we present our results for two carbon stars, V1942 Sgr and V CrB, for which the confusion is not a serious problem, and which have H I properties that differ radically. Both are N-type carbon stars (CGCS 4229 and CGCS 3652, respectively) that have already been detected in CO rotational lines (Olofsson et al. 1993a). However, the only previously available CO spectrum of V1942 Sgr had been of low signal-to-noise ratio, and for our study it appeared essential to also obtain new CO data of high quality.
2 V1942 Sgr
V1942 Sgr is classified as a long-period irregular variable
(type Lb). Lebzelter & Obbrugger (2009) compared
the light-curve properties of semi-regular (SR) and Lb variables, and
concluded that Lb stars can be seen as an extension of the SRs that
have shorter periods and smaller amplitudes.
V1942 Sgr is a carbon star on the TP-AGB with a C/O ratio of
around 1.12 (Olofsson et al. 1993b). Bergeat
et al. (2001)
estimate that its effective temperature, ,
is 2960 K.
The parallax measured by Hipparcos (
mas,
van Leeuwen 2007)
places it at 535 pc, implying a luminosity of 5200
.
From the data obtained by IRAS in the mid-infrared, there is no direct
evidence that the star is undergoing mass loss (the low resolution
8-22
m
spectrum is featureless).
However, Olofsson et al. (1993a)
discovered emission in the CO(1-0) rotational line centered on
=
-31.5 km s-1, close to the
expected radial velocity of V1942 Sgr (
=
-32.0 km s-1 from the General
Catalogue of Stellar Radial Velocities). The expansion velocity,
=
12.4 km s-1, is surprisingly
high for an Lb variable. From this spectrum, Schöier & Olofsson
(2001)
derive a mass-loss rate of
yr-1
(at 535 pc).
Extended emission associated with V1942 Sgr was discovered by
IRAS (Young et al. 1993a).
The 60
m data show
a resolved shell of external radius 3.2', i.e., 0.50 pc.
2.1 H I observations
The H I emission was
observed with the Nançay Radio Telescope (NRT), between March 2007 and
July 2009, for a total of 85 h. The NRT beamwidth (FWHM)
at 21 cm is 4' in right ascension (RA) and 22' in declination
(Dec). An on-source frequency-switched spectrum is presented in
Fig. 1.
The main emission peaks at 50 K around =
0 km s-1. A narrow emission
feature with a peak of
0.3 K,
centered on -33 km s-1, is
visible on top of the 0.4 K blue wing of the main peak near
0 km s-1.
![]() |
Figure 1: Frequency-switched H I 21 cm spectrum obtained with the NRT on the position of V1942 Sgr. The spectrum enlarged by a factor 20 is also shown as a dashed line. The emission from V1942 Sgr is clearly detected at -33 km s-1. |
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Position-switched spectra were also obtained with an on-position taken
at the position of V1942 Sgr and off-positions, at 2',
4',
6',
8',
10',
12',
16',
24', and
32'.
The comparison between the spectra obtained for different values of the
throw shows that the interstellar emission varies linearly with offset
in the velocity range from -100 to -20 km s-1.
It means that the H I background
emission around V1942 Sgr shows a gradient in RA that is
constant for each velocity. This situation is similar to that
encountered for EP Aqr and Y CVn (Le Bertre & Gérard 2004,
Figs. 3 and 7). In such a case, the source emission
can be extracted from the position-switched spectra by subtracting the
contribution of the interstellar emission, which is estimated by
interpolation between the two extreme off-positions.
The intensity of the emission detected from the source in the
position-switched spectra is constant with throw from 4' to
32'.
Therefore, the source is confined mostly to the central beam (i.e.,
2' in RA;
see GL2006, Sect. 2.1). The spectrum obtained at the star's
position is shown in Fig. 2. It has a shape
similar to that obtained for Y CVn (Libert et al. 2007) with a
narrow emission line superimposed on a pedestal extending from -39 to
-27 km s-1. The narrow
emission is centered on
=
-32.9 km s-1 and has a
quasi-Gaussian profile of width 2.95 km s-1
(FWHM) and peak intensity 168 mJy. The pedestal is
also centered on
km s-1
and has an intensity of
mJy.
