6,719 research outputs found
An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters
The main aim of the present work is to derive an empirical mass-loss (ML) law
for Population II stars in first and second ascent red giant branches. We used
the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8
micron range of a carefully chosen sample of 15 Galactic globular clusters
spanning the entire metallicity range and sampling the vast zoology of
horizontal branch (HB) morphologies. We complemented the IRAC photometry with
near-infrared data to build suitable color-magnitude and color-color diagrams
and identify mass-losing giant stars. We find that while the majority of stars
show colors typical of cool giants, some stars show an excess of mid-infrared
light that is larger than expected from their photospheric emission and that is
plausibly due to dust formation in mass flowing from them. For these stars, we
estimate dust and total (gas + dust) ML rates and timescales. We finally
calibrate an empirical ML law for Population II red and asymptotic giant branch
stars with varying metallicity. We find that at a given red giant branch
luminosity only a fraction of the stars are losing mass. From this, we conclude
that ML is episodic and is active only a fraction of the time, which we define
as the duty cycle. The fraction of mass-losing stars increases by increasing
the stellar luminosity and metallicity. The ML rate, as estimated from
reasonable assumptions for the gas-to-dust ratio and expansion velocity,
depends on metallicity and slowly increases with decreasing metallicity. In
contrast, the duty cycle increases with increasing metallicity, with the net
result that total ML increases moderately with increasing metallicity, about
0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars,
we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.Comment: 17 pages, 9 figures, in press on A&
An Extended Star Formation History for the Galactic Center from Hubble Space Telescope/NICMOS Observations
We present Hubble Space Telescope (HST) Near-Infrared Camera and Multiobject
Spectrometer (NICMOS) observations as evidence that continuous star formation
has created much of the central stellar cusp of the Galaxy. The data are the
deepest ever obtained for a Galactic Center (GC) population, being 50%
complete for \mnk, or initial stellar masses 2 \Msun. We use
Geneva and Padova stellar evolution models to produce synthetic luminosity
functions for burst and continuous star formation scenarios, finding that the
observations are fit best by continuous star formation at a rate that is
consistent with the recent star formation activity that produced the three
massive young clusters in the central 50 \pc. Further, it is not possible to
fit the observations with ancient burst models, such as would be appropriate
for an old population like that in Baade's Window or NGC6528
The Terzan 5 puzzle: discovery of a third, metal-poor component
We report on the discovery of 3 metal-poor giant stars in Terzan 5, a complex
stellar system in the the Galactic bulge, known to have two populations at
[Fe/H]=-0.25 and +0.3. For these 3 stars we present new echelle spectra
obtained with NIRSPEC at Keck II, which confirm their radial velocity
membership and provide average [Fe/H]=-0.79 dex iron abundance and
[alpha/Fe]=+0.36 dex enhancement. This new population extends the metallicity
range of Terzan~5 0.5 dex more metal poor, and it has properties consistent
with having formed from a gas polluted by core collapse supernovae.Comment: Accepted for publication on ApJ Lette
Detections of massive stars in the cluster MCM2005b77, in the star-forming regions GRS G331.3400.36 (S62) and GRS G337.9200.48 (S36)
Large infrared and millimeter wavelength surveys of the Galactic plane have
unveiled more than 600 new bubble HII regions and more than 3000 candidate star
clusters. We present a study of the candidate clusters MCM2005b72, DBS2003-157,
DBS2003-172, and MCM2005b77, based on near-infrared spectroscopy taken with
SofI on the NTT and infrared photometry from the 2MASS, VVV, and GLIMPSE
surveys. We find that (1) MCM2005b72 and DBS2003-157 are subregions of the same
star-forming region, HII GRS G331.34-00.36 (bubble S62). MCM2005b72 coincides
with the central part of this HII region, while DBS2003-157 is a bright
mid-infrared knot of the S62 shell. We detected two O-type stars at extinction
\Aks=1.0-1.3 mag. Their spectrophotometric properties are consistent with the
near-kinematic distance to GRS G331.34-00.36 of 3.9pm0.3 kpc. (2) DBS2003-172
coincides with a bright mid-infrared knot in the S36 shell (GRS G337.92-00.48),
where we detected a pair of candidate He I stars embedded in a small cometary
nebula. (3) The stellar cluster MCM2005b77 is rich in B-type stars, has an
average Aks of 0.91 mag, and is adjacent to the HII region IRAS 16137-5025. The
average spectrophotometric distance of kpc matches the
near-kinematic distance to IRAS 16137-5025 of 5.2pm0.1 kpc.Comment: 22 pages, 11 Figures, ApJ accepte
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