23rd Issue, August 1998
This HTML document contains all paper abstracts published in the
22nd issue of the Magellanic Clouds Newsletter with full references and
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Contents
Announcements:
- Radio survey of X-ray sources available at CDS
Refereed Papers:
- de Freitas Pacheco: Evolutionary Models for the Magellanic Clouds:
I. The Large Cloud
The Astronomical Journal, accepted
- Maragoudaki et al.: The LMC Stellar Complexes in Luminosity Slices.
Star formation indicators
Astronomy and Astrophysics (Letters), accepted
- de Koter et al.: An Empirical Isochrone of Very Massive Stars in R136A
The Astrophysical Journal, accepted
- Bauer et al.: A slope variation in the Period-Luminosity relation for
short period SMC Cepheids
Astronomy & Astrophysics, accepted
- Udalski: Optical Gravitational Lensing Experiment. Population Effects
on the Mean Brightness of the Red Clump Stars
Acta Astronomica, submitted
- Larwood: On the precession of accretion discs in X-ray binaries
Monthly Notices of The Royal Astronomical Society, accepted
- Afonso et al.: EROS 2 intensive observation of the caustic crossing
of microlensing event MACHO SMC-98-1
Astronomy and Astrophysics, submitted
- Albrow et al.: The Relative Lens-Source Proper Motion in MACHO 98-SMC-1
Astrophysical Journal Letters, submitted
- Alcock et al.: Discovery and Characterization of a Caustic Crossing
Microlensing Event in the SMC
The Astrophysical Journal, submitted
Conference Proceedings:
- Oey: Superbubbles in the Magellanic Clouds
Review paper to appear in New Views of the Magellanic Clouds, IAU Symp.
190 (July 12-17, 1998), eds. Y.-H. Chu, N. Suntzeff, J. Hesser, & D.
Bohlender.
Announcements
Radio survey of X-ray sources available at CDS
FITS files of all the 28 deep radio images of fields in the Magellanic
Clouds, from Fender, Southwell & Tzioumis `A radio survey of supersoft,
transient and persistent X-ray sources in the Magellanic clouds', MNRAS,
1998, 298, 692 are available for downloading at
ftp://cdsarc.u-strasbg.fr/pub/cats/J/MNRAS/298/692/
These include images of the SNR N49 (possibly associated with SGR 0525-66.
Robert P. Fender, Astronomical Institute `Anton Pannekoek'
Center for High-Energy Astrophysics, University of Amsterdam
The Netherlands
Refereed Papers
Evolutionary Models for the Magellanic Clouds:
I. The Large Cloud
J.A. de Freitas Pacheco
Observatoire de la Côte d'Azur, BP4299,
F-06304, Nice Cedex 4, France
An evolutionary model is developed for the Large Magellanic Cloud, taking
into account constraints in the star formation history imposed by
recent data on color-magnitude diagrams of field stars. The present
model has a ``closed'' geometry, since the onset of a galactic wind
in the early phases of the LMC is questioned. The code followed
the evolution of several primary elements as
O, Mg, Si, S, Ar, Ca and Fe, including the contribution
of both type Ia and type II supernovae to the chemical enrichment.
An initial mass function steeper than the solar neighborhood
seems to be necessary
to produce negative [O/Fe] ratios, but this conclusion depends on
the adopted yields and on the adopted star formation history.
The signature of the past enhanced star
formation period 2-3 Gyr ago, is clearly seen in the behavior of the oxygen
and sulphur evolution, representing a possible observational test for
the proposed model.
Accepted by: The Astronomical Journal
The LMC Stellar Complexes in Luminosity Slices
Star formation indicators
F. Maragoudaki (1,2), M. Kontizas (1), E. Kontizas (2),
A. Dapergolas (2), and D.H. Morgan (3)
(1) Section of Astrophysics, Astronomy & Mechanics,
Dept. of Physics, University of Athens, GR-157 83 Athens, Greece
(2)
Astronomical Institute, National Observatory of Athens, P.O. Box 20048,
GR-118 10 Athens, Greece
(3)
UK Schmidt Telescope Unit, Royal Observatory of Edinburgh, EH9 3HJ, UK
An approach towards the investigation of the star formation mechanisms in
galaxies can be achieved through the search of stellar complexes and the
determination of their properties. A method has been developed for the
detection of stellar complexes and the derivation of their fundamental
properties in the LMC.
