21 research outputs found
Tracing the development of dust around evolved stars: The case of 47 Tuc
We observed mid-infrared (7.5-22 mum) spectra of AGB stars in the globular
cluster 47 Tuc with the Spitzer telescope and find significant dust features of
various types. Comparison of the characteristics of the dust spectra with the
location of the stars in a logP-K-diagram shows that dust mineralogy and
position on the AGB are related. A 13 mum feature is seen in spectra of low
luminosity AGB stars. More luminous AGB stars show a broad feature at 11.5 mum.
The spectra of the most luminous stars are dominated by the amorphous silicate
bending vibration centered at 9.7 mum. For 47 Tuc AGB stars, we conclude that
early on the AGB dust consisting primarily of Mg-, Al- and Fe oxides is formed.
With further AGB evolution amorphous silicates become the dominant species.Comment: 2 figures, accepted for publication in ApJ Letter
Dust, pulsation, chromospheres and their role in driving mass loss from red giants in Galactic globular clusters
Context: Mass loss from red giants in old globular clusters affects the
horizontal branch (HB) morphology and post-HB stellar evolution including the
production of ultraviolet-bright stars, dredge up of nucleosynthesis products
and replenishment of the intra-cluster medium. Studies of mass loss in globular
clusters also allows one to investigate the metallicity dependence of the mass
loss from cool, low-mass stars down to very low metallicities.
Aims: We present an analysis of new VLT/UVES spectra of 47 red giants in the
Galactic globular clusters 47 Tuc (NGC 104), NGC 362, omega Cen (NGC 5139), NGC
6388, M54 (NGC 6715) and M15 (NGC 7078). The spectra cover the wavelength
region 6100-9900A at a resolving power of R = 110,000. Some of these stars are
known to exhibit mid-infrared excess emission indicative of circumstellar dust.
Our aim is to detect signatures of mass loss, identify the mechanism(s)
responsible for such outflows, and measure the mass-loss rates.
Methods: We determine for each star its effective temperature, luminosity,
radius and escape velocity. We analyse the H-alpha and near-infrared calcium
triplet lines for evidence of outflows, pulsation and chromospheric activity,
and present a simple model for estimating mass-loss rates from the H-alpha line
profile. We compare our results with a variety of other, independent methods.
Results: We argue that a chromosphere persists in Galactic globular cluster
giants and controls the mass-loss rate to late-K/early-M spectral types, where
pulsation becomes strong enough to drive shock waves at luminosities above the
RGB tip. This transition may be metallicity-dependent. We find mass-loss rates
of ~10^-7 to 10^-5 solar masses per year, largely independent of metallicity.Comment: 23 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
Discovery of long-period variable stars in the very-metal-poor globular cluster M15
We present a search for long-period variable (LPV) stars among giant branch
stars in M15 which, at [Fe/H] ~ -2.3, is one of the most metal-poor Galactic
globular clusters. We use multi-colour optical photometry from the 0.6-m Keele
Thornton and 2-m Liverpool Telescopes. Variability of delta-V ~ 0.15 mag is
detected in K757 and K825 over unusually-long timescales of nearly a year,
making them the most metal-poor LPVs found in a Galactic globular cluster. K825
is placed on the long secondary period sequence, identified for metal-rich
LPVs, though no primary period is detectable. We discuss this variability in
the context of dust production and stellar evolution at low metallicity, using
additional spectra from the 6.5-m Magellan (Las Campanas) telescope. A lack of
dust production, despite the presence of gaseous mass loss raises questions
about the production of dust and the intra-cluster medium of this cluster.Comment: 13 pages, 9 figures, accepted by MNRA
Dust Production and Mass Loss in the Galactic Globular Cluster NGC 362
We investigate dust production and stellar mass loss in the Galactic globular
cluster NGC 362. Due to its close proximity to the Small Magellanic Cloud
(SMC), NGC 362 was imaged with the IRAC and MIPS cameras onboard the Spitzer
Space Telescope as part of the Surveying the Agents of Galaxy Evolution
(SAGE-SMC) Spitzer Legacy program. We detect several cluster members near the
tip of the Red Giant Branch that exhibit infrared excesses indicative of
circumstellar dust and find that dust is not present in measurable quantities
in stars below the tip of the Red Giant Branch. We modeled the spectral energy
distribution (SED) of the stars with the strongest IR excess and find a total
cluster dust mass-loss rate of 3.0(+2.0/-1.2) x 10^-9 solar masses per year,
corresponding to a gas mass-loss rate of 8.6(+5.6/-3.4) x 10^-6 solar masses
per year, assuming [Fe/H] = -1.16. This mass loss is in addition to any
dust-less mass loss that is certainly occurring within the cluster. The two
most extreme stars, variables V2 and V16, contribute up to 45% of the total
cluster dust-traced mass loss. The SEDs of the more moderate stars indicate the
presence of silicate dust, as expected for low-mass, low-metallicity stars.