The peak intensity at the central position (178 mJy) agrees with that
measured in the frequency-switched spectrum (cf. Fig. 1, with a
conversion factor of 2.15 K/Jy for the NRT at 21 cm). We also
searched for H I emission
at blueshifted velocities as low as -48 km s-1,
and redshifted velocities as high as -18 km s-1 (see
the CO spectra in Sect. 2.2). We set an upper limit of
2 mJy on emission over this range. Nevertheless, we suspect
that there are residual features at -46, -26, and
-24 km s-1, possibly peaking
at
3 mJy.
![]() |
Figure 2: H I line profile of V1942 Sgr. The spectrum has been smoothed to a resolution of 0.32 km s-1. The dashed line is a fit obtained with the model described in Sect. 4.1. |
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The spectra of the source in the off-positions are then determined by
subtracting the individual position-switched spectra (on-off) and the
contribution of the interstellar emission (assuming that it varies
linearly with RA) from the central spectrum.
Furthermore, we obtained data for on-positions at 11' north and 11'
south, and off-positions at 2',
4', and
12', and
then on-positions at 22' north and 22' south, and off-positions at
8'. All of
these data were used to construct the flux density map of the source
presented in Fig. 3.
![]() |
Figure 3: Map of the 21 cm H I emission of V1942 Sgr. In each box, the label at upper left indicates the position (RA, Dec) with respect to the central star in arcminutes. |
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In this map, we see that the intensity at -2' west is almost the same
as on the star, and therefore conclude that the source is slightly
offset west from the stellar position. Assuming a Gaussian distribution
of the intensity, we estimate that the H I source
is centered on 0.6' (0.1')
west in RA and on 0' (
1')
in Dec. The size (FWHM) would then be
in RA and <5' in Dec. The integrated flux in the map is
0.71 Jy
km s-1.
Assuming that the emission is optically thin and atomic hydrogen is at
a temperature well above the background (
0.4 K + 4.2 K, Reich
& Reich 1986),
and using the standard relation,
=
,
with
in
,
d in pc, and
in Jy
km s-1,
this flux translates to 0.048
of atomic hydrogen at 535 pc.
![]() |
Figure 4: CO (2-1, upper panel) and (1-0, lower panel) spectra of V1942 Sgr obtained with the IRAM-30 m telescope. The fits used to derive the wind parameters are also shown (see Table 1). |
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2.2 CO observations
Table 1: CO line parameters of V1942 Sgr.
CO observations of V1942 Sgr were obtained at the IRAM 30-m telescope equipped with the EMIR (Eight MIxer Receiver) on June 23, 2009 in average weather conditions (precipitable water vapor, pwv


![]() |
Figure 5: Frequency-switch H I 21 cm spectrum obtained with the NRT on the position of V CrB. The bar indicates the velocity range of the CO emission. |
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The spectra are shown in Fig. 4. These new
spectra indicate
that the line profiles are composite with two components centered on
the same central velocity, but with different widths, similar to those
observed by Knapp et al. (1998)
and Winters et al. (2003)
in several late-type giants, mostly oxygen-rich stars of the SR
variability type. The emission extends from -48 to
-18 km s-1, and therefore we
confirm the large expansion velocity (12 km s-1)
estimated by Olofsson et al. (1993a).
Each line profile was fitted with two parabolae to derive
representative expansion velocities (Table 1). We estimate the
mass-loss rates and photodissociation radii associated with
each component using the same approach as in Winters et al. (2003).
We adopt a CO/H mass ratio of .
The differences in the mass-loss rates and photodissociation radii
estimated from the two lines are comparable to the uncertainties in the
fits.
3 V CrB
V CrB is a metal-poor (
)
carbon star on the TP-AGB with a C/O ratio around 1.10 (Abia
et al. 2001).
It is a Mira of period 358 days.
Using the period-luminosity relation for carbon Miras of Whitelock
et al. (2006),
Guandalini (2009, in prep) determines a luminosity of 5600
and a distance of 547 pc. The temperature
is estimated to be 2090 K (Bergeat et al. 2001). At such a
low temperature (i.e., less than 2500 K), molecular hydrogen is
expected to be the dominant species in the atmosphere and in the inner
envelope of V CrB (Glassgold & Huggins 1983). Indeed,
photospheric H2 has been detected in the
near-infrared (2.122
m)
by Johnson et al. (1983).
