57 research outputs found
On the physical association of the peculiar emission: Line stars HD 122669 and HD 122691
Spectroscopic and photometric observations indicate a physical association between the peculiar early-type emission-line stars HD 122669 and HD 122691. The latter has undergone a drastic change in the strength of its emission lines during the past twenty years. There is some indication that both stars vary with shorter time scales
On the usefulness of finding charts Or the runaway carbon stars of the Blanco & McCarthy field 37
We have been recently faced with the problem of cross--identifying stars
recorded in historical catalogues with those extracted from recent fully
digitized surveys (such as DENIS and 2MASS). Positions mentioned in the old
catalogues are frequently of poor precision, but are generally accompanied by
finding charts where the interesting objects are flagged. Those finding charts
are sometimes our only link with the accumulated knowledge of past literature.
While checking the identification of some of these objects in several
catalogues, we had the surprise to discover a number of discrepancies in recent
works.The main reason for these discrepancies was generally the blind
application of the smallest difference in position as the criterion to identify
sources from one historical catalogue to those in more recent surveys. In this
paper we give examples of such misidentifications, and show how we were able to
find and correct them.We present modern procedures to discover and solve
cross--identification problems, such as loading digitized images of the sky
through the Aladin service at CDS, and overlaying entries from historical
catalogues and modern surveys. We conclude that the use of good finding charts
still remains the ultimate (though time--consuming) tool to ascertain
cross--identifications in difficult cases.Comment: 4 pages, 1 figure, accepted by A&
The Three-Dimensional Circumstellar Environment of SN 1987A
We present the detailed construction and analysis of the most complete map to
date of the circumstellar environment around SN 1987A, using ground and
space-based imaging from the past 16 years. PSF-matched difference-imaging
analyses of data from 1988 through 1997 reveal material between 1 and 28 ly
from the SN. Careful analyses allows the reconstruction of the probable
circumstellar environment, revealing a richly-structured bipolar nebula. An
outer, double-lobed ``Peanut,'' which is believed to be the contact
discontinuity between red supergiant and main sequence winds, is a prolate
shell extending 28 ly along the poles and 11 ly near the equator. Napoleon's
Hat, previously believed to be an independent structure, is the waist of this
Peanut, which is pinched to a radius of 6 ly. Interior to this is a cylindrical
hourglass, 1 ly in radius and 4 ly long, which connects to the Peanut by a
thick equatorial disk. The nebulae are inclined 41\degr south and 8\degr east
of the line of sight, slightly elliptical in cross section, and marginally
offset west of the SN. From the hourglass to the large, bipolar lobes, echo
fluxes suggest that the gas density drops from 1--3 cm^{-3} to >0.03 cm^{-3},
while the maximum dust-grain size increases from ~0.2 micron to 2 micron, and
the Si:C dust ratio decreases. The nebulae have a total mass of ~1.7 Msun. The
geometry of the three rings is studied, suggesting the northern and southern
rings are located 1.3 and 1.0 ly from the SN, while the equatorial ring is
elliptical (b/a < 0.98), and spatially offset in the same direction as the
hourglass.Comment: Accepted for publication in the ApJ Supplements. 38 pages in
apjemulate format, with 52 figure
NGC 346 in The Small Magellanic Cloud. IV. Triggered Star Formation in the HII Region N66
Stellar feedback, expanding HII regions, wind-blown bubbles, and supernovae
are thought to be important triggering mechanisms of star formation. Stellar
associations, being hosts of significant numbers of early-type stars, are the
loci where these mechanisms act. In this part of our photometric study of the
star-forming region NGC346/N66 in the Small Magellanic Cloud, we present
evidence based on previous and recent detailed studies, that it hosts at least
two different events of triggered star formation and we reveal the complexity
of its recent star formation history. In our earlier studies of this region
(Papers I, III) we find that besides the central part of N66, where the bright
OB stellar content of the association NGC346 is concentrated, an arc-like
nebular feature, north of the association, hosts recent star formation. This
feature is characterized by a high concentration of emission-line stars and
Young Stellar Objects, as well as embedded sources seen as IR-emission peaks
that coincide with young compact clusters of low-mass pre-main sequence stars.
