372 research outputs found
Spectroscopy of the bright optical counterparts of X-ray sources in the direction of M 31. II
A recent survey of the Local Group spiral galaxy M 31 with XMM-Newton yielded
a large number of X-ray sources. This is the second in a series of papers with
the aim of identifying the optical counterparts of these X-ray sources. We have
obtained optical spectra for 21 bright optical counterparts of 20 X-ray sources
in the direction of M 31, using the 1.3-m Skinakas telescope in Crete, Greece.
For 17 of the 20 X-ray sources, we have identified the optical counterpart as a
normal late type star (of type F or later) in the foreground (i.e. in the Milky
Way). For two more sources there were two possible optical counterparts in each
case, while two more objects have X-ray properties that are not compatible with
the spectral characteristics of late type non-flaring stars.Comment: Accepted for publication by Astronomy & Astrophysics (7 pages, 8
figures, and 2 tables
Ca II Triplet Spectroscopy of Giants in SMC Star Clusters: Abundances, Velocities and the Age-Metallicity Relation
We have obtained spectra at the Ca II triplet of individual red giants in
seven SMC star clusters whose ages range from ~4 to 12 Gyr. The spectra have
been used to determine mean abundances for six of the star clusters to a
typical precision of 0.12 dex. When combined with existing data for other
objects, the resulting SMC age-metallicity relation is generally consistent
with that for a simple model of chemical evolution, scaled to the present-day
SMC mean abundance and gas mass fraction. Two of the clusters (Lindsay 113 and
NGC 339), however, have abundances that ~0.5 dex lower than that expected from
the mean age-metallicity relation. It is suggested that the formation of these
clusters, which have ages of ~5 Gyr, may have involved the infall of uneriched
gas, perhaps from the Magellanic Stream. The spectra also yield radial
velocities for the seven clusters. The resulting velocity dispersion is 16 +/-
4 km/sec, consistent with those of the SMC planetary nebula and carbon star
populations.Comment: 28 pages including 4 figure
The Low- and Intermediate-Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae
We present a study on the central stars (CSs) of Planetary Nebulae (PNe)
observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging
Spectrograph instrument on-board the HST. The stellar magnitudes have been
measured using broad-band photometry, and Zanstra analysis of the nebulae
provided the stellar temperatures. From the location of the CSs on the HR
diagram, and by comparing the observed CSs with current models of stellar
evolution, we infer the CSs masses. We examine closely the possibility of light
contamination in the bandpass from an unrecognized stellar companion, and we
establish strong constraints on the existence and nature of any binary
companion. We find an average mass of 0.63 Msun, which is similar to the mass
obtained for a sample of CSs in the LMC (0.65 Msun). However, the SMC and LMC
CS mass distributions differ slightly, the SMC sample lacking an
intermediate-mass stellar population (0.65 to 0.75 Msun). We discuss the
significance and possible reasons for the difference between the two mass
distributions. In particular, we consider the differences in the star formation
history between the clouds and the mass-loss rate dependence on metallicity.Comment: 30 pages, 6 figures, 5 tables. To be published in ApJ (October 20
Long-term global distribution of earth's shortwave radiation budget at the top of atmosphere
The mean monthly shortwave (SW) radiation budget at the top of atmosphere (TOA) was computed on 2.5° longitude-latitude resolution for the 14-year period from 1984 to 1997, using a radiative transfer model with long-term climatological data from the International Satellite Cloud Climatology Project (ISCCP-D2) supplemented by data from the National Centers for Environmental Prediction – National Center for Atmospheric Research (NCEP-NCAR) Global Reanalysis project, and other global data bases such as TIROS Operational Vertical Sounder (TOVS) and Global Aerosol Data Set (GADS). The model radiative fluxes at TOA were validated against Earth Radiation Budget Experiment (ERBE) S4 scanner satellite data (1985–1989). The model is able to predict the seasonal and geographical variation of SW TOA fluxes. On a mean annual and global basis, the model is in very good agreement with ERBE, overestimating the outgoing SW radiation at TOA (OSR) by 0.93 Wm<sup>-2</sup> (or by 0.92%), within the ERBE uncertainties. At pixel level, the OSR differences between model and ERBE are mostly within ±10 Wm<sup>-2</sup>, with ±5 Wm<sup>-2</sup> over extended regions, while there exist some geographic areas with differences of up to 40 Wm<sup>-2</sup>, associated with uncertainties in cloud properties and surface albedo. The 14-year average model results give a planetary albedo equal to 29.6% and a TOA OSR flux of 101.2 Wm<sup>-2</sup>. A significant linearly decreasing trend in OSR and planetary albedo was found, equal to 2.3 Wm<sup>-2</sup> and 0.6% (in absolute values), respectively, over the 14-year period (from January 1984 to December 1997), indicating an increasing solar planetary warming. This planetary SW radiative heating occurs in the tropical and sub-tropical areas (20° S–20° N), with clouds being the most likely cause. The computed global mean OSR anomaly ranges within ±4 Wm<sup>-2</sup>, with signals from El Niño and La Niña events or Pinatubo eruption, whereas significant negative OSR anomalies, starting from year 1992, are also detected
