2,045 research outputs found
Observations of Lick Standard Stars Using the SCORPIO Multi-Slit Unit at the SAO 6-m Telescope
We present Lick line-index measurements of standard stars from the list of
Worthey. The spectra were taken with the multi-slit unit of the SCORPIO
spectrograph at the 6-m Special Astrophysical observatory telescope. We
describe in detail our method of analysis and explain the importance of using
the Lick index system for studying extragalactic globular clusters. Our results
show that the calibration of our instrumental system to the standard Lick
system can be performed with high confidence.Comment: 12 pages, 3 figure
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
Toy models of crossed Andreev reflection
We propose toy models of crossed Andreev reflection in multiterminal hybrid
structures containing out-of-equilibrium conductors. We apply the description
to two possible experiments: (i) to a device containing a large quantum dot
inserted in a crossed Andreev reflection circuit. (ii) To a device containing
an Aharonov-Bohm loop inserted in a crossed Andreev reflection circuit.Comment: 5 pages, 9 figures, minor modification
An HST/WFPC2 survey of bright young clusters in M31. I. VdB0, a massive star cluster seen at t â 25 Myr
Aims. We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We obtained shallow (to B ~ 25) photometry of individual stars in 20 candidate clusters. We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case.
Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of VdB0 and the completeness of the final samples was estimated using an extensive set of artificial stars experiments. The reddening, the age and the metallicity of the cluster were estimated by comparing the observed color magnitude diagram (CMD) with theoretical isochrones. Structural parameters were obtained from model-fitting to the intensity profiles measured within circular apertures on the WFPC2 images.
Results. Under the most conservative assumptions, the stellar mass of VdB0 is M> 2.4 x 10^4 M_â , but our best estimates lie in the range â4-9 x 10^4 M_â. The CMD of VdB0 is best reproduced by models having solar metallicity and age â25 Myr. Ages less than â12 Myr and greater than â60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants (âł18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example.
Conclusions. VdB0 is significantly brighter (âł1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ((r_h = 7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31, it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~4 Gyr
The Velocity Function of Galaxies
We present a galaxy circular velocity function, Psi(log v), derived from
existing luminosity functions and luminosity-velocity relations. Such a
velocity function is desirable for several reasons. First, it enables an
objective comparison of luminosity functions obtained in different bands and
for different galaxy morphologies, with a statistical correction for dust
extinction. In addition, the velocity function simplifies comparison of
observations with predictions from high-resolution cosmological N-body
simulations.
We derive velocity functions from five different data sets and find rough
agreement among them, but about a factor of 2 variation in amplitude. These
velocity functions are then compared with N-body simulations of a LCDM model
(corrected for baryonic infall) in order to demonstrate both the utility and
current limitations of this approach. The number density of dark matter halos
and the slope of the velocity function near v_*, the circular velocity
corresponding to an ~L_* spiral galaxy, are found to be comparable to that of
observed galaxies. The primary sources of uncertainty in construction of
Psi(log v) from observations and N-body simulations are discussed and
explanations are suggected to account for these discrepancies.Comment: Latex. 28 pages, 4 figures. Accepted by Ap
LP 349-25: a new tight M8V binary
We present the discovery of a tight M8V binary, with a separation of only 1.2
astronomical units, obtained with the PUEO and NACO adaptive optics systems,
respectively at the CFHT and VLT telescopes. The estimated period of LP 349-25
is approximately 5 years, and this makes it an excellent candidate for a
precise mass measurement.Comment: Accepted by Astronomy and Astrophysics Letter
The Distance to SN 1999em from the Expanding Photosphere Method
We present optical and IR spectroscopy of the first two months of evolution
of the Type II SN 1999em. We combine these data with high-quality optical/IR
photometry beginning only three days after shock breakout, in order to study
the performance of the ``Expanding Photosphere Method'' (EPM) in the
determination of distances. With this purpose we develop a technique to measure
accurate photospheric velocities by cross-correlating observed and model
spectra. The application of this technique to SN 1999em shows that we can reach
