40,386 research outputs found
The Discovery of a Second Luminous Low Mass X-ray Binary in the Globular Cluster M15
We report an observation by the Chandra X-ray Observatory of 4U2127+119, the
X-ray source identified with the globular cluster M15. The Chandra observation
reveals that 4U2127+119 is in fact two bright sources, separated by 2.7". One
source is associated with AC211, the previously identified optical counterpart
to 4U2127+119, a low mass X-ray binary (LMXB). The second source, M15-X2, is
coincident with a 19th U magnitude blue star that is 3.3" from the cluster
core. The Chandra count rate of M15-X2 is 2.5 times higher than that of AC211.
Prior to the 0.5" imaging capability of Chandra the presence of two so closely
separated bright sources would not have been resolved. The optical counterpart,
X-ray luminosity and spectrum of M15-X2 are consistent with it also being an
LMXB system. This is the first time that two LMXBs have been seen to be
simultaneously active in a globular cluster. The discovery of a second active
LMXB in M15 solves a long standing puzzle where the properties of AC211 appear
consistent with it being dominated by an extended accretion disk corona, and
yet 4U2127+119 also shows luminous X-ray bursts requiring that the neutron star
be directly visible. The resolution of 4U2127+119 into two sources suggests
that the X-ray bursts did not come from AC211, but rather from M15-X2. We
discuss the implications of this discovery for understanding the origin and
evolution of LMXBs in GCs as well as X-ray observations of globular clusters in
nearby galaxies.Comment: 18 pages, 4 figures, Accepted for publication in Ap J Letter
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The Abundances Of Neutron-Capture Species In The Very Metal-Poor Globular Cluster M15: A Uniform Analysis Of Red Giant Branch And Red Horizontal Branch Stars
The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Delta(RHB-RGB) approximate to 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.National Science Foundation AST 07-07447, AST 09-08978Astronom
Minimization of multi-penalty functionals by alternating iterative thresholding and optimal parameter choices
Inspired by several recent developments in regularization theory,
optimization, and signal processing, we present and analyze a numerical
approach to multi-penalty regularization in spaces of sparsely represented
functions. The sparsity prior is motivated by the largely expected
geometrical/structured features of high-dimensional data, which may not be
well-represented in the framework of typically more isotropic Hilbert spaces.
In this paper, we are particularly interested in regularizers which are able to
correctly model and separate the multiple components of additively mixed
signals. This situation is rather common as pure signals may be corrupted by
additive noise. To this end, we consider a regularization functional composed
by a data-fidelity term, where signal and noise are additively mixed, a
non-smooth and non-convex sparsity promoting term, and a penalty term to model
the noise. We propose and analyze the convergence of an iterative alternating
algorithm based on simple iterative thresholding steps to perform the
minimization of the functional. By means of this algorithm, we explore the
effect of choosing different regularization parameters and penalization norms
in terms of the quality of recovering the pure signal and separating it from
additive noise. For a given fixed noise level numerical experiments confirm a
significant improvement in performance compared to standard one-parameter
regularization methods. By using high-dimensional data analysis methods such as
Principal Component Analysis, we are able to show the correct geometrical
clustering of regularized solutions around the expected solution. Eventually,
for the compressive sensing problems considered in our experiments we provide a
guideline for a choice of regularization norms and parameters.Comment: 32 page
Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling
We analyze HST/STIS spectra (see Paper I) of the central region of the dense
globular cluster M15. We infer the velocities of 64 individual stars,
two-thirds of which have their velocity measured for the first time. This
triples the number of stars with measured velocities in the central 1 arcsec of
M15 and doubles the number in the central 2 arcsec. Combined with existing
ground-based data we obtain the radial profiles of the projected kinematical
quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few
arcsec, somewhat higher than the values of 10-12 km/s inferred previously from
ground-based data. To interpret the results we construct dynamical models based
on the Jeans equation, which imply that M15 must have a central concentration
of non-luminous material. If this is due to a single black hole, then its mass
is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the
relation between M_BH and sigma_RMS that has been established for galaxies.
Also, the existence of intermediate-mass black holes in globular clusters is
consistent with several scenarios for globular cluster evolution proposed in
the literature. Therefore, these results may have important implications for
our understanding of the evolution of globular clusters, the growth of black
holes, the connection between globular cluster and galaxy formation, and the
nature of the recently discovered `ultra-luminous' X-ray sources in nearby
galaxies. Instead of a single black hole, M15 could have a central
concentration of dark remnants (e.g., neutron stars) due to mass segregation.