Photographic plates taken with the 1.2m U.K. Schmidt Telescope have been
digitized by the APM and SuperCosmos machines to produce homogeneous
data for extended regions. Star counts have been performed for extended areas
in selected luminosity slices and colours (U, HeII, R filters). Isodensity
contours
have been used to identify the various structures with enhanced stellar number
density (3 sigma above the mean background density).
About 50 large stellar groupings have been revealed showing: 1) hierarchical
structure, where the smallest are found within the large ones. 2) their size
distribution has peaks at 250±50pc (aggregates) and 600±50pc
(complexes), there are also a few cases with size 1200± 50pc
(supercomplexes). 3) there is evidence that at the fainter magnitudes these
structures are aligned to a general trend whereas at the most bright end
(B1, O spectral types) they become more clumpy and symmetrical in shape.
The relationship between the above complexes' properties and the various
scenarios of star formation are discussed.
Accepted by: Astronomy and Astrophysics (Letters)
An Empirical Isochrone of Very Massive Stars in R136A
Alex de Koter (1,2), Sara R. Heap (3), and Ivan Hubeny (3,4)
(1) Advanced Computer Concepts, Code 681,
Goddard Space Flight Center, Greenbelt MD 20771, USA
(2) Astronomical Institute `Anton Pannekoek', University
of Amsterdam, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands
(3) Laboratory for Astronomy and Solar Physics, Code 681,
Goddard Space Flight Center, Greenbelt MD 20771, USA
(4) AURA, National Optical Astronomy Observatories,
Code 681, Goddard Space Flight Center, Greenbelt MD 20771, USA
We report on a detailed spectroscopic study of twelve very massive
and luminous stars (M >~ 35 Mo) in the core of the compact cluster
R136a, near the center of the 30 Doradus complex. The three brightest
stars of the cluster, viz: R136a1, R136a2 and R136a3, have been
investigated earlier by de Koter, Heap, & Hubeny (1997). Low-resolution
spectra (< 200 km/s) of the program stars were obtained
with the GHRS
and FOS spectrographs on the Hubble Space Telescope. These
instruments covered the spectral range from 1200 to 1750 Å and from
3200 to 6700 Å respectively.
Fundamental stellar parameters were obtained by fitting the
observations by model spectra calculated with the unified isa-wind
code of de Koter et al. supplemented by synthetic data calculated using
the program tlusty.
We find that the stars are almost exclusively of spectral type O3.
They occupy only a relatively narrow range in
effective temperatures between 40 and 46 kK. The
reason for these similar T_eff's is that the isochrone of
these very massive stars, which we determined to be at ~ 2 Myr,
runs almost vertically in the HR-diagram.
We present a quantitative method of determining the effective
temperature of O3-type stars based on the strength of the
OV 1371 Å line.
Present-day evolutionary calculations by Meynet et al. (1994)
imply that the program stars have initial masses in the range of M_i
~ 37 to 76 Mo. The observed mass-loss rates are up to three
(two) times higher than is assumed in these evolution tracks when
adopting a metallicity Z = 0.004 (0.008) for the LMC.
The high observed mass-loss rates imply that already at an age
of ~ 2 Myr the most luminous of our program stars will have
lost a significant fraction of their respective initial masses.
For the least luminous stars investigated in this paper, the
observed mass loss agrees with the prediction by the
theory of radiation-driven winds (Kudritzki et al. 1989). However,
for increasing luminosity the observed mass loss becomes larger,
reaching up to three to four times what is expected from the
theory. Such an increasing discrepancy fits in with the results
of de Koter et al. (1997) where an observed over predicted mass
loss ratio of up to eight was reported for the brightest members
of the R136a cluster, for which M_i ~ 100 Mo
was found.
The failure of the theory is also present when one compares
observed over predicted wind momentum as a function of wind
performance number. This strongly indicates that the shortcoming
of the present state of the theory is connected to the neglect
of effects of multiple photon momentum transfer.