This Mira is presently undergoing mass loss, since, for instance, it
exhibits clear SiC dust emission at 11.3
m (Goebel
et al. 1981).
The source has also been detected in the CO(1-0) and CO(2-1) rotational
lines by Olofsson et al. (1993a), and
more recently in CO(3-2) by Knapp et al. (1998). In contrast
to V1942 Sgr, only one velocity component is visible. The
central velocity is at
=
-99.0 km s-1, the expansion
velocity,
=
6.5 km s-1, and the mass-loss
rate,
yr-1
(at 547 pc). With the Plateau-de-Bure IRAM interferometer,
Neri et al. (1998)
measured a source size of 7'' in CO (1-0). On the other hand,
IRAS has not detected extended far-infrared emission associated with
V CrB (Young et al. 1993a, their
Table 1).
V CrB was also observed for a total of 44 h in H I with the NRT. The frequency-switched spectrum shows no feature around the expected velocity of -99.0 km s-1 (Fig. 5), and only weak interstellar emission of at most 0.2 K around -99 km s-1.
We obtained H I data in
the position-switch mode of observation with off-positions at 4',
6',
8',
10',
12',
16',
24', and
32'.
As for V1942 Sgr we find that the interstellar emission varies
linearly with offset, in the velocity range from -120 to
-80 km s-1. We are thus
confident that the interstellar contamination can be corrected for
accurately. The source is not detected at the star's position and at
5' in RA
(Fig. 6,
in which we have averaged the spectra obtained at +4' and +6',
and at -4' and -6', to improve the sensitivity). By
integrating over the velocity range defined by the CO emission (i.e.,
-106 to -92 km s-1), an upper
limit of 4 mJy
km s-1
can be set on the intensity of the H I emission
at the V CrB's position in the area defined by the
NRT beam. This translates to an upper limit of
in atomic hydrogen at 547 pc. Since the source was not found
to be extended by IRAS, we do not expect to find much material outside
the NRT beam.
![]() |
Figure 6: H I spectra obtained for V CrB ( middle), at +5' east ( top) and -5' west ( bottom) after correction for interstellar contamination (Sect. 3). The spectra have been smoothed to a resolution of 0.64 km s-1. The bar indicates the velocity range of the CO emission. |
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4 Interpretation
4.1 V1942 Sgr
The CO line profiles observed in V1942 Sgr have a characteristic composite profile. This kind of profile has been interpreted as evidence of a succession of mass-loss events with different outflow velocities by Knapp et al. (1998) and Winters et al. (2003). However, the interferometric data obtained for EP Aqr, a source with such profiles, are difficult to explain with this scenario (Winters et al. 2007). Furthermore, in other cases, X Her (Kahane & Jura 1996) and RS Cnc (Libert et al. 2009), there is evidence that the broad components originate in a bipolar flow. Bipolar flows are believed to develop at the end of the AGB phase when the stars are about to begin their evolution towards the planetary nebula phase (e.g., Sahai et al. 2007).
The H I spectrum
obtained at the star's position shows a pedestal of width
10 km s-1 that could be a
counterpart to the narrow CO (1-0 and 2-1) components that
have about the same width. The mass in hydrogen corresponding to this
pedestal is
.
Assuming 90% in H and 10% in 4He,
in number (i.e. a mean molecular weight,
,
of 1.3), and adopting a mass-loss rate of
yr-1
on the basis of the narrow CO components (see Table 1), the timescale
would be
years,
and the radius 0.31 pc (
). The stellar effective
temperature (2960 K) is high enough for us to expect that most
of the hydrogen is in atomic form.
On the other hand, there is no H I
counterpart to the broad CO components at a level of
2 mJy. This seems to indicate that the broad components
correspond to a quite recent phenomenon. Adopting an upper limit of
2 mJy, the flux is <0.07 Jykm s-1,
and the mass in atomic hydrogen is at most
.
The timescale is then
104 years.