Surprisingly, the SED shapes of the stars with the strongest mass-loss rates
appear to require the presence of amorphous carbon dust, possibly in
combination with silicate dust, despite their oxygen-rich nature. These results
corroborate our previous findings in omega Centauri.Comment: 13 pages, 11 figures. Accepted to Ap
Fundamental parameters, integrated RGB mass loss and dust production in the Galactic globular cluster 47 Tucanae
Fundamental parameters and time-evolution of mass loss are investigated for
post-main-sequence stars in the Galactic globular cluster 47 Tucanae (NGC 104).
This is accomplished by fitting spectral energy distributions (SEDs) to
existing optical and infrared photometry and spectroscopy, to produce a true
Hertzsprung--Russell diagram. We confirm the cluster's distance as 4611 (+213,
-200) pc and age as 12 +/- 1 Gyr. Horizontal branch models appear to confirm
that no more RGB mass loss occurs in 47 Tuc than in the more-metal-poor omega
Centauri, though difficulties arise due to inconsistencies between the models.
Using our SEDs, we identify those stars which exhibit infrared excess, finding
excess only among the brightest giants: dusty mass loss begins at a luminosity
of ~ 1000 Lsun, becoming ubiquitous above 2000 Lsun. Recent claims of dust
production around lower-luminosity giants cannot be reproduced, despite using
the same archival Spitzer imagery.Comment: 22 pages, 17 figures, accepted ApJ
The Very Slow Wind From the Pulsating Semiregular Red Giant L2 Pup
We have obtained 11.7 and 17.9 micron images at the Keck I telescope of the
circumstellar dust emission from L2 Pup, one of the nearest (D = 61 pc)
mass-losing, pulsating, red giants that has a substantial infrared excess. We
propose that the wind may be driven by the stellar pulsations with radiation
pressure on dust being relatively unimportant, as described in some recent
calculations. L2 Pup may serve as the prototype of this phase of stellar
evolution where it could lose about 15% of its initial main sequence mass.Comment: ApJ, in pres
X Her and TX Psc: Two cases of ISM interaction with stellar winds observed by Herschel
The asymptotic giant branch (AGB) stars X Her and TX Psc have been imaged at
70 and 160 microns with the PACS instrument onboard the Herschel satellite, as
part of the large MESS (Mass loss of Evolved StarS) Guaranteed Time Key
Program. The images reveal an axisymmetric extended structure with its axis
oriented along the space motion of the stars. This extended structure is very
likely to be shaped by the interaction of the wind ejected by the AGB star with
the surrounding interstellar medium (ISM). As predicted by numerical
simulations, the detailed structure of the wind-ISM interface depends upon the
relative velocity between star+wind and the ISM, which is large for these two
stars (108 and 55 km/s for X Her and TX Psc, respectively). In both cases,
there is a compact blob upstream whose origin is not fully elucidated, but that
could be the signature of some instability in the wind-ISM shock. Deconvolved
images of X Her and TX Psc reveal several discrete structures along the
outermost filaments, which could be Kelvin-Helmholtz vortices. Finally, TX Psc
is surrounded by an almost circular ring (the signature of the termination
shock?) that contrasts with the outer, more structured filaments. A similar
inner circular structure seems to be present in X Her as well, albeit less
clearly.Comment: 11 pages, Astronomy & Astrophysics, in pres
Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Large Magellanic Cloud
Reliable models of the thermally pulsing asymptotic giant branch (TP-AGB) phase are of critical importance across astrophysics, including our interpretation of the spectral energy distribution of galaxies, cosmic dust production, and enrichment of the interstellar medium. With the aim of improving sets of stellar isochrones that include a detailed description of the TP-AGB phase, we extend our recent calibration of the AGB population in the Small Magellanic Cloud (SMC) to the more metal-rich Large Magellanic Cloud (LMC). We model the LMC stellar populations with the trilegal code, using the spatially resolved star formation history derived from the VISTA survey. We characterize the efficiency of the third dredge-up by matching the star counts and the Ks-band luminosity functions of the AGB stars identified in the LMC. In line with previous findings, we confirm that, compared to the SMC, the third dredge-up in AGB stars of the LMC is somewhat less efficient, as a consequence of the higher metallicity. The predicted range of initial mass of C-rich stars is between Mi 48 1.7 and 3 M 99 at Zi = 0.008. We show how the inclusion of new opacity data in the carbon star spectra will improve the performance of our models. We discuss the predicted lifetimes, integrated luminosities, and mass-loss rate distributions of the calibrated models. The results of our calibration are included in updated stellar isochrones publicly available
