340 research outputs found
Discovery of a Shell of Neutral Atomic Hydrogen Surrounding the Carbon Star IRC+10216
We have used the Robert C. Byrd Green Bank Telescope to perform the most
sensitive search to date for neutral atomic hydrogen (HI) in the circumstellar
envelope (CSE) of the carbon star IRC+10216. Our observations have uncovered a
low surface brightness HI shell of diameter ~1300" (~0.8 pc), centered on
IRC+10216. The HI shell has an angular extent comparable to the far
ultraviolet-emitting astrosphere of IRC+10216 previously detected with the
GALEX satellite, and its kinematics are consistent with circumstellar matter
that has been decelerated by the local interstellar medium. The shell appears
to completely surround the star, but the highest HI column densities are
measured along the leading edge of the shell, near the location of a previously
identified bow shock. We estimate a total mass of atomic hydrogen associated
with IRC+10216 CSE of M_HI~3x10e-3 M_sun. This is only a small fraction of the
expected total mass of the CSE (<1%) and is consistent with the bulk of the
stellar wind originating in molecular rather than atomic form, as expected for
a cool star with an effective temperature T_eff<~2200 K. HI mapping of a 2 deg
x 2 deg region surrounding IRC+10216 has also allowed us to characterize the
line-of-sight interstellar emission in the region and has uncovered a link
between diffuse FUV emission southwest of IRC+10216 and the Local Leo Cold
Cloud.Comment: Accepted to MNRAS. 16 pages. A version with full resolution figures
may be found at
http://www.haystack.mit.edu/hay/staff/lmatthew/Matthews_etal_IRC+10216.pd
Infrared point source variability between the Spitzer and MSX surveys of the Galactic mid-plane
We present a list of 552 sources with suspected variability, based on a
comparison of mid-infrared photometry from the GLIMPSE I and MSX surveys, which
were carried out nearly a decade apart. We were careful to address issues such
as the difference in resolution and sensitivity between the two surveys, as
well as the differences in the spectral responses of the instruments. We
selected only sources where the IRAC 8.0 and MSX 8.28 micron fluxes differ by
more than a factor of two, in order to minimize contamination from sources
where the difference in fluxes at 8 micron is due to a strong 10 micron
silicate feature. We present a subset of 40 sources for which additional
evidence suggests variability, using 2MASS and MIPSGAL data. Based on a
comparison with the variability flags in the IRAS and MSX Point-Source Catalogs
we estimate that at least a quarter of the 552 sources, and at least half of
the 40 sources are truly variable. In addition, we tentatively confirm the
variability of one source using multi-epoch IRAS LRS spectra. We suggest that
most of the sources in our list are likely to be Asymptotic Giant Branch stars.Comment: 47 pages, 12 Figures, 3 Tables, accepted for publication in A
An HI Imaging Survey of Asymptotic Giant Branch Stars
We present an imaging study of a sample of eight asymptotic giant branch
(AGB) stars in the HI 21-cm line. Using observations from the Very Large Array,
we have unambiguously detected HI emission associated with the extended
circumstellar envelopes of six of the targets. The detected HI masses range
from M_HI ~ 0.015-0.055 M_sun. The HI morphologies and kinematics are diverse,
but in all cases appear to be significantly influenced by the interaction
between the circumstellar envelope and the surrounding medium. Four stars (RX
Lep, Y UMa, Y CVn, and V1942 Sgr) are surrounded by detached HI shells ranging
from 0.36 to 0.76 pc across. We interpret these shells as resulting from
material entrained in a stellar outflow being abruptly slowed at a termination
shock where it meets the local medium. RX Lep and TX Psc, two stars with
moderately high space velocities (V_space>56 km/s), exhibit extended gaseous
wakes (~0.3 and 0.6 pc in the plane of the sky), trailing their motion through
space. The other detected star, R Peg, displays a peculiar "horseshoe-shaped"
HI morphology with emission extended on scales up to ~1.7 pc; in this case, the
circumstellar debris may have been distorted by transverse flows in the local
interstellar medium. We briefly discuss our new results in the context of the
entire sample of evolved stars that has been imaged in HI to date.Comment: Accepted to AJ. A version with full resolution figures is available
at http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_HI_survey.pd
Carbon stars in the IRTS survey
We have identified 139 cool carbon stars in the near-infrared
spectro-photometric survey of the InfraRed Telescope in Space (IRTS) from the
conspicuous presence of molecular absorption bands at 1.8, 3.1 and 3.8 microns.