The H I spectra obtained
on V1942 Sgr are very similar to those obtained by
Le Bertre & Gérard (2004) and Libert
et al. (2007)
on Y CVn, a well-documented carbon star with a detached shell
discovered by IRAS (Young et al. 1993a) and imaged
by ISO (Izumiura et al. 1996). At the
central position, we detect a pedestal of width
10 km s-1, twice the expansion
velocity measured for the narrow CO components. At all positions, we
detect a narrow line of width, km s-1.
This narrow profile proves that the stellar wind from
V1942 Sgr is decelerated at some distance from the central
star. Young et al. (1993b)
interpreted the detached shells detected by IRAS at 60
m as the
effects of a slowing-down of stellar outflows by the surrounding
interstellar matter. Elaborating on this hypothesis and using the
formalism of Lamers & Cassinelli (1999), Libert
et al. (2007)
developed a model in which the inner radius of a detached shell
corresponds to the location where the stellar wind is abruptly slowed
down from
to
/4. The outer radius
corresponds to the location where external matter is compressed by the
expanding shell (bow shock). They applied this model to Y CVn and
obtained excellent fits to the H I
line profiles observed at different pointings, on and around the star's
position.
We use the same model for V1942 Sgr. For the freely
expanding wind (r<r1),
we adopt a velocity of 5.0 km s-1,
i.e., half the width of the pedestal, which also corresponds to the
narrow CO components. The broad CO components have no obvious
counterpart in H I. They probably
trace a short-lived structure restricted to the central part of the
circumstellar shell that has no effect on the large scales probed at
21 cm. The mass-loss rate,
yr-1,
is adopted from the CO line fitting (Table 1). The central
velocity is taken to be -32.9 km s-1.
We obtain a good fit to the different H I
line profiles (Figs. 2
and 7)
with the parameters given in Table 2. The external
radius, ,
that we have adopted is compatible with that derived by Young
et al. (1993a)
from IRAS data at 60
m,
,
but not the internal radius (r1
= 2' versus
).
In our model, r1 is
constrained by the parameters obtained from the low velocity CO
components and by the intensity of the H I pedestal.
We assume that the inner shell is too small compared to the IRAS beam
at 60
m
(
)
to have been reliably constrained.
Table 2: Model parameters (d = 535 pc).
![]() |
Figure 7: Comparison between the H I line profiles obtained on V1942 Sgr and the detached-shell model discussed in Sect. 4.1. Top: average of the two spectra at +2' (east) and -2' (west). Middle: average of the two spectra at +11' (north) and -11' (south). Bottom: average of the four spectra at (+2', +11'), (-2', +11'), (+2', -11'), and (-2', -11'). |
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![]() |
Figure 8:
Atomic hydrogen column density profile for the V1942 Sgr
model.
The vertical lines mark the radii r1 (0.31 pc),
|
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4.2 V CrB
V CrB was not detected in H I.
Since there is no significant Galactic confusion, we are quite
confident in our upper limit of
in atomic hydrogen. For a source losing atomic hydrogen with a
mass-loss rate of
yr-1,
this would correspond to a timescale of 2000 years (
). However,
the stellar effective temperature is so low (2090 K) that
hydrogen should be in molecular form in the atmosphere and outwards
(Glassgold & Huggins 1983),
until it is photodissociated by the interstellar radiation field. To
estimate the distance,
,
at which this happens, we follow the approach of Morris & Jura (1983). Assuming a
mean intensity for the ultraviolet radiation between 912 and
1100 Å of
photons cm-2 s-1 sr-1,
and that 0.11 of all absorptions lead to a dissociation, we infer that
,
where
is in pc and
in
yr-1.
For a mass-loss rate of
yr-1,
we obtain
=
0.2 pc (
1.2'),
which corresponds to a dynamical time of
years
(
=
6.5 km s-1).
Therefore, the non-detection of V CrB in H I
implies that it has not been undergoing mass loss at the present rate
for more than years.
Furthermore, when comparing with V1942 Sgr, which is at the
same distance, we can state that V CrB has not experienced the
same phase of mass loss during the past
years,
because if it had it would have been easily detected in a way similar
to V1942 Sgr.