Accepted by: The Astrophysical Journal
For preprints, contact:
dekoter@astro.uva.nl
or by anonymous ftp at
helios.astro.uva.nl, cd
pub/alex/r136aempiso.ps
A slope variation in the Period-Luminosity relation
for short period SMC Cepheids
F. Bauer (1,2), C. Afonso (2), J.N. Albert (3),
J. Andersen (4), R. Ansari (3),
E. Aubourg (2), P. Bareyre (1,2), J.P. Beaulieu (5),
A. Bouquet (1), S. Char (6),
X. Charlot (2), F. Couchot (3), C. Coutures (2), F. Derue (3),
R. Ferlet (5),
C. Gaucherel (2), J.F. Glicenstein (2), B. Goldman (2,7,8),
A. Gould (9), D. Graff (2,10),
M. Gros (2), J. Haissinski (3), J.C. Hamilton (1),
D. Hardin (2), J. de Kat (2),
T. Lasserre (2), E. Lesquoy (2), C. Loup (5),
C. Magneville (2), B. Mansoux (3),
J.B. Marquette (5), E. Maurice (11), A. Milsztajn (2),
M. Moniez (3),
N. Palanque-Delabrouille (2), O. Perdereau (3),
L. Prevot (11), C. Renault (2),
N. Regnault (3), J. Rich (2), M. Spiro (2),
A. Vidal-Madjar (5), L. Vigroux (2), S. Zylberajch (2)
The EROS collaboration
(1) Collège de France, PCC, IN2P3 CNRS, 11 place Marcelin Berthelot,
75231 Paris Cedex, France
(2) CEA, DSM, DAPNIA, Centre d'Études de Saclay, 91191
Gif-sur-Yvette, Cedex, France
(3) Laboratoire de l'Accélérateur Linéaire,
IN2P3 CNRS et Université Paris-Sud, BP~34 91898 Orsay Cedex, France
(4) Astronomical Observatory, Copenhagen University, Juliane Maries Vej 30,
2100 Copenhagen, Denmark
(5) Institut d'Astrophysique de Paris, INSU CNRS, 98 bis Boulevard Arago,
75014 Paris, France
(6) Universidad de la Serena, Facultad de Ciencias, Departamento de Fisica,
Casilla 554, La Serena, Chile
(7) Dept. Astronomía, Universidad de Chile, Casilla 36-D, Santiago,
Chile
(8) European Southern Observatory, Casilla 19001, Santiago 19, Chile
(9) Department of Astronomy, Ohio State University, Columbus, OH 43210,
USA
(10) Physics Department, Ohio State University, Columbus, OH 43210, USA
(11) Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex
04, France
We present the Period-Luminosity relations from 290 Cepheids towards the LMC
and 590 Cepheids towards the SMC. The two data sets were obtained
using the two wide field CCD cameras of the EROS 2 microlensing survey.
We observe a significant slope change of the period-luminosity relation for
the SMC fundamental mode Cepheids with periods shorter than 2 days.
Many possible experimental biases have been investigated, but none
can account for this effect.
We discuss three possible explanations of this slope change.
Submitted to: Astronomy & Astrophysics
Optical Gravitational Lensing Experiment.
Population Effects on the Mean Brightness
of the Red Clump Stars
A. Udalski
Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa,
Poland
We present an empirical test indicating that the mean I-band
magnitude of the red clump stars, used as the standard candle in the
recent distance determinations to the Magellanic Clouds and other
objects, is age independent for intermediate age (2-10 Gyr) stars.
Fifteen star clusters of age ~ 1.5-12 Gyr from the LMC and SMC
(ESO121SC03, SL663, NGC 2155, NGC 2121, SL388, SL862, NGC 121, L1,
Kron3, NGC 416, L113, NGC 339, L11, NGC 419, NGC 411) were observed and
their color-magnitude diagrams are presented. The mean I-band
brightness of the red clump in these clusters is constant and its mean
extinction-free magnitude is: I_0=17.88±0.05 mag and I_0=18.31±
0.07 mag at the mean metallicity of -0.8 dex and -1.2 dex for the
LMC and SMC clusters, respectively. For older objects (> 10 Gyr) the
brightness of the red clump, which converts into the red part of the
horizontal branch, fades by about 0.3-0.4 mag, setting an important
limitation on the red clump stars method of distance determination.
The red clump distance moduli to the Magellanic Clouds from the new
independent data set are: m-M=18.18±0.06 mag and m-M=18.65±0.08
mag for the LMC and SMC, respectively, in very good agreement with
previous determinations.