The Spitzer Spectroscopic Survey of S-type Stars
S-type AGB stars are thought to be in the transitional phase between M-type
and C-type AGB stars. Because of their peculiar chemical composition, one may
expect a strong influence of the stellar C/O ratio on the molecular chemistry
and the mineralogy of the circumstellar dust. In this paper, we present a large
sample of 87 intrinsic galactic S-type AGB stars, observed at infrared
wavelengths with the Spitzer Space Telescope, and supplemented with
ground-based optical data. On the one hand, we derive the stellar parameters
from the optical spectroscopy and photometry, using a grid of model
atmospheres. On the other, we decompose the infrared spectra to quantify the
flux-contributions from the different dust species. Finally, we compare the
independently determined stellar parameters and dust properties. For the stars
without significant dust emission, we detect a strict relation between the
presence of SiS absorption in the Spitzer spectra and the C/O ratio of the
stellar atmosphere. These absorption bands can thus be used as an additional
diagnostic for the C/O ratio. For stars with significant dust emission, we
define three groups, based on the relative contribution of certain dust species
to the infrared flux. We find a strong link between group-membership and C/O
ratio. We show that these groups can be explained by assuming that the
dust-condensation can be cut short before silicates are produced, while the
remaining free atoms and molecules can then form the observed magnesium
sulfides or the carriers of the unidentified 13 and 20 micron features.
Finally, we present the detection of emission features attributed to molecules
and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons
and magnesium sulfide grains. We show that we often detect magnesium sulfides
together with molecular SiS and we propose that it is formed by a reaction of
SiS molecules with Mg.Comment: Accepted for publication in A&
Surveying the Agents of Galaxy Evolution in the Tidally-Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC) II. Cool Evolved Stars
We investigate the infrared (IR) properties of cool, evolved stars in the
Small Magellanic Cloud (SMC), including the red giant branch (RGB) stars and
the dust-producing red supergiant (RSG) and asymptotic giant branch (AGB) stars
using observations from the Spitzer Space Telescope Legacy program entitled:
"Surveying the Agents of Galaxy Evolution in the Tidally-stripped, Low
Metallicity SMC", or SAGE-SMC. The survey includes, for the first time, full
spatial coverage of the SMC bar, wing, and tail regions at infrared (IR)
wavelengths (3.6 - 160 microns). We identify evolved stars using a combination
of near-IR and mid-IR photometry and point out a new feature in the mid-IR
color-magnitude diagram that may be due to particularly dusty O-rich AGB stars.
We find that the RSG and AGB stars each contribute ~20% of the global SMC flux
(extended + point-source) at 3.6 microns, which emphasizes the importance of
both stellar types to the integrated flux of distant metal-poor galaxies. The
equivalent SAGE survey of the higher-metallicity Large Magellanic Cloud
(SAGE-LMC) allows us to explore the influence of metallicity on dust
production. We find that the SMC RSG stars are less likely to produce a large
amount of dust (as indicated by the [3.6]-[8] color). There is a higher
fraction of carbon-rich stars in the SMC, and these stars appear to able to
reach colors as red as their LMC counterparts, indicating that C-rich dust
forms efficiently in both galaxies. A preliminary estimate of the dust
production in AGB and RSG stars reveals that the extreme C-rich AGB stars
dominate the dust input in both galaxies, and that the O-rich stars may play a
larger role in the LMC than in the SMC.Comment: Accepted for publication in AJ. 25 pages, 36 figures, Table 4 will be
available electronically from A