This reasoning assumes that molecular hydrogen is not self-protected within small-scale structures that might develop in the stellar outflow. However, the non-detection by IRAS of an extended emission around V CrB (Young et al. 1993a) is consistent with our conclusion that mass loss started only recently.
4.3 Discussion
The V1942 Sgr proper motion measured by Hipparcos is
10.98 mas yr-1 in RA and
-5.10 mas yr-1 in Dec. When
corrected for solar motion towards apex and for a distance of
535 pc, it translates to 6.45 mas in RA and
-2.28 mas in Dec. This implies a motion in the plane of the
sky at a velocity of 17 km s-1,
at a position angle, PA = 110.
Accounting for the radial velocity,
=
-33 km s-1, we obtain a
3D space velocity of 37 km s-1.
The offset with respect to the central star that we find in the
H I map might therefore
be an effect of the motion of V1942 Sgr relative to the
surrounding ISM. Such a deformation in H I
has already been noted in several cases: Mira (Matthews et al.
2008),
RX Lep (Libert et al. 2008), and RS Cnc
(MR2007 and Libert et al. 2009). GL2006
noted also that many H I sources
are offset with respect to the central stars. A visual inspection of
the IRAS map at 60
m
of V1942 Sgr (after the Improved Reprocessing of the IRAS
Survey: Miville-Deschênes
& Lagache 2005)
indicates that the image is slightly elongated in RA and shifted west
by
1/2 pixel
(
), in agreement with our
H I map. Finally, we note that
the central velocity in H I is
km s-1,
whereas in CO it is
km s-1.
This effect is small but consistent with an interaction between the
external shell of V1942 Sgr and its local ISM. Shifts in
velocity of the H I emission
towards the LSR have already been reported in several red giants
(GL2006, Matthews et al. 2008).
From their study of circumstellar shells resolved by IRAS, Young et al. (1993b) find that, among nearby AGB stars detected in CO, Miras, in contrast to semi-regulars, are in general unresolved. They suggest that the latter have been losing matter for a longer time than the former. In their H I survey of evolved stars, GL2006 obtained results that agree with this suggestion. Although their sample is small, several Miras with high mass-loss rates could not be detected in H I, whereas SRs were often easily detected. The high quality data that we have obtained for V1942 Sgr and V CrB strengthen the case of SRs undergoing mass loss for a longer time than Miras. One normally assumes that SRs evolve into Miras, and it is puzzling to find no relics of this SR phase around several Miras. SRs might evolve directly in the post-AGB phase, as suggested also by the presence of bipolar outflows that has been reported in several cases (Kahane & Jura 1996; Libert et al. 2009).
Young et al. (1993b)
also find that extended sources are observed preferentially around
carbon stars and GL2006 obtained a higher rate of detection of carbon
stars in H I.
However, the case of V CrB seems to suggest that some stars
could reach the carbon-rich stage without undergoing substantial mass
loss previously.
In a systematic investigation of the relations between mass loss and
red giant characterstics, Winters et al. (2000) find that
the mass-loss rate depends critically on stellar parameters such as the
effective temperature, which controls the dust formation, and the
luminosity, which controls the radiation pressure. V CrB may
have switched only recently from the B-regime (with a low and,
presently, undetected wind) to the A-regime with a wind at a few 10-7 yr-1.
Although both V1942 Sgr and V CrB are carbon
stars on the TP-AGB phase, their history of mass loss during the past years
seem to differ dramatically. If it is correct that the bipolar shaping
is a signpost of the end of the AGB, V1942 Sgr (and also
sources with composite CO line profiles) might be on the verge of
leaving this phase. Both sources have similar C/O abundance
ratios, 1.12 for V1942 Sgr (Olofsson et al. 1993b) and 1.10
for V CrB (Abia et al. 2001), and similar
luminosities, 5200 and 5600
,
respectively. Both sources also have a low 12C/13C abundance
ratio, 30 for V1942 Sgr (Abia & Isern 1997) and 10 for
V CrB (Abia et al. 2001), compared
to
40
for the majority of carbon stars in the AGB phase. The explanation of
these low abundance ratios is not known, but could be due to a
non-standard mixing process occurring in low-mass stars at the base of
the convective stellar envelope (``cool bottom processing'', Nollett
et al. 2003).
