93 research outputs found
Three New Long Period X-ray Pulsars Discovered in the Small Magellanic Cloud
The Small Magellanic Cloud is increasingly an invaluable laboratory for
studying accreting and isolated X-ray pulsars. We add to the class of compact
SMC objects by reporting the discovery of three new long period X-ray pulsars
detected with the {\it Chandra X-ray Observatory}. The pulsars, with periods of
152, 304 and 565 seconds, all show hard X-ray spectra over the range from 0.6 -
7.5 keV. The source positions of the three pulsars are consistent with known
H-alpha emission sources, indicating they are likely to be Be type X-ray binary
star systems.Comment: Accepted for publication in the Astrophysical Journa
XSHOOTER spectroscopy of the enigmatic planetary nebula Lin49 in the Small Magellanic Cloud
We performed a detailed spectroscopic analysis of the fullerene C60-containing planetary nebula (PN) Lin49 in the Small Magellanic Cloud (SMC) using XSHOOTER at the European Southern Observatory Very Large Telescope and the Spitzer/Infrared Spectrograph instruments. We derived nebular abundances for nine elements. We used TLUSTY to derive photospheric parameters for the central star. Lin49 is C-rich and metal-deficient PN (Z ∼ 0.0006). The nebular abundances are in good agreement with asymptotic giant branch nucleosynthesis models for stars with initial mass 1.25 M⊙ and metallicity Z = 0.001. Using the TLUSTY synthetic spectrum of the central star to define the heating and ionizing source, we constructed the photoionization model with CLOUDY that matches the observed spectral energy distribution (SED) and the line fluxes in the UV to far-IR wavelength ranges simultaneously. We could not fit the ∼1–5 μm SED using a model with 0.005–0.1-μm-sized graphite grains and a constant hydrogen density shell owing to the prominent near-IR excess, while at other wavelengths the model fits the observed values reasonably well. We argue that the near-IR excess might indicate either (1) the presence of very small particles in the form of small carbon clusters, small graphite sheets, or fullerene precursors, or (2) the presence of a high-density structure surrounding the central star. We found that SMC C60 PNe show a near-IR excess component to lesser or greater degree. This suggests that these C60 PNe might maintain a structure nearby their central star
An Investigation of Be/X-ray Pulsars with OGLE-III Data
We have studied five seasons of OGLE-III data for eight SMC Be/X-ray pulsars
for which no other survey data were available. We have determined orbital
periods for four of these binary systems, one of which also shows nonradial
pulsations. Optical identification of SMC X-2 is reconsidered, but no periods
were found for either of the two possible candidates
Space Telescope Imaging Spectrograph slitless observations of Small Magellanic Cloud Planetary Nebulae: a study on morphology, emission line intensity, and evolution
A sample of 27 Planetary Nebulae (PNs) in the Small Magellanic Clouds (SMC)
have been observed with the Hubble Space Telescope Imaging Spectrograph
(HST/STIS) to determine their morphology, size, and the spatial variation of
the ratios of bright emission lines. The morphologies of SMC PNs are similar to
those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs
are asymmetric, compared to half in the LMC. The low metallicity environment of
the SMC seems to discourage the onset of bipolarity in PNs. We measured the
line intensity, average surface brightness (SB), and photometric radius of each
nebula in halpha, hbeta, [O III] lambda4959 and 5007, [NII] 6548 and 6584, [S
II] lambda6716 and 5731, He I 6678, and [OI] 6300 and 6363. We show that the
surface brightness to radius relationship is the same as in LMC PNs, indicating
its possible use as a distance scale indicator for Galactic PNs. We determine
the electron densities and the ionized masses of the nebulae where the [S II]
lines were measured accurately, and we find that the SMC PNs are denser than
the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3
Msun. We also found that the median [O III]/hbeta intensity ratio in the SMC is
about half than the corresponding LMC median. We use Cloudy to model the
dependence of the [O III]/hbeta ratio on the oxygen abundance. Our models
encompass very well the average observed physical quantities. We suggest that
the SMC PNs are principally cooled by the carbon lines, making it hard to study
their excitation based on the optical lines at our disposal.Comment: Accepted for publication in the Astrophysical Journal, 30 pages, 13
figures, 6 tables. For high resolution version of Figs 1 to 6, see
http://archive.stsci.edu/hst/mcpn/home.htm
Spectrophotometry of HII Regions, Diffuse Ionized Gas and Supernova Remnants in M31: The Transition from Photo- to Shock-Ionization
We present results of KPNO 4-m optical spectroscopy of discrete emission-line
nebulae and regions of diffuse ionized gas (DIG) in M31. Long-slit spectra of
16 positions in the NE half of M31 were obtained over a 5-15 kpc range in
radial distance from the center of the galaxy. The spectra have been used to
confirm 16 supernova remnant candidates from the Braun & Walterbos (1993)
catalog. The slits also covered 46 HII regions which show significant
differences among the various morphological types (center-brightened, diffuse,
rings). Radial gradients in emission-line ratios such as [OIII]/H and
[OII]/[OIII] are observed most prominently in the center-brightened HII
regions. These line ratio trends are either much weaker or completely absent in
the diffuse and ring nebulae. The line ratio gradients previously seen in M31
SNRs (Blair, Kirshner, & Chevalier 1981; 1982) are well reproduced by our new
data. The spectra of center-brightened HII regions and SNRs confirm previous
determinations of the radial abundance gradient in M31. We use diagnostic
diagrams which separate photoionized gas from shock-ionized gas to compare the
spectral properties of HII regions, SNRs and DIG. This analysis strengthens
earlier claims (Greenawalt, Walterbos, & Braun 1997) that the DIG in the disk
of M31 is photoionized by a dilute radiation field.Comment: 45 pages, 9 figures, 7 tables, to appear in the Astronomical Journal
(December 1999
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
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
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
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
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