26 research outputs found
The First Empirical Mass Loss Law for Population II Giants
Using the Spitzer IRAC camera we have obtained mid-IR photometry of the red
giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars
show an excess of mid-infrared light above that expected from their
photospheric emission. This is plausibly due to dust formation in mass flowing
from these stars. This mass loss extends down to the level of the horizontal
branch and increases with luminosity. The mass loss is episodic, occurring in
only a fraction of stars at a given luminosity. Using a simple model and our
observations we derive mass loss rates for these stars. Finally, we obtain the
first empirical mass loss formula calibrated with observations of Population II
stars. The dependence on luminosity of our mass loss rate is considerably
shallower than the widely used Reimers Law. The results presented here are the
first from our Spitzer survey of a carefully chosen sample of 17 Galactic
Globular Clusters, spanning the entire metallicity range from about one
hundredth up to almost solar
Deep Hubble Space Telescope WFPC2 Photometry of NGC 288. I. Binary Systems and Blue Stragglers
We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the main sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn. We have unequivocally detected a significant population of binary systems with a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (rh~=1.6rc) the binary fraction (fb) lies in the range 0.08-0.38 regardless of the assumed distribution of mass ratios, F(q). The most probable fb lies between 0.10 and 0.20 depending on the adopted F(q). On the other hand, in the outer region (r>=1rh), fb must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of blue stragglers (BSs) is exceptionally high, suggesting that the BS production mechanism via binary evolution can be very efficient. A large population of BSs is possible even in low-density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSs in the color-magnitude diagram is not compatible with a rate of BS production that has been constant in time, if it is assumed that all the BSs are formed by the merging of two stars. Based on observations made with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal GO-6804
Dust is forming along the red giant branch of 47 TUC
We present additional evidence that dust is really forming along the red
giant branch (RGB) of 47 Tuc at luminosities ranging from above the horizontal
branch to the RGB-tip (Origlia et al. 2007). The presence of dust had been
inferred from an infrared excess in the (K-8) color, with K measured from high
spatial resolution ground based near-IR photometry and "8" referring to
Spitzer-IRAC 8 micron photometry. We show how (K-8) is a far more sensitive
diagnostic for detecting tiny circumstellar envelopes around warm giants than
colors using only the Spitzer-IRAC bands, for example the (3.6-8) color used by
Boyer et al. (2010). In addition, we also show high resolution HST-ACS I band
images of the giant stars which have (K-8) color excess. These images clearly
demonstrate that Boyer et al (2010) statement that our detections of color
excess associated with stars below the RGB-tip arise from blends and artefacts
is simply not valid.Comment: 12 pages, 4 figure
ISOCAM observations of Galactic Globular Clusters: mass loss along the Red Giant Branch
Deep images in the 10 micron spectral region have been obtained for five
massive Galactic globular clusters, NGC 104 (=47 Tuc), NGC 362, NGC 5139 (omega
Cen), NGC 6388, NGC 7078 (=M15) and NGC 6715 (=M54) in the Sagittarius Dwarf
Spheroidal using ISOCAM in 1997. A significant sample of bright giants have an
ISOCAM counterpart but only < 20% of these have a strong mid-IR excess
indicative of dusty circumstellar envelopes. From a combined physical and
statistical analysis we derive mass loss rates and frequency. We find that i)
significant mass loss occurs only at the very end of the Red Giant Branch
evolutionary stage and is episodic, ii) the modulation timescales must be
greater than a few decades and less than a million years, and iii) mass loss
occurrence does not show a crucial dependence on the cluster metallicity.Comment: 26 pages, 9 figure
Another Faint UV Object Associated with a Globular Cluster X-Ray Source: The Case of M92
The core of the metal poor Galactic Globular Cluster M92 (NGC 6341) has been
observed with WFPC2 on the Hubble Space Telescope through visual, blue and
mid-UV filters in a program devoted to study the evolved stellar population in
a selected sample of Galactic Globular Clusters. In the UV color magnitude diagram we have discovered a faint `UV-dominant'
object. This star lies within the error box of a Low Luminosity Globular
Cluster X-ray source (LLGCX) recently found in the core of M92. The properties
of the UV star discovered in M92 are very similar to those of other UV stars
found in the core of some clusters (M13, 47 Tuc, M80, etc)---all of them are
brighter in the UV than in the visible and are located in the vicinity of a
LLGCX. We suggest that these stars are a new sub-class of cataclysmic
variables.Comment: 21 pages, 4 figures. Astrophysical journal in pres