Weak dependence of the mean I-band brightness of the red clump on
metallicity and its independence of age for intermediate age population
(2-10 Gyr) of stars as well as the most precise calibration as compared
to other standard candle candidates makes the red clump stars method one
of the most accurate steps in the distance scale ladder.
Submitted to: Acta Astronomica
On the precession of accretion discs in X-ray binaries
J. Larwood (1,2)
(1) Isaac Newton Institute for Mathematical Sciences,
20 Clarkson Road, Cambridge CB3 0EH. United Kingdom
(2) Astronomy Unit, School of Math. Sciences,
Queen Mary & Westfield College, Mile End Road, London E1 4NS, UK
In this letter recent results on the nodal precession of accretion discs in
close binaries are applied to the discs in some X-ray binary systems. The ratio
between the tidally forced precession period and the binary orbital period is
given, as well as the condition required for the rigid precession of gaseous
Keplerian discs. Hence the minimum precessional period that may be supported by
a fluid Keplerian disc is determined. It is concluded that near rigid body
precession of tilted accretion discs can occur and generally reproduce
observationally inferred precession periods, for reasonable system parameters.
In particular long periods in SS433, Her X-1, LMC X-4 and SMC X-1 can be fit by
the tidal model. It is also found that the precession period that has been
tentatively put forward for Cyg X-2 cannot be accommodated by a tidally
precessing disc model for any realistic choice of system parameters.
Accepted by: Monthly Notices of The Royal Astronomical Society
EROS 2 intensive observation of the caustic crossing
of microlensing event MACHO SMC-98-1
Afonso et al. (EROS collaboration)
We report on intensive photometric monitoring on 18 June 1998
of MACHO SMC-98-1, a binary-lens microlensing event seen toward the
Small Magellanic Cloud (SMC). The observations cover 5.3 hours (UT
5:17 - 10:37), and show a sharp drop of 1.8 mag during the first 1.8
hours, followed by an abrupt flattening at UT 7:08 ± 0:02. We
interpret the kink at 7:08 as the end of the second caustic crossing
(when the source first moved completely outside the caustic). These
results indicate that µ sin phi >~ 1.5 km/s/kpc at the
2 sigma level, where µ is
the proper motion of the lens (relative to the line of sight to the source),
and phi is the unknown (and so random) angle of the caustic crossing.
Hence, the lens probably does not lie in either the Galactic halo
or disk and so is most likely in the SMC itself. Our data can be combined
with those of other groups to give more precise constraints on the proper
motion (and hence the nature) of the lens.
Submitted to: Astronomy and Astrophysics
The Relative Lens-Source Proper Motion in MACHO 98-SMC-1
M.D. Albrow (1),
J.-P. Beaulieu (2),
J.A.R. Caldwell (3),
D.L. DePoy (4),
M. Dominik (2),
B.S. Gaudi (4),
A. Gould (4),
J. Greenhill (5),
K. Hill (5),
S. Kane (5,6),
R. Martin (7),
J. Menzies (3),
R.M. Naber (2),
J.-W. Pel (2),
K.R. Pollard (1),
P.D. Sackett (2),
K.C. Sahu (6),
P. Vermaak (3),
R. Watson (5),
A. Williams (7),
(The PLANET Collaboration)
and
R.W. Pogge (4)
(1) Univ. of Canterbury, Dept. of Physics & Astronomy,
Private Bag 4800, Christchurch, New Zealand
(2) Kapteyn Astronomical Institute, Postbus 800,
9700 AV Groningen, The Netherlands
(3) South African Astronomical Observatory, P.O. Box 9,
Observatory 7935, South Africa
(4) Ohio State University, Department of Astronomy, Columbus,
OH 43210, USA.
(5) Univ. of Tasmania, Physics Dept., G.P.O. 252C,
Hobart, Tasmania 7001, Australia
(6) Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD. 21218, USA
(7) Perth Observatory, Walnut Road, Bickley, Perth 6076, Australia
We present photometric and spectroscopic data for the second microlensing
event seen toward the Small Magellanic Cloud (SMC), MACHO-98-SMC-001.