Among them 14 are new, bright (K ~ 4.0-7.0), carbon stars. We find a trend
relating the 3.1 microns band strength to the K-L' color index, which is known
to correlate with mass-loss rate. This could be an effect of a relation between
the depth of the 3.1 microns feature and the degree of development of the
extended stellar atmosphere where dust starts to form.Comment: accepted by the PASP; December 7, 200
The Central Regions of M31 in the 3 - 5 micron Wavelength Region
Images obtained with NIRI on the Gemini North telescope are used to
investigate the photometric properties of the central regions of M31 in the 3 -
5 micron wavelength range. The light distribution in the central arcsecond
differs from what is seen in the near-infrared in the sense that the difference
in peak brigh tness between P1 and P2 is larger in M' than in K'; no obvious
signature of P3 is dete cted in M'. These results can be explained if there is
a source of emission that contributes ~ 20% of the peak M' light of P1 and has
an effective temperature of no more than a few hundred K that is located
between P1 and P2. Based on the red K-M' color of this source, it is suggested
that the emission originates in a circumstellar dust shell surrounding a single
bright AGB star. A similar bright source that is ~ 8 arcsec from the center of
the galaxy is also detected in M'. Finally, the (L', K-L') color-magnitude
diagram of unblended stars shows a domin ant AGB population with photometric
characteristics that are similar to those of the most luminous M giants in the
Galactic bulge.Comment: To appear in the Astronomical Journa
HI emission from the red giant Y CVn with the VLA and FAST
Imaging studies with the VLA have revealed HI emission associated with the
extended circumstellar shells of red giants. We analyse the spectral map
obtained on Y CVn, a J-type carbon star on the AGB. The HI line profiles can be
interpreted with a model of a detached shell resulting from the interaction of
a stellar outflow with the local interstellar medium. We reproduce the spectral
map by introducing a distortion along a direction corresponding to the star's
motion in space. We then use this fitting to simulate observations expected
from the FAST radiotelescope, and discuss its potential for improving
ourdescription of the outer regions of circumstellar shells.Comment: accepted for publication in RA
Dust-enshrouded Asymptotic Giant Branch Stars in the Solar Neighbourhood
A study is made of a sample of 58 dust-enshrouded Asymptotic Giant Branch
(AGB) stars (including 2 possible post AGB stars), of which 27 are carbon-rich
and 31 are oxygen-rich. These objects were originally identified by Jura &
Kleinmann as nearby (within about 1 kpc of the sun) AGB stars with high
mass-loss rates, greater than 1E-6 solar masses per year. Ground-based
near-infrared photometry, data obtained by IRAS and kinematic data from the
literature are combined to investigate the properties of these stars. The light
amplitude in the near-infrared is found to be correlated with period, and this
amplitude decreases with increasing wavelength. Statistical tests show that
there is no reason to suspect any difference in the period distributions of the
carbon- and oxygen-rich stars for periods less than 1000 days, and no
carbon-rich star has a period longer than 1000 days. The colours are consistent
with those of cool stars with evolved circumstellar dust-shells. Luminosities
and distances are estimated using a period-luminosity relation. Mass-loss
rates, estimated from the 60 micron fluxes, show a correlation with pulsation
period and is tightly correlated with the K-[12] colour. The kinematics and
scale-height of the sample shows that the sources with periods less than 1000
days must have low mass main-sequence progenitors. It is argued that the three
oxygen-rich stars with periods over 1000 days probably had intermediate mass
main-sequence progenitors with remaining stars having an average progenitor
mass of about 1.3 solar masses. The average lifetime of stars in this phase is
estimated to be about 4.0E4 years, indicating they will undergo at most one
more thermal pulse before leaving the AGB.Comment: 27 pages, 20 figures, accepted for MNRA
HI and CO in the circumstellar environment of the oxygen-rich AGB star RX Lep
Circumstellar shells around AGB stars are built over long periods of time
that may reach several million years. They may therefore be extended over large
sizes (~1 pc, possibly more), and different complementary tracers are needed to
describe their global properties. In the present work, we combined 21-cm HI and
CO rotational line data obtained on an oxygen-rich semi-regular variable, RX
Lep, to describe the global properties of its circumstellar environment. With
the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile
is parabolic and implies an expansion velocity of ~4.2 km/s and a mass-loss
rate ~1.7 10^-7 Msun/yr (d = 137 pc). The HI line at 21 cm was detected with
the Nancay Radiotelescope on the star position and at several offset positions.