Magellanic Cloud Periphery Carbon Stars IV: The SMC
The kinematics of 150 carbon stars observed at moderate dispersion on the
periphery of the Small Magellanic Cloud are compared with the motions of
neutral hydrogen and early type stars in the Inter-Cloud region. The
distribution of radial velocities implies a configuration of these stars as a
sheet inclined at 73+/-4 degrees to the plane of the sky. The near side, to the
South, is dominated by a stellar component; to the North, the far side contains
fewer carbon stars, and is dominated by the neutral gas. The upper velocity
envelope of the stars is closely the same as that of the gas. This
configuration is shown to be consistent with the known extension of the SMC
along the line of sight, and is attributed to a tidally induced disruption of
the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr
ago. The dearth of gas on the near side of the sheet is attributed to ablation
processes akin to those inferred by Weiner & Williams (1996) to collisional
excitation of the leading edges of Magellanic Stream clouds. Comparison with
pre LMC/SMC encounter kinematic data of Hardy, Suntzeff, & Azzopardi (1989) of
carbon stars, with data of stars formed after the encounter, of Maurice et al.
(1989), and Mathewson et al. (a986, 1988) leaves little doubt that forces other
than gravity play a role in the dynamics of the H I.Comment: 30 pages; 7 figures, latex compiled, 1 table; to appear in AJ (June
2000
Recent discoveries of supersoft X-ray sources in M 31
Classical novae (CNe) have recently been reported to represent the major
class of supersoft X-ray sources (SSSs) in the central area of our neighbouring
galaxy M 31. This paper presents a review of results from recent X-ray
observations of M 31 with XMM-Newton and Chandra. We carried out a dedicated
optical and X-ray monitoring program of CNe and SSSs in the central area of M
31. We discovered the first SSSs in M 31 globular clusters (GCs) and their
connection to the very first discovered CN in a M 31 GC. This result may have
an impact on the CN rate in GCs. Furthermore, in our optical and X-ray
monitoring data we discovered the CN M31N 2007-11a, which shows a very short
SSS phase of 29 - 52 days. Short SSS states (durations < 100 days) of CNe
indicate massive white dwarfs (WDs) that are candidate progenitors of
supernovae type Ia. In the case of M31N 2007-11a, the optical and X-ray light
curves suggest a binary containing a WD with M_WD > 1.0 M_sun. Finally, we
present the discovery of the SSS counterpart of the CN M31N 2006-04a. The X-ray
light curve of M31N 2006-04a shows short-time variability, which might indicate
an orbital period of about 2 hours.Comment: 4 pages, 1 figure; Proc. of workshop "Supersoft X-ray Sources - New
Developments", ESAC, May 2009; accepted for publication in Astronomische
Nachrichte
J004457+4123 (Sharov 21): not a remarkable nova in M31 but a background quasar with a spectacular UV flare
We announce the discovery of a quasar behind the disk of M31, which was
previously classified as a remarkable nova in our neighbour galaxy. The paper
is primarily aimed at the outburst of J004457+4123 (Sharov 21), with the first
part focussed on the optical spectroscopy and the improvement in the
photometric database. Both the optical spectrum and the broad band spectral
energy distribution of Sharov 21 are shown to be very similar to that of
normal, radio-quiet type 1 quasars. We present photometric data covering more
than a century and resulting in a long-term light curve that is densely sampled
over the past five decades. The variability of the quasar is characterized by a
ground state with typical fluctuation amplitudes of ~0.2 mag around B~20.5,
superimposed by a singular flare of ~2 yr duration (observer frame) with the
maximum at 1992.81 where the UV flux has increased by a factor of ~20. The
total energy in the flare is at least three orders of magnitudes higher than
the radiated energy of the most luminous supernovae, provided that it comes
from an intrinsic process and the energy is radiated isotropically. The profile
of the flare light curve appears to be in agreement with the standard
predictions for a stellar tidal disruption event where a ~10 M_sun giant star
was shredded in the tidal field of a ~2...5 10^8 M_sun black hole. The short
fallback time derived from the light curve requires an ultra-close encounter
where the pericentre of the stellar orbit is deep within the tidal disruption
radius. Gravitational microlensing provides an alternative explanation, though
the probability of such a high amplification event is very low.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11
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