308 research outputs found
A second black hole candidate in a M31 globular cluster is identified with XMM-Newton
We use arguments developed in previous work to identify a second black hole
candidate associated with a M31 globular cluster, Bo 144, on the basis of X-ray
spectral and timing properties. The 2002 XMM-Newton observation of the
associated X-ray source (hereafter XBo 144) revealed behaviour that is common
to all low-mass X-ray binaries (LMXBs) in the low-hard state. Studies have
shown that neutron star LMXBs exhibit this behaviour at 0.01-1000 keV
luminosities <=10% of the Eddington limit (L_Edd). However, the unabsorbed
0.3-10 keV XBo 144 luminosity was ~0.30 L_Edd for a 1.4 M_sun neutron star, and
the expected 0.01-1000 keV luminosity is 3-7 times higher. We therefore
identify XBo 144 as a black hole candidate. Furthermore, it is the second black
hole candidate to be consistent with formation via tidal capture of a mean
sequence donor in a GC; such systems were previously though non-existent,
because the donor was thought to be disrupted during the capture process.Comment: Accepted for publication in MRAS letters. Four pages, three figure
An updated survey of globular clusters in M31. II Newly discovered bright and remote clusters
We present the first results of a large spectroscopic survey of candidate
globular clusters located in the extreme outskirts of the nearby M31 galaxy. We
obtained low resolution spectra of 48 targets selected from the XSC of 2MASS,
as in Galleti et al. (2005). The observed candidates have been robustly
classified according to their radial velocity and by verifying their
extended/point-source nature from ground-based optical images. Among the 48
observed candidates clusters we found 5 genuine remote globular clusters. One
of them has been already identified independently by Mackey et al. (2007),
their GC1; the other four are completely new discoveries: B516, B517, B518,
B519. The newly discovered clusters lie at projected distance 40 kpc<~R_p<~100
kpc from the center of M31, and have absolute integrated magnitude
-9.5<M_V<-7.5. For all the observed clusters we have measured the strongest
Lick indices and we have obtained spectroscopic metallicity estimates.
Mackey-GC1, Martin-GC1, B517 and B518 have spectra typical of old and metal
poor globular clusters ([Fe/H]<~ -1.3); B519 appears old but quite metal-rich
([Fe/H]~-0.5); B516 presents very strong Balmer absorption lines: if this is
indeed a cluster it should have a relatively young age (likely <2 Gyr). The
present analysis nearly doubles the number of M31 globulars at R_p> 40 kpc. At
odds with the Milky Way, M31 appears to have a significant population of very
bright globular clusters in its extreme outskirts.Comment: 16 pages including 6 pages published only in the electronic edition
of the Journal. Accepted for publication in A&
FABRICATION AND COMPARATIVE STUDY OF HORIZONTAL AND VERTICAL ELECTROSPUN PROTEIN-POLYSACCHARIDE NANOFIBER BIOMATERIALS
The use of biocompatible and biodegradable composite materials for biomedical applications has attracted the attention of many researchers in the past few years. In this study, we fabricated nanofibers of silk fibroin and cellulose and its derivatives to amalgamate their unique properties into a single material. The production of these nanofibers via electrospinning is of particular interest, and whereas several studies have been done on normal nanofibers, the formation of branched nanofibers is an exciting area not currently explored. Blend solutions are formed by dissolving silk and cellulose/cellulose acetate in formic acid separately and mixing to achieve the desired ratios. Samples are electrospun in both the vertical and horizontal directions before undergoing water annealing treatment and characterization using the SEM, FTIR, TGA, and DSC. From SEM images, we find that samples spun vertically exhibit branching structures, whereas samples spun horizontally form normal nanofibers. Structural analysis shows that samples with high silk content retain the beta sheet structures and samples with high cellulose/cellulose acetate content show decreased content of random side chain groups. Thermal analysis shows that vertically spun samples are stronger than horizontally spun samples; this observation is likely due to the branching of the nanofibers. These results show that electrospinning can be used to fabricate branched nanofibers of silk-cellulose/cellulose acetate blends, a material that boasts attractive properties conducive to biomedical applications
