55 research outputs found
A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We conducted an extensive search for Wolf-Rayet stars (W-Rs) in the SMC,
using the same interference filter imaging techniques that have proved
successful in finding W-Rs in more distant members of the Local Group.
Photometry of some 1.6 million stellar images resulted in some 20 good
candidates, which we then examined spectroscopically. Two of these indeed
proved to be newly found W-Rs, bringing the total known in the SMC from 9 to
11. Other finds included previously unknown Of-type stars (one as early as
O5f?p)),the recovery of the Luminous Blue Variable S18, and the discovery of a
previously unknown SMC symbiotic star. More important, however, is the fact
that there does not exist a significant number of W-Rs waiting to be discovered
in the SMC. The number of W-Rs in the SMC is a factor of 3 lower than in the
LMC (per unit luminosity), and we argue this is the result of the SMC's low
metallicity on the evolution of the most massive stars.Comment: Accepted by Astrophysical Journal. Postscript version available via
ftp.lowell.edu/pub/massey/smcwr.ps.gz Revised version contains slightly
revised spectral types for the Of stars but is otherwise unchange
Discovery of a new Transient X-ray Pulsar in the Small Magellanic Cloud
Rossi X-Ray Timing Explorer observations of the Small Magellanic Cloud have
revealed a previously unknown transient X-ray pulsar with a pulse period of
95s. Provisionally designated XTE SMC95, the pulsar was detected in three
Proportional Counter Array observations during an outburst spanning 4 weeks in
March/April 1999. The pulse profile is double peaked reaching a pulse fraction
\~0.8. The source is proposed as a Be/neutron star system on the basis of its
pulsations, transient nature and characteristically hard X-ray spectrum. The
2-10 keV X-ray luminosity implied by our observations is > 2x10^37 erg/s which
is consistent with that of normal outbursts seen in Galactic systems. This
discovery adds to the emerging picture of the SMC as containing an extremely
dense population of transient high mass X-ray binaries.Comment: Accepted by A&A. 7 pages, 6 figure
Change of the ice rheology with climatic transitions – implication on ice flow modelling and dating of the EPICA Dome C core
International audienceThe study of the distribution of the crystallographic orientations (the fabric) along ice cores supplies information on the past and current ice flows of ice-sheets. Beside the usually observed formation of a vertical single maximum fabric, the EPICA Dome Concordia ice core (EDC) shows an abrupt and unexpected strenghtening of its fabric during termination II around 1750 m depth. Such strengthenings were already observed for sites located on an ice-sheet. This suggests that horizontal shear could occur along the EDC core. Moreover, the change in the fabric leads to a modification of the viscosity between neighbouring ice layers. Through the use of an anisotropic ice flow model, we quantify the change in viscosity and investigate its implication on ice flow and dating
Change in ice rheology during climate variations – implications for ice flow modelling and dating of the EPICA Dome C core
The study of the distribution of crystallographic orientations (i.e., the fabric) along ice cores provides information on past and current ice flow in ice-sheets. Besides the usually observed formation of a vertical single maximum fabric, the EPICA Dome C ice core (EDC) shows an abrupt and unexpected strengthening of its fabric during termination II around 1750 m depth. Such strengthening has already been observed for sites located on an ice-sheet flank. This suggests that horizontal shear could occur along the EDC core. Moreover, the change in the fabric leads to a modification of the effective viscosity between neighbouring ice layers. Through the use of an anisotropic ice flow model, we quantify the change in effective viscosity and investigate its implication for ice flow and dating
The binary period and outburst behaviour of the SMC X-ray binary pulsar system SXP504
A probable binary period has been detected in the optical counterpart to the
X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AXJ0054.8-7244 = SXP504
in the Small Magellanic Cloud. This source was detected by Chandra on 04 Jul
2002 and subsequently observed by XMM-Newton on 18 Dec 2003. The source is
coincident with an Optical Gravitational Lensing (OGLE) object in the
lightcurves of which several optical outburst peaks are visible at ~ 268 day
intervals. Timing analysis shows a period of 268.6 +/- 0.1 days at > 99%
significance. Archival Rossi X-ray Timing Explorer (RXTE) data for the 504s
pulse-period has revealed detections which correspond closely with predicted or
actual peaks in the optical data. The relationship between this orbital period
and the pulse period of 504s is within the normal variance found in the Corbet
diagram.Comment: Accepted by MNRAS. 1 LATEX page. 4 figure
An XMM-Newton view of Planetary Nebulae in the Small Magellanic Cloud. The X-ray luminous central star of SMP SMC 22
During an X-ray survey of the Small Magellanic Cloud, carried out with the
XMM-Newton satellite, we detected significant soft X-ray emission from the
central star of the high-excitation planetary nebula SMP SMC 22. Its very soft
spectrum is well fit with a non local thermodynamical equilibrium model
atmosphere composed of H, He, C, N, and O, with abundances equal to those
inferred from studies of its nebular lines. The derived effective temperature
of 1.5x10^5 K is in good agreement with that found from the optical/UV data.