The linear shell size is relatively small, ~0.1 pc, but we detect a trail
extending southward to ~0.5 pc. The line profiles are approximately Gaussian
with an FWHM ~3.8 km/s and interpreted with a model developed for the detached
shell around the carbon-rich AGB star Y CVn. Our HI spectra are well-reproduced
by assuming a constant outflow (Mloss = 1.65 10^-7 Msun/yr) of ~4 10^4 years
duration, which has been slowed down by the external medium. The spatial offset
of the HI source is consistent with the northward direction of the proper
motion, lending support to the presence of a trail resulting from the motion of
the source through the ISM, as already suggested for Mira, RS Cnc, and other
sources detected in HI. The source was also observed in SiO (3 mm) and OH (18
cm), but not detected. The properties of the external parts of circumstellar
shells around AGB stars should be dominated by the interaction between stellar
outflows and external matter for oxygen-rich, as well as for carbon-rich,
sources, and the 21-cm HI line provides a very useful tracer of these regions.Comment: 15 pages, 9 figures, accepted for publication in A&
CO and HI observations of an enigmatic cloud
An isolated HI cloud with peculiar properties has recently been discovered by
Dedes, Dedes, & Kalberla (2008, A&A, 491, L45) with the 300-m Arecibo
telescope, and subsequently imaged with the VLA. It has an angular size of ~6',
and the HI emission has a narrow line profile of width ~ 3 km/s.
We explore the possibility that this cloud could be associated with a
circumstellar envelope ejected by an evolved star.
Observations were made in the rotational lines of CO with the IRAM-30m
telescope, on three positions in the cloud, and a total-power mapping in the HI
line was obtained with the Nancay Radio Telescope.
CO was not detected and seems too underabundant in this cloud to be a
classical late-type star circumstellar envelope. On the other hand, the HI
emission is compatible with the detached-shell model that we developed for
representing the external environments of AGB stars.
We propose that this cloud could be a fossil circumstellar shell left over
from a system that is now in a post-planetary-nebula phase. Nevertheless, we
cannot rule out that it is a Galactic cloud or a member of the Local Group,
although the narrow line profile would be atypical in both cases.Comment: Accepted for publication in Astronomy and Astrophysic
Circumstellar HI and CO around the carbon stars V1942 Sgr and V CrB
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 highly 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 are
investigating the mass loss history of AGB stars by observing the molecular and
atomic emissions of their circumstellar envelopes. Methods. In this work we
have selected two stars that are on the thermally pulsing phase of the AGB
(TP-AGB) and for which high quality data in the CO rotation lines and in the
atomic hydrogen line at 21 cm could be obained. Results. V1942 Sgr, a carbon
star of the Irregular variability type, shows a complex CO line profile that
may originate from a long-lived wind at a rate of ~ 10^-7 Msol/yr, and from a
young (< 10^4 years) fast outflow at a rate of ~ 5 10^-7 Msol/yr. Intense HI
emission indicates a detached shell with 0.044 Msol 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 during ~ 6 10^5 years. On
the other hand, the carbon Mira V CrB is presently undergoing mass loss at a
rate of 2 10^-7 Msol/yr, but was not detected in HI. The wind is mostly
molecular, and was active for at most 3 10^4 years, with an integrated mass
loss of at most 6.5 10^-3 Msol. Conclusions. Although both sources are carbon
stars on the TP-AGB, they appear to develop mass loss under very different
conditions, and a high rate of mass loss may not imply a high integrated mass
loss.Comment: Accepted for publication in Astron. Astrophy
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