The extended structure of the remote cluster B514 in M31. Detection of extra-tidal stars
We present a study of the density profile of the remote M31 globular cluster
B514, obtained from HST/ACS observations. Coupling the analysis of the
distribution of the integrated light with star counts we can reliably follow
the profile of the cluster out to r~35", corresponding to ~130pc. The profile
is well fitted, out to ~15 core radii, by a King Model having C=1.65. With an
estimated core radius r_c=0.38", this corresponds to a tidal radius of r_t~17"
(~65pc). We find that both the light and the star counts profiles show a
departure from the best fit King model for r>~8" - as a surface brightness
excess at large radii, and the star counts profile shows a clear break in
correspondence of the estimated tidal radius. Both features are interpreted as
the signature of the presence of extratidal stars around the cluster. We also
show that B514 has a half-light radius significantly larger than ordinary
globular clusters of the same luminosity. In the M_V vs. log r_h plane, B514
lies in a region inhabited by peculiar clusters, like Omega Cen, G1, NGC2419
and others, as well as by the nuclei of dwarf elliptical galaxies.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
HST/ACS colour-magnitude diagrams of M31 globular clusters
With the aim of increasing the sample of M31 clusters for which a colour
magnitude diagram is available, we searched the HST archive for ACS images
containing objects included in the Revised Bologna Catalogue of M31 globular
clusters. Sixty-three such objects were found. We used the ACS images to
confirm or revise their classification and we obtained useful CMDs for 11 old
globular clusters and 6 luminous young clusters. We obtained simultaneous
estimates of the distance, reddening, and metallicity of old clusters by
comparing their observed field-decontaminated CMDs with a grid of template
clusters of the Milky Way. We estimated the age of the young clusters by
fitting with theoretical isochrones. For the old clusters, we found
metallicities in the range -0.4<=[Fe/H]<=-1.9, that generally agree with
existing spectroscopic extimates. At least four of them display a clear blue
HB, indicating ages >10 Gyr. All six candidate young clusters are found to have
ages <1Gyr. With the present work the total number of M31 GCs with reliable
optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for
the young ones. The old clusters show similar characteristics to those of the
MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ~-0.6
and located at a large projected distance from the centre of M31 and from the
galaxy major axis. Metal-rich globulars at large galactocentric distances are
rare both in M31 and in the MW. B407, in addition, has a velocity in stark
contrast with the rotation pattern shared by the bulk of M31 clusters of
similar metallicity. This, along with other empirical evidence, supports the
hypothesis that the cluster is physically associated with a substructure in the
M31 halo that has been interpreted as the relic of a merging event.Comment: 19 pages, 14 figures, 6 tables. Accepted for publication on Astronomy
and Astrophysic
An updated survey of globular clusters in M 31. III. A spectroscopic metallicity scale for the Revised Bologna Catalog
We present a new homogeneous set of metallicity estimates based on Lick
indices for 245 old globular clusters of the M31 galaxy comprised in the
Revised Bologna Catalog. The metallicity distribution of the M31 globular
clusters is briefly discussed and compared with that of the Milky Way. Simple
parametric statistics suggests that the [Fe/H] distribution is likely not
unimodal. The strong correlation between metallicity and kinematics found in
previous studies is confirmed. The most metal-rich GCs tend to be packed at the
center of the system and share the galactic rotation as traced by the HI disk.
Although the velocity dispersion around the curve increases with decreasing
metallicity, also clusters with [Fe/H]<-1.0 display a clear rotational pattern,
at odds with their Milky Way counterparts.Comment: 25 pages with 17 figures and 2 Appendices - To appear on Astronomy &
Astrophysic
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