The unabsorbed flux in the 0.1-0.5 keV range is about 3x10^{-11} erg cm^-2
s^-1, corresponding to a luminosity of 1.2x10^37 erg/s at the distance of 60
kpc. We also searched for X-ray emission from a large number of SMC planetary
nebulae, confirming the previous detection of SMP SMC 25 with a luminosity of
(0.2-6)x10^35 erg/s (0.1-1 keV). For the remaining objects that were not
detected, we derived flux upper limits corresponding to luminosity values from
several tens to hundreds times smaller than that of SMP SMC 22. The
exceptionally high X-ray luminosity of SMP SMC 22 is probably due to the high
mass of its central star, quickly evolving toward the white dwarf's cooling
branch, and to a small intrinsic absorption in the nebula itself.Comment: Accepted for publication on Astronomy and Astrophysic
A Synoptic X-ray Study of M31 with the Chandra-HRC
We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot
images, each covering most of the M31 disk. The data cover a total baseline of
2.5 years and contain a mean effective exposure of 17 ks. We have measured the
mean fluxes and long-term lightcurves for 166 objects detected in these data.
At least 25% of the sources show significant variability. The cumulative
luminosity function (CLF) of the disk sources is well-fit by a power-law with a
slope comparable to those observed in typical elliptical galaxies. The CLF of
the bulge is a broken power law similar to measurements made by previous
surveys. We note several sources in the southwestern disk with L_X > 10^{37}
erg/s . We cross-correlate all of our sources with published optical and radio
catalogs, as well as new optical data, finding counterpart candidates for 55
sources. In addition, 17 sources are likely X-ray transients. We analyze
follow-up HST WFPC2 data of two X-ray transients, finding F336W (U-band
equivalent) counterparts. In both cases, the counterparts are variable. In one
case, the optical counterpart is transient with F336W = 22.3 +/- 0.1 mag. The
X-ray and optical properties of this object are consistent with a ~10 solar
mass black hole X-ray nova with an orbital period of ~20 days. In the other
case, the optical counterpart varies between F336W = 20.82 +/- 0.06 mag and
F336W = 21.11 +/- 0.02 mag. Ground-based and HST observations show this object
is bright (V = 18.8 +/- 0.1) and slightly extended. Finally, the frequency of
bright X-ray transients in the M31 bulge suggests that the ratio of neutron
star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.Comment: 68 pages (27 text), 8 tables, 16 figures, 1 appendix, accepted by
ApJ; accepted version contains reorganized text, new tables and figures, and
updated result
The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wavebands
We present the initial results from the Spitzer Survey of the Small
Magellanic Cloud (S3MC), which imaged the star-forming body of the Small
Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the
F_8/F_24 ratio (an estimate of PAH abundance) has large spatial variations and
takes a wide range of values that are unrelated to metallicity but
anticorrelated with 24 um brightness and F_24/F_70 ratio. This suggests that
photodestruction is primarily responsible for the low abundance of PAHs
observed in star-forming low-metallicity galaxies. We use the S3MC images to
compile a photometric catalog of ~400,000 mid- and far-infrared point sources
in the SMC. The sources detected at the longest wavelengths fall into four main
categories: 1) bright 5.8 um sources with very faint optical counterparts and
very red mid-infrared colors ([5.8]-[8.0]>1.2), which we identify as YSOs. 2)
Bright mid-infrared sources with mildly red colors (0.16<[5.8]-[8.0]<0.6),
identified as carbon stars. 3) Bright mid-infrared sources with neutral colors
and bright optical counterparts, corresponding to oxygen-rich evolved stars.
And, 4) unreddened early B stars (B3 to O9) with a large 24 um excess. This
excess is reminiscent of debris disks, and is detected in only a small fraction
of these stars (<5%). The majority of the brightest infrared point sources in
the SMC fall into groups one to three. We use this photometric information to
produce a catalog of 282 bright YSOs in the SMC with a very low level of
contamination (~7%).Comment: Accepted for publication in The Astrophysical Journal. Given the
draconian figure file-size limits implemented in astro-ph, readers are
encouraged to download the manuscript with full quality images from
http://celestial.berkeley.edu/spitzer/publications/s3mcsurvey.pd
Rifampicin mono-resistant tuberculosis in France: a 2005–2010 retrospective cohort analysis
The XMM-Newton survey of the Small Magellanic Cloud: XMMUJ010633.1-731543 and XMMUJ010743.1-715953, two new Be/X-ray binary systems
In the course of the XMM-Newton survey of the Small Magellanic Cloud (SMC),
two new bright X-ray sources were discovered exhibiting the spectral
characteris- tics of High Mass X-ray Binaries - but revealing only weak
evidence for pulsations in just one of the objects(at 153s in
XMMUJ010743.1-715953). The accurate X- ray source locations permit the
identification of these X-ray source with Be stars, thereby strongly suggesting
these systems are new Be/X-ray binaries. From blue spectra the proposed
classification for XMMUJ010633.1-731543 is B0.5-1Ve and for
XMMUJ010743.1-715953 it is B2IV-Ve.Comment: MNRAS (accepted), 12 pages, 17 figures, 4 table
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