an average uncertainty of 11% in velocity from an individual spectrum. Our
analysis shows that EPM is quite robust to the effects of dust. In particular,
the distances derived from the VI filters change by only 7% when the adopted
visual extinction in the host galaxy is varied by 0.45 mag. The superb time
sampling of the BVIZJHK light-curves of SN 1999em permits us to study the
internal consistency of EPM and test the dilution factors computed from
atmosphere models for Type II plateau supernovae. We find that, in the first
week since explosion, the EPM distances are up to 50% lower than the average,
possibly due the presence of circumstellar material. Over the following 65
days, on the other hand, our tests lend strong credence to the atmosphere
models, and confirm previous claims that EPM can produce consistent distances
without having to craft specific models to each supernova. This is particularly
true for the VI filters which yield distances with an internal consistency of
4%. From the whole set of BVIZJHK photometry, we obtain an average distance of
7.5+/-0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of
the internal precision of the method obtained from the analysis of the first 70
days of the supernova evolution.Comment: 68 pages, 15 tables, 22 figures, to appear in Ap
Expected coalescence rates of NS-NS binaries for laser beam interferometers
The coalescence rate of two neutron stars (NS) is revisited. For estimation
of the number of bound NS-NS and the probability of their coalescence in a
timescale , the galactic star formation history, directly derived from
observations, and the evolution of massive stars are considered. The newly
established galactic merging rate is ,
while the local merging rate, including the contribution of elliptical
galaxies, is about a factor of two higher, . Using
the present data basis on galaxy distribution in the local universe and the
expected sensitivity of the first generation of laser beam interferometers, we
estimate that one event should occur every 125 years for LIGO and one event
each 148 years for VIRGO. The situation is considerably improved for
advanced-LIGO since we predict that 6 events per year should be detected
whereas for a recently proposed VIRGO new configuration, the event rate might
increase up to 3 events every two years.Comment: 15 pages, 3 figures accepted for publication in IJMP
A ROSAT HRI survey of bright nearby galaxies
We use the extensive public archive of ROSAT High Resolution Imager (HRI)
observations to carry out a statistical investigation of the X-ray properties
of nearby galaxies. Specifically we focus on the sample of 486 bright (B_T <
12.5) northern galaxies studied by Ho, Filippenko and Sargent (HFS) in the
context of their exploration of the optical spectroscopic properties of nearby
galactic nuclei. Over 20% of HFS galaxies are encompassed in ROSAT HRI fields
of reasonable (> 10ks) exposure. The X-ray sources detected within the optical
extent of each galaxy are categorised as either nuclear or non-nuclear
depending on whether the source is positioned within or outside of a 25
arcsecond radius circle centred on the optical nucleus. A nuclear X-ray source
is detected in over 70% of the galaxies harbouring either a Seyfert or LINER
nucleus compared to a detection rate of only ~40% in less active systems. The
correlation of the H alpha luminosity with nuclear X-ray luminosity previously
observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the
regime of ultra-low luminosity (L(x)~10^38 - 10^40 erg/s) active galactic
nuclei (AGN). The inferred accretion rates for this sample of low-luminosity
AGN are significantly sub-Eddington. In total 142 non-nuclear sources were
detected. In combination with published data for M31 this leads to a luminosity
distribution (normalised to an optical blue luminosity of L(B) = 10^10
L(solar)) for the discrete X-ray source population in spiral galaxies of the
form dN/dL38 = 1.0 +/- 0.2 L38^-1.8, where L38 is the X-ray luminosity in units
of 10^38 erg/s. The implied L(x)/L(B) ratio is ~1.1 x 10^39 erg/s/(10^10
L(solar)). The nature of the substantial number of ``super-luminous''
non-nuclear objects detected in the survey is discussed.Comment: 20 pages, 7 figures, accepted for publication in MNRAS. Also
available from http://www.star.le.ac.uk/~tro/papers/xhfs.p
The rotation curve and mass-distribution in highly flattened galaxies
A new method is developed which permits the reconstruction of the
surface-density distribution in the galactic disk of finite radius from an
arbitrary smooth distribution of the angular velocity via two simple
quadratures. The existence of upper limits for disk's mass and radius during
the analytic continuation of rotation curves into the hidden (non-radiating)
part of the disk is demonstrated.Comment: 13 pages, 2 figure
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