27 research outputs found
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
The Mass Function of Main Sequence Stars in NGC6397 from Near IR and Optical High Resolution HST Observations
We have investigated the properties of the stellar mass function in the
globular cluster NGC6397 using a large set of HST observations that include
WFPC2 images in V and I, obtained at ~4' and 10' radial distances, and a series
of deep images in the J and H bands obtained with the NIC2 and NIC3 cameras of
NICMOS pointed to regions located ~4.5' and ~3.2' from the center. These
observations span the region from ~1 to ~3 times the cluster's half-light
radius. All luminosity functions, derived from color magniutde diagrams,
increase with decreasing luminosity up to a peak at M_I~8.5 or M_H~7 and then
precipitously drop well before photometric incompleteness becomes significant.
Within the observational uncertainties, at M_I~12 or M_H~10.5 (~0.09 Msun) the
luminosity functions are compatible with zero. By applying the best available
mass- luminosity relation appropriate to the metallicity of NGC6397 to both the
optical and IR data, we obtain a mass function that shows a break in slope at
\~0.3 Msun. No single exponent power-law distribution is compatible with these
data, regardless of the value of the exponent. We find that a dynamical model
of the cluster can simultaneously reproduce all the luminosity functions
observed throughout the cluster only if the IMF rises as m**-1.6 in the range
0.8-0.3 Msun and then drops as m**0.2 below ~0.3 Msun. Adopting a more physical
log-normal distribution for the IMF, all these data taken together imply a best
fit distribution with characteristic mass m_c~0.3 and sigma~1.8.Comment: 18 pages, 6 figures (ps). Accepted for publication in Ap
Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar B1821-24
We report here the results of the first Chandra X-Ray Observatory
observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are
detected, of which 12 lie within one core radius of the center. We show that
the apparently extended X-ray core emission seen with the ROSAT HRI is due to
the superposition of multiple discrete sources for which we determine the X-ray
luminosity function down to a limit of about 6xE30 erg/s. For the first time
the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is
measured and found to be best described by a power law with photon index ~ 1.2.
Marginal evidence of an emission line centered at 3.3 keV in the pulsar
spectrum is found, which could be interpreted as cyclotron emission from a
corona above the pulsar's polar cap if the the magnetic field is strongly
different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0
keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest
unidentified sources is presented. Based on the spectral parameters of the
brightest of these sources, we suggest that it is a transiently accreting
neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum
with a hydrogen neutron star atmosphere model yields the effective temperature
T_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty =
14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter,
unresolved X-ray emission from the central core of M28. Using the
Chandra-derived positions, we also report on the result of searching archival
Hubble Space Telescope data for possible optical counterparts.Comment: Accepted for publication in ApJ; 22 pages, 8 figures, 5 table
On the He burning phases of the Carina dSph
We performed a detailed comparison between predicted He burning phases and multiband photometry of the Carina dwarf spheroidal galaxy. We found a good agreement with the predictions computed assuming an alpha -enhanced chemical mixture, indicating a mean metallicity {[Fe/H]}̃-1.8 with a raw observed peak-to-peak spread in iron abundance of 0.4±0.2 dex
GRAWITA: VLT Survey Telescope observations of the gravitational wave sources GW150914 and GW151226
We report the results of deep optical follow-up surveys of the first two gravitational-wave sources, GW150914 and GW151226, done by the GRAvitational Wave Inaf TeAm Collaboration (GRAWITA). The VLT Survey Telescope (VST) responded promptly to the gravitational wave alerts sent by the LIGO and Virgo Collaborations, monitoring a region of 90 and 72 deg2 for GW150914 and GW151226, respectively, and repeated the observations over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the r band. The paper describes the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Several transients have been discovered but no candidates are recognized to be related to the gravitational wave events. Interestingly, among many contaminant supernovae, we find a possible correlation between the supernova VSTJ57.77559-59.13990 and GRB 150827A detected by Fermi-GBM. The detection efficiency of VST observations for different types of electromagnetic counterparts of gravitational wave events is evaluated for the present and future follow-up surveys
Supplement: "Localization and broadband follow-up of the gravitational-wave transient GW150914" (2016, ApJL, 826, L13)
This Supplement provides supporting material for Abbott et al. (2016a). We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands