2,241 research outputs found
Carbon-rich dust production in metal-poor galaxies in the Local Group
We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax,
and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the
Spitzer Space Telescope. The spectra show significant quantities of dust around
the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous
comparisons of carbon stars with similar pulsation properties in the Galaxy and
the Magellanic Clouds revealed no evidence that metallicity affected the
production of dust by carbon stars. However, the more metal-poor stars in the
current sample appear to be generating less dust. These data extend two known
trends to lower metallicities. In more metal-poor samples, the SiC dust
emission weakens, while the acetylene absorption strengthens. The bolometric
magnitudes and infrared spectral properties of the carbon stars in Fornax are
consistent with metallicities more similar to carbon stars in the Magellanic
Clouds than in the other dwarf spheroidals in our sample. A study of the carbon
budget in these stars reinforces previous considerations that the dredge-up of
sufficient quantities of carbon from the stellar cores may trigger the final
superwind phase, ending a star's lifetime on the asymptotic giant branch.Comment: ApJ, in press, 21 pages, 12 figures. Replaced Fig 12, corrected two
reference
Hubble Space Telescope imaging of the compact elliptical galaxy M32 reveals a dearth of carbon stars
We present new {\em Hubble Space Telescope} WFC3/IR medium-band photometry of
the compact elliptical galaxy M32, chemically resolving its thermally pulsating
asymptotic giant branch stars. We find 2829 M-type stars and 57 C stars. The
carbon stars are likely contaminants from M31. If carbon stars are present in
M32 they are so in very low numbers. The uncorrected C/M ratio is 0.020
0.003; this drops to less than 0.007 after taking into account contamination
from M31. As the mean metallicity of M32 is just below solar, this low ratio of
C to M stars is unlikely due to a metallicity ceiling for the formation of
carbon stars. Instead, the age of the AGB population is likely to be the
primary factor. The ratio of AGB to RGB stars in M32 is similar to that of the
inner disc of M31 which contain stars that formed 1.5--4 Gyr ago. If the M32
population is at the older end of this age then its lack of C-stars may be
consistent with a narrow mass range for carbon star formation predicted by some
stellar evolution models. Applying our chemical classifications to the dusty
variable stars identified with {\em Spitzer}, we find that the x-AGB candidates
identified with {\em Spitzer} are predominately M-type stars. This
substantially increases the lower limit to the cumulative dust-production rate
in M32 to 1.97 .Comment: 10 pages, 7 figures, submitted MNRAS 7/12/2
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
An Infrared Census of DUST in Nearby Galaxies with Spitzer (DUSTiNGS). IV. Discovery of High-redshift AGB Analogs
Carbon enrichment of the evolved stars in the Sagittarius dwarf spheroidal
We present spectra of 1142 colour-selected stars in the direction of the
Sagittarius Dwarf Spheroidal (Sgr dSph) galaxy, of which 1058 were taken with
VLT/FLAMES multi-object spectrograph and 84 were taken with the SAAO Radcliffe
1.9-m telescope grating spectrograph. Spectroscopic membership is confirmed (at
>99% confidence) for 592 stars on the basis of their radial velocity, and
spectral types are given. Very slow rotation is marginally detected around the
galaxy's major axis. We identify five S stars and 23 carbon stars, of which all
but four carbon stars are newly-determined and all but one (PQ Sgr) are likely
Sgr dSph members. We examine the onset of carbon-richness in this metal-poor
galaxy in the context of stellar models. We compare the stellar death rate (one
star per 1000-1700 years) to known planetary nebula dynamical ages and find
that the bulk population produce the observed (carbon-rich) planetary nebulae.
We compute average lifetimes of S and carbon stars as 60-250 and 130-500 kyr,
compared to a total thermal-pulsing asymptotic giant branch lifetime of
530-1330 kyr. We conclude by discussing the return of carbon-rich material to
the ISM.Comment: 14 pages, 10 figures, accepted MNRA
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
An Optical Readout TPC (O-TPC) for Studies in Nuclear Astrophysics With Gamma-Ray Beams at HIgS
We report on the construction, tests, calibrations and commissioning of an
Optical Readout Time Projection Chamber (O-TPC) detector operating with a
CO2(80%) + N2(20%) gas mixture at 100 and 150 Torr. It was designed to measure
the cross sections of several key nuclear reactions involved in stellar
evolution. In particular, a study of the rate of formation of oxygen and carbon
during the process of helium burning will be performed by exposing the chamber
gas to intense nearly mono-energetic gamma-ray beams at the High Intensity
Gamma Source (HIgS) facility. The O-TPC has a sensitive target-drift volume of
30x30x21 cm^3. Ionization electrons drift towards a double parallel grid
avalanche multiplier, yielding charge multiplication and light emission.
Avalanche induced photons from N2 emission are collected, intensified and
recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional
track images. The event's time projection (third coordinate) and the deposited
energy are recorded by photomultipliers and by the TPC charge-signal,
respectively. A dedicated VME-based data acquisition system and associated data
analysis tools were developed to record and analyze these data. The O-TPC has
been tested and calibrated with 3.183 MeV alpha-particles emitted by a 148Gd
source placed within its volume with a measured energy resolution of 3.0%.
Tracks of alpha and 12C particles from the dissociation of 16O and of three
alpha-particles from the dissociation of 12C have been measured during initial
in-beam test experiments performed at the HIgS facility at Duke University. The
full detection system and its performance are described and the results of the
preliminary in-beam test experiments are reported.Comment: Supported by the Richard F. Goodman Yale-Weizmann Exchange Program,
ACWIS, NY, and USDOE grant Numbers: DE-FG02-94ER40870 and DE-FG02-97ER4103
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