The lens is a binary. We resolve the
caustic crossing and find that the source took 2 Delta t = 8.5 hours to
transit the caustic. We measure the
source temperature T_eff=8000 K both spectroscopically and from the
color (V-I)_0~ 0.22. We find two acceptable binary-lens models.
In the first, the source crosses the caustic at phi=43°.2
and the unmagnified source magnitude is I_s=22.15.
The angle implies that the lens
crosses the source radius in time t_*=Delta t sin phi = 2.92 hours.
The magnitude (together with the temperature) implies that the angular
radius of the source is theta_* = 0.089 µas. Hence, the proper motion is
µ=theta_*/t_*=1.26 km/s/kpc. For the second solution, the
corresponding parameters are
phi=30°.6, I_s=21.81, t_*=2.15 hours,
theta_* = 0.104 µas, µ=theta_*/t_*=2.00 km/s/kpc.
Both proper-motion estimates are slower than 99.5% of the proper motions
expected for halo lenses. Both are consistent with an ordinary binary lens
moving at ~ 75-120 km/s within the SMC itself.
We conclude that the lens is most likely in the SMC proper.
Submitted to: Astrophysical Journal Letters
Discovery and Characterization
of a Caustic Crossing
Microlensing Event in the SMC
C. Alcock, R.A. Allsman, D. Alves, T.S. Axelrod, A.C. Becker,
D.P. Bennett,
K.H. Cook, A.J. Drake, K.C. Freeman, K. Griest, L.J. King,
M.J. Lehner,
S.L. Marshall, D. Minniti, B.A. Peterson, M.R. Pratt, P.J.
Quinn, S.H. Rhie,
A.W. Rodgers, P.B. Stetson, C.W. Stubbs, W. Sutherland,
A. Tomaney, T. Vandehei
We present photometric observations and analysis of the second
microlensing event detected towards the Small Magellanic Cloud (SMC),
MACHO Alert 98-SMC-1. This event was detected early enough to allow
intensive observation of the lightcurve. These observations revealed
98-SMC-1 to be the first caustic crossing, binary microlensing event
towards the Magellanic Clouds to be discovered in progress.
Frequent coverage of the evolving lightcurve allowed an accurate
prediction for the date of the source crossing out of the lens caustic
structure. The caustic crossing temporal width, along with the
angular size of the source star, measures the proper motion of the
lens with respect to the source, and thus allows an estimate of the
location of the lens. Lenses located in the Galactic halo would have
a velocity projected to the SMC of v ~ 1500 km/s, while an
SMC lens would typically have v ~ 60 km/s. The event
lightcurve allows us to obtain a unique fit to the parameters of the
binary lens, and to estimate the proper motion of the lensing system.
We have performed a joint fit to the MACHO/GMAN data presented here,
including recent EROS data of this event. These
joint data are sufficient to constrain the time tstar for the lens
to move an angle equal to the source angular radius; tstar = 0.116
± 0.010 days. We estimate a radius for the lensed source of
rstar = 1.4 ± 0.1 rsun from its unblended color and magnitude.
This yields a projected velocity of v = 84 ± 9 km/s. Only
0.15 % of halo lenses would be expected to have a v value at
least as small as this, while 31% of SMC lenses would be expected to
have v as large as this. This implies that the lensing system
is more likely to reside in the SMC than in the Galactic halo.
Similar observations of future Magellanic Cloud microlensing events
will help to determine the contribution of Machos to the Galaxy's dark
halo.
Submitted to: The Astrophysical Journal
Conference Proceedings
Superbubbles in the Magellanic Clouds
M. S. Oey (1)
(1) Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, U.K.
Superbubbles that result from the stellar winds and supernovae of
OB associations probably play a fundamental role in
the structure and energetics of the ISM in
star-forming galaxies. Their influence may also dominate the
relationship between the different interstellar gas
phases. How do superbubbles form and evolve? How do they affect the
local and global ISM? The Magellanic Clouds provide a superior
opportunity to study this shell-forming activity, since both
stellar content and gaseous structure can be examined in detail.
Here, the results of recent studies of superbubbles in the Magellanic
Clouds are reviewed.
Review paper to appear in New Views of the Magellanic
Clouds, IAU Symp. 190 (July 12-17, 1998), eds. Y.-H. Chu, N. Suntzeff,
J. Hesser, & D. Bohlender.
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