518 research outputs found
Orbital and physical parameters of eclipsing binaries from the All-Sky Automated Survey catalogue - VI. AK Fornacis - a rare, bright K-type eclipsing binary
We present the results of the combined photometric and spectroscopic analysis
of a bright (V=9.14), nearby (d=31 pc), late-type detached eclipsing binary AK
Fornacis. This P=3.981 d system has not been previously recognised as a
double-lined spectroscopic binary, and this is the first full physical model of
this unique target. With the FEROS, CORALIE and HARPS spectrographs we
collected a number of high-resolution spectra in order to calculate radial
velocities of both components of the binary. Measurements were done with our
own disentangling procedure and the TODCOR technique, and were later combined
with the photometry from the ASAS and SuperWASP archives. We also performed an
atmospheric analysis of the component spectra with the Spectroscopy Made Easy
(SME) package. Our analysis shows that AK For consists of two active, cool
dwarfs having masses of and
M and radii of and
R, slightly less metal abundant than the Sun. Parameters of both
components are well reproduced by the models.
AK For is the brightest system among the known eclipsing binaries with K or M
type stars. Its orbital period is one of the longest and rotational velocities
one of the lowest, which allows us to obtain very precise radial velocity
measurements. The precision in physical parameters we obtained places AK For
among the binaries with the best mass measurements in the literature. It also
fills the gap in our knowledge of stars in the range of 0.5-0.8 M, and
between short and long-period systems. All this makes AK For a unique benchmark
for understanding the properties of low-mass stars.Comment: 9 pages, 11 figures, 3 tables, accpeted for publication in A&
Dynamical Formation of Millisecond Pulsars in Globular Clusters
The cumulative luminosity distribution functions (CLFs) of radio millisecond
pulsars (MSPs) in globular clusters (GCs) and in the Galactic field at a
frequency of 1.4 GHz have been examined. Assuming a functional form, where is the number of MSPs and is the luminosity at 1.4 GHz, it
is found that the CLFs significantly differ with a steeper slope, , in GCs than in the Galactic field (), suggesting a
different formation or evolutionary history of MSPs in these two regions of the
Galaxy. To probe the production mechanism of MSPs in clusters, a search of the
possible relationships between the MSP population and cluster properties was
carried out. The results of an investigation of 9 GCs indicate positive
correlations between the MSP population and the stellar encounter rate and
metallicity. This provides additional evidence suggesting that stellar
dynamical interactions are important in the formation of the MSP population in
GCs.Comment: 18 pages, 3 figures, 4 tables, accepted for publication in Ap
Globular cluster systems in fossil groups: NGC6482, NGC1132 and ESO306-017
We study the globular cluster (GC) systems in three representative fossil
group galaxies: the nearest (NGC6482), the prototype (NGC1132) and the most
massive known to date (ESO306-017). This is the first systematic study of GC
systems in fossil groups. Using data obtained with the Hubble Space Telescope
Advanced Camera for Surveys in the F475W and F850LP filters, we determine the
GC color and magnitude distributions, surface number density profiles, and
specific frequencies. In all three systems, the GC color distribution is
bimodal, the GCs are spatially more extended than the starlight, and the red
population is more concentrated than the blue. The specific frequencies seem to
scale with the optical luminosities of the central galaxy and span a range
similar to that of the normal bright elliptical galaxies in rich environments.
We also analyze the galaxy surface brightness distributions to look for
deviations from the best-fit S\'ersic profiles; we find evidence of recent
dynamical interaction in all three fossil group galaxies. Using X-ray data from
the literature, we find that luminosity and metallicity appear to correlate
with the number of GCs and their mean color, respectively. Interestingly,
although NGC6482 has the lowest mass and luminosity in our sample, its GC
system has the reddest mean color, and the surrounding X-ray gas has the
highest metallicity.Comment: 16 pages, 13 figures. Accepted for publication in A&
The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency
We study the rich globular cluster (GC) system in the center of the massive
cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational
lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude
I_814=29 with >90% completeness and sample the brightest ~5% of the GC system.
Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we
estimate a total population of N(GC_total) = 162,850 GCs within a projected
radius of 400kpc. As many as half may comprise an intracluster component. Even
with the sizable uncertainties, which mainly result from the uncertain GCLF
parameters, this is by far the largest GC system studied to date. The specific
frequency S_N is high, but not uncommon for central galaxies in massive
clusters, rising from S_N~5 near the center to ~12 at large radii. Passive
galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass
profiles of the GCs, stars, intracluster gas, and lensing-derived total mass,
and we compare the mass fractions as a function of radius. The estimated mass
in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar
mass of the Milky Way. The shape of the GC mass profile appears intermediate
between those of the stellar light and total cluster mass. Despite the extreme
nature of this system, the ratios of the GC mass to the baryonic and total
masses, and thus the GC formation efficiency, are typical of those in other
rich clusters when comparing at the same physical radii. The GC formation
efficiency is not constant, but varies with radius, in a manner that appears
similar for different clusters; we speculate on the reasons for this similarity
in profile.Comment: 13 pages, 11 figures; accepted for publication in Ap
On the globular cluster formation history of NGC 5128
We deduce the globular cluster formation history of the nearby elliptical NGC
5128 by using a phenomenological chemical enrichment model to reproduce its
observed metallicity distribution function (MDF). We derive the observed MDF
using recently obtained U and B photometry of the NGC 5128 GC system, with
(U-B) used as the metallicity indicator. We test whether the observed MDF can
be reproduced using a single starburst at high redshift followed by passive
evolution (monolithic scenario). Our results strongly indicate that this
scenario is incapable of reproducing the observed MDF of the NGC 5128 GC
system. At least one more major starburst is required. A persistent feature of
our best-fit models is a strong GC formation episode in the first 1-2 Gyrs,
followed by a second formation episode typically after 10-12 Gyrs. Roughly
one-third of the stellar mass in the GC system is created in this late
starburst, producing clusters that are potentially less than 1 Gyr old.
Although we do not have adequate resolution in the model or in the observed MDF
to make a reliable claim about the possibility of additional GC formation
episodes, our results suggest that the GCs in this galaxy did not form solely
in a monolithic collapse and seem to support observational evidence which
indicates that NGC 5128 is a merger remnant.Comment: replaced with accepted versio
The Mass-Radius Relationship for Very Low Mass Stars: Four New Discoveries from the HATSouth Survey
We report the discovery of four transiting F-M binary systems with companions
between 0.1-0.2 Msun in mass by the HATSouth survey. These systems have been
characterised via a global analysis of the HATSouth discovery data, combined
with high-resolution radial velocities and accurate transit photometry
observations. We determined the masses and radii of the component stars using a
combination of two methods: isochrone fitting of spectroscopic primary star
parameters, and equating spectroscopic primary star rotation velocity with
spin-orbit synchronisation. These new very low mass companions are HATS550-016B
(0.110 -0.006/+0.005 Msun, 0.147 -0.004/+0.003 Rsun), HATS551-019B (0.17
-0.01/+0.01 Msun, 0.18 -0.01/+0.01 Rsun), HATS551-021B (0.132 -0.005/+0.014
Msun, 0.154 -0.008/+0.006 Rsun), HATS553-001B (0.20 -0.02/+0.01 Msun, 0.22
-0.01/+0.01 Rsun). We examine our sample in the context of the radius anomaly
for fully-convective low mass stars. Combining our sample with the 13 other
well-studied very low mass stars, we find a tentative 5% systematic deviation
between the measured radii and theoretical isochrone models.Comment: 17 pages, 8 figures, accepted for publication in MNRA
Early-type galaxies at large galactocentric radii - I. Stellar kinematics and photometric properties
We present the results of a combined analysis of the kinematic and
photometric properties at large galactocentric radii of a sample of 14
low-luminosity early-type galaxies in the Fornax and Virgo clusters. From
Gemini South GMOS long-slit spectroscopic data we measure radial profiles of
the kinematic parameters v_{rot}, sigma, h_{3}, and h_{4} out to ~ 1 - 3
effective radii. Multi-band imaging data from the HST/ACS are employed to
evaluate surface brightness profiles and isophotal shape parameters of
ellipticity, position angle and discyness/boxiness. The galaxies are found to
host a cold and old stellar component which extend to the largest observed
radii and that is the dominant source of their dynamical support. The
prevalence of discy-shaped isophotes and the radial variation of their
ellipticity are signatures of a gradual gas dissipation. An early star-forming
collapse appears to be the main mechanism acting in the formation of these
objects. Major mergers are unlikely to have occurred in these galaxies. We can
not rule out a minor merging origin for these galaxies, but a comparison of our
results with model predictions of different merger categories places some
constraints on the possible merger progenitors. These merger events are
required to happen at high-redshift (i.e., z > 1), between progenitors of
different mass ratio (at least 3:1) and containing a significant amount of gas
(i.e., > 10 percent). A further scenario is that the low-luminosity galaxies
were originally late-type galaxies, whose star formation has been truncated by
removal of gas and subsequently the disc has been dynamically heated by high
speed encounters in the cluster environment.Comment: 19 pages, 16 figures (Contact author for high resolution version of
figures 1,2,3), MNRAS, accepted. The second paper of the series "Early-type
galaxies at large galactocentric radii - II. Metallicity gradients and the
[Z/H]-mass, [alpha/Fe] mass relations" can be found at arXiv:1006.169
- …