96 research outputs found
Nearby main sequence stars with cool circumstellar material
The discovery of the so-called Vega phenomenon was one of the most important and unexpected results of the IRAS mission. Several nearby main sequence stars were found to possess clouds of solid grains emitting strongly in the far-IR. Three of these objects were marginally resolved by IRAS. This phenomenon appears to be widespread and not limited to proto-planetary epochs. Possible connection of this phenomenon to the existing of planets is discussed
VINCI-VLTI measurements of HR 4049: the physical size of the circumbinary envelope
We present the first detection of the envelope which surrounds the post-AGB
binary source HR 4049. VINCI-VLTI K-band interferometric observations of this
source imply the existence of a large structure with a Gaussian angular FWHM
22.4 +/- 1.4 mas or uniform disk diameter of 34.9 +/- 1.9 mas. With the
Hipparcos parallax of 1.50 +/- 0.64 mas these values correspond to a physical
size of 14.9 (+11.1,-4.4) AU and 23.3 (+17.3,-7.0) AU, respectively. Our
measurements, covering an azimuth range of about 60 degrees, for the
sky-projected baseline, provide information on the geometry of the emitting
region and show that there is only a slight variation of the measured angular
values along the different directions sampled. Thus, our results are consistent
with a spherical geometry of the envelope. However, we cannot completely rule
out the existence of an asymmetric envelope (like the circumbinary disk
envisaged by some recent models) because of the limited spatial frequency and
azimuth range covered by the observations.Comment: 4 pages, 4 postscript figures, accepted letter for A&
Why haven't loose globular clusters collapsed yet?
We report on the discovery of a surprising observed correlation between the
slope of the low-mass stellar global mass function (GMF) of globular clusters
(GCs) and their central concentration parameter c=log(r_t/r_c), i.e. the
logarithmic ratio of tidal and core radii. This result is based on the analysis
of a sample of twenty Galactic GCs with solid GMF measurements from deep HST or
VLT data. All the high-concentration clusters in the sample have a steep GMF,
most likely reflecting their initial mass function. Conversely,
low-concentration clusters tend to have a flatter GMF implying that they have
lost many stars via evaporation or tidal stripping. No GCs are found with a
flat GMF and high central concentration. This finding appears
counter-intuitive, since the same two-body relaxation mechanism that causes
stars to evaporate and the cluster to eventually dissolve should also lead to
higher central density and possibly core-collapse. Therefore, more concentrated
clusters should have lost proportionately more stars and have a shallower GMF
than low concentration clusters, contrary to what is observed. It is possible
that severely depleted GCs have also undergone core collapse and have already
recovered a normal radial density profile. It is, however, more likely that GCs
with a flat GMF have a much denser and smaller core than suggested by their
surface brightness profile and may well be undergoing collapse at present. In
either case, we may have so far seriously underestimated the number of post
core-collapse clusters and many may be lurking in the Milky Way.Comment: Four pages, one figure, accepted for publication in ApJ Letter
Very Large Telescope Observations of the peculiar globular cluster NGC6712. Discovery of a UV, H-alpha excess star in the core
We present results from multi-band observations in the central region of the
cluster NGC6712 with the ESO-Very Large Telescope. Using high resolution images
we have identified three UV-excess stars. In particular two of them are within
the cluster core, a few arcsec apart: the first object is star "S" which
previous studies identified as the best candidate to the optical counterpart to
the luminous X-ray source detected in this cluster. The other UV object shows
clearcut H-alpha emission and, for this reason, is an additional promising
interacting binary candidate (a quiescent LMXB or a CV). The presence of two
unrelated interacting binary systems a few arcsec apart in the core of this
low-density cluster is somewhat surprising and supports the hypothesis that the
(internal) dynamical history of the cluster and/or the (external) interaction
with the Galaxy might play a fundamental role in the formation of these
peculiar objects.Comment: 15 pages, 3 figures. ApJL in pres
On the temporal evolution of the stellar mass function in Galactic clusters
We show that we can obtain a good fit to the present day stellar mass
functions (MFs) of a large sample of young and old Galactic clusters in the
range 0.1 - 10 Msolar with a tapered power law distribution function with an
exponential truncation of the form dN/dm \propto m^alpha [1 -
exp-(m/m_c)^beta]. The average value of the power-law index alpha is -2, that
of beta is 2.5, whereas the characteristic mass m_c is in the range 0.1 - 0.8
Msolar and does not seem to vary in any systematic way with the present cluster
parameters such as metal abundance, total cluster mass or central
concentration. However, m_c shows a remarkable correlation with the dynamical
age of the cluster, namely m_c/Msolar ~ 0.15 + 0.5 tau_dyn^0.75, where tau_dyn
is the dynamical age taken as the ratio of cluster age and dissolution time.
The small scatter seen around this correlation is consistent with the
uncertainties on the estimated value of tau_dyn. We attribute the observed
trend to the onset of mass segregation via two-body relaxation in a tidal
environment, causing the preferential loss of low-mass stars from the cluster
and hence a drift of the characteristic mass m_c towards higher values. If
dynamical evolution is indeed at the origin of the observed trend, it would
seem plausible that high-concentration globular clusters, now with median m_c ~
0.33 Msolar, were born with a stellar MF very similar to that measured today in
the youngest Galactic clusters and with a value of m_c ~ 0.15 Msolar. This
hypothesis is consistent with the absence of a turn-over in the MF of the
Galactic bulge down to the observational limit at ~0.2 Msolar and, if correct,
it would carry the implication that the characteristic mass is not set by the
thermal Jeans mass of the cloud.Comment: 7 pages, 2 figures, accepted for publication in the Astrophysical
Journa
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