5 Conclusions
The combination of high velocity resolution CO and H I data provides a promising tool to investigate the history of mass loss by evolved stars. The low level of Galactic H I emission and the absence of small-scale structure in this emission have allowed us to obtain H I data of high quality for V1942 Sgr and V CrB with the NRT. We have also obtained high quality CO (1-0) and (2-1) spectra of V1942 Sgr with the IRAM 30-m telescope.
For V1942 Sgr, the CO spectra exhibit composite
profiles revealing a low velocity wind of 10-7
yr-1
and a high velocity wind, possibly bipolar, of
yr-1.
A comparison with the H I spectrum
shows that this high velocity wind is recent with an age of at most 104 years.
On the other hand, the low velocity wind appears to have filled a
cavity of
0.2 pc
in radius and created the detached shell, which was discovered by IRAS,
over a period of
years.
Follow-up observations with the VLA and ALMA would help us to refine
this scenario, or possibly develop a new scheme. The narrowness of the
H I line profile in
V1942 Sgr provides new evidence that AGB stellar winds are
slowed down by their surrounding medium, as surmised by Young
et al. (1993b).
For V CrB, the CO spectra that have been published
reveal an outflow with expansion velocity, 6.5 km s-1,
and mass-loss rate,
yr-1.
The non-detection in H I of
V CrB places an upper limit of
years
on the age of this outflow. In the case of a star with low effective
temperature, molecular hydrogen data are obviously needed to constrain
the history of mass loss more accurately.
The Nançay Radio Observatory is the Unité scientifique de Nançay of the Observatoire de Paris, associated as Unité de Service et de Recherche (USR) No. B704 to the French Centre National de la Recherche Scientifique (CNRS). The Nançay Observatory also gratefully acknowledges the financial support of the Conseil Régional de la Région Centre in France. We thank the IRAM Director, P. Cox, for allowing the CO observations of V1942 Sgr to be made on Director's time (D01-09). IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). We are grateful to C. Abia, M. Busso, R. Guandalini, and A. Jorissen for enlightening discussions, and to the referee for careful comments that helped us to improve the manuscript. Thisresearch has made use of the SIMBAD and VizieR databases, operated at CDS, Strasbourg, France and of the NASA's Astrophysics Data System.
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All Tables
Table 1: CO line parameters of V1942 Sgr.
Table 2: Model parameters (d = 535 pc).
All Figures
![]() |
Figure 1: Frequency-switched H I 21 cm spectrum obtained with the NRT on the position of V1942 Sgr. The spectrum enlarged by a factor 20 is also shown as a dashed line. The emission from V1942 Sgr is clearly detected at -33 km s-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: H I line profile of V1942 Sgr. The spectrum has been smoothed to a resolution of 0.32 km s-1. The dashed line is a fit obtained with the model described in Sect. 4.1. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Map of the 21 cm H I emission of V1942 Sgr. In each box, the label at upper left indicates the position (RA, Dec) with respect to the central star in arcminutes. |
Open with DEXTER | |
In the text |
![]() |
Figure 4: CO (2-1, upper panel) and (1-0, lower panel) spectra of V1942 Sgr obtained with the IRAM-30 m telescope. The fits used to derive the wind parameters are also shown (see Table 1). |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Frequency-switch H I 21 cm spectrum obtained with the NRT on the position of V CrB. The bar indicates the velocity range of the CO emission. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: H I spectra obtained for V CrB ( middle), at +5' east ( top) and -5' west ( bottom) after correction for interstellar contamination (Sect. 3). The spectra have been smoothed to a resolution of 0.64 km s-1. The bar indicates the velocity range of the CO emission. |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Comparison between the H I line profiles obtained on V1942 Sgr and the detached-shell model discussed in Sect. 4.1. Top: average of the two spectra at +2' (east) and -2' (west). Middle: average of the two spectra at +11' (north) and -11' (south). Bottom: average of the four spectra at (+2', +11'), (-2', +11'), (+2', -11'), and (-2', -11'). |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Atomic hydrogen column density profile for the V1942 Sgr
model.
The vertical lines mark the radii r1 (0.31 pc),
|
Open with DEXTER | |
In the text |
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