However, the best-fitting Fokker-Planck models that have previously been
constructed for M15 do not predict a central mass concentration that is
sufficient to explain the observed kinematics.[ABRIDGED]Comment: 43 pages, LaTeX, with 14 PostScript figures. Astronomical Journal, in
press (Dec 2002). Please note that the results reported here are modified by
the Addendum available at astro-ph/0210158 (Astronomical Journal, in press,
Jan 2003). This second version submitted to astro-ph is identical to first,
with the exception of the preceeding remar
Characterizing the Chemistry of the Milky Way Stellar Halo: Detailed Chemical Analysis of a Metal-Poor Stellar Stream
We present the results of a detailed abundance analysis of one of the
confirmed building blocks of the Milky Way stellar halo, a
kinematically-coherent metal-poor stellar stream. We have obtained high
resolution and high S/N spectra of 12 probable stream members using the MIKE
spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the
2dCoude spectrograph on the Smith Telescope at McDonald Observatory. We have
derived abundances or upper limits for 51 species of 46 elements in each of
these stars. The stream members show a range of metallicity (-3.4 < [Fe/H] <
-1.5) but are otherwise chemically homogeneous, with the same star-to-star
dispersion in [X/Fe] as the rest of the halo. This implies that, in principle,
a significant fraction of the Milky Way stellar halo could have formed from
accreted systems like the stream. The stream stars show minimal evolution in
the alpha or Fe-group elements over the range of metallicity. This stream is
enriched with material produced by the main and weak components of the rapid
neutron-capture process and shows no evidence for enrichment by the slow
neutron-capture process.Comment: v2: Removed references to M15 after learning that the source
kinematic data for M15 were incorrect in an earlier paper. M15 is not related
to this stream. (ApJ, accepted; 31 pages, 18 figures, 11 tables
A Na I Absorption Map of the Small-Scale Structure in the Interstellar Gas Toward M15
Using the DensePak fiber optic array on the KPNO WIYN telescope, we have
obtained high S/N echelle spectra of the Na I D wavelength region toward the
central 27" x 43" of the globular cluster M15 at a spatial resolution of 4".
The spectra exhibit significant interstellar Na I absorption at LSR velocities
of +3 km/s (LISM component) and +68 km/s (IVC component). Both components vary
appreciably in strength on these scales. The derived Na I column densities
differ by a factor of 4 across the LISM absorption map and by a factor of 16
across the IVC map. Assuming distances of 500 pc and 1500 pc for the LISM and
IVC clouds, these maps show evidence of significant ISM structure down to the
minimum scales of 2000 AU and 6000 AU probed in these absorbers. The
smallest-scale N(Na I) variations observed in the M15 LISM and IVC maps are
typically comparable to or higher than the values found at similar scales in
previous studies of interstellar Na I structure toward binary stars. The
physical implications of the small and larger-scale Na I features observed in
the M15 maps are discussed in terms of variations in the H I column density as
well as in the Na ionization equilibrium.Comment: 11 pages, 3 figures, accepted for publication in ApJ Letter
Ba & Eu Abundances in M15 giant stars
To investigate the Ba and Eu abundances for a sample of 63 giant stars in the
globular cluster M15. This is the largest sample of M15 giants stars for which
Ba abundances have been determined and, due to the target selection of the
original research programme, the Ba abundances are complete along the red giant
branch. Stellar parameters were taken from the previous key study and a
microturbulence-surface gravity relation was determined for precise measurement
of the Ba line at 6496.898 Angstroms, which has a high sensitivity to
microturbulence. Element abundances for Ba, La, Eu, Ca, Ni and Fe were
calculated using spectrum synthesis and equivalent widths techniques. A bimodal
distribution in Ba, Eu and La abundances was found within the sample. The low
Ba,Eu,La mode had mean abundances of =-2.41+/-0.16,
=-1.80+/-0.08 and =-2.19+/-0.13 while the high Ba,Eu,La mode
had mean abundances of =-2.00+/-0.16, =-1.65+/-0.13 and
=-1.95+/-0.11. Both modes are indicative of a pollution scenario
dominated by the r-process, hence contributions from explosive nucleosynthesis
of massive stars. There may be evidence of further enhancement by another heavy
element process and of potential anticorrelations in Na-O for both modes
indicating a complex formation and evolution history for M15.Comment: 20 pages, 15 figure
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