All these objects indicate that the northern arc of N66 encompasses the most
current star formation event in the region. We present evidence that this star
formation is the product of a different mechanism than that in the general area
of the association, and that it is triggered by a wind-driven expanding HII
region (or bubble) blown by a massive supernova progenitor, and possibly other
bright stars, a few Myr ago. We propose a scenario according to which this
mechanism triggered star formation away from the bar of N66, while in the bar
of N66 star formation is introduced by the photo-ionizing OB stars of the
association itself.Comment: Astrophysical Journal, In Press. 10 pages, 4 figures, emulateapj
LaTeX style. Figures with Scaled-down resolution. Related Press Releases:
http://www.nasa.gov/mission_pages/spitzer/news/spitzer-20081008.html and
http://www.eso.org/public/outreach/press-rel/pr-2008/pr-34-08.htm
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
The Evolution of Massive Stars. I. Red Supergiants in the Magellanic Clouds
We investigate the red supergiant (RSG) content of the SMC and LMC using
multi-object spectroscopy on a sample of red stars previously identified by
{\it BVR} CCD photometry. We obtained high accuracy ( km s) radial
velocities for 118 red stars seen towards the SMC and 167 red stars seen
towards the LMC, confirming most of these (89% and 95%, respectively) as red
supergiants (RSGs). Spectral types were also determined for most of these RSGs.
We find that the distribution of spectral types is skewed towards earlier type
at lower metallicities: the average (median) spectral type is K5-7 I in the
SMC, M1 I in the LMC, and M2 I in the Milky Way. We argue that RSGs in the
Magellanic Clouds are 100deg (LMC) and 300deg (SMC) cooler than Galactic RSGs
of the same spectral type. We compare the distribution of RSGs in the H-R
diagram to that of various stellar evolutionary models; we find that none of
the models produce RSGs as cool and luminous as what is actually observed. In
all of our H-R diagrams, however, there is an elegant sequence of decreasing
effective temperatures with increasing luminosities; explaining this will be an
important test of future stellar evolutionary models.Comment: Version with eps figures embedded can be obtained from
ftp://ftp.lowell.edu/pub/massey/rsgs.ps.gz Accepted by the Astronomical
Journa
A New View of the Circumstellar Environment of SN 1987A
We summarize the analysis of a uniform set of both previously-known and
newly-discovered scattered-light echoes, detected within 30" of SN 1987A in ten
years of optical imaging, and with which we have constructed the most complete
three-dimensional model of the progenitor's circumstellar environment.
Surrounding the SN is a richly-structured bipolar nebula. An outer,
double-lobed ``peanut,'' which we believe is the contact discontinuity between
the red supergiant and main sequence winds, is a prolate shell extending 28 ly
along the poles and 11 ly near the equator. Napoleon's Hat, previously believed
to be an independent structure, is the waist of this peanut, which is pinched
to a radius of 6 ly. Interior, the innermost circumstellar material lies along
a cylindrical hourglass, 1 ly in radius and 4 ly long, which connects to the
peanut by a thick equatorial disk. The nebulae are inclined 41o south and 8o
east of the line of sight, slightly elliptical in cross section, and marginally
offset west of the SN. The 3-D geometry of the three circumstellar rings is
studied, suggesting the equatorial ring is elliptical (b/a<0.98), and spatially
offset in the same direction as the hourglass. Dust-scattering models suggest
that between the hourglass and bipolar lobes: the gas density drops from 1--3
cm^{-3} to >0.03 cm^{-3}; the maximum dust-grain size increases from ~0.2
micron to 2 micron; and the Si:C dust ratio decreases. The nebulae have a total
mass of ~1.7 Msun, yielding a red-supergiant mass loss around 5*10^{-6} Msun
yr^{-1}.Comment: Accepted for publication in ApJ 2/14/05. 16 pages in emualteapj
forma
A UBVR CCD Survey of the Magellanic Clouds
We present photometry and a preliminary interpretation of a UBVR survey of
the Large and Small Magellanic Clouds. We determine improved values for the
relative number of blue and red supergiants. We also compare the relative
number of Red Supergiants (RSGs) and Wolf-Rayet stars, demonstrating a strong,
tight correlation with metallicity, and reinvestigate the initial mass function
slope of massive stars found in the field.Comment: complete postscript (including embedded figures) can be found at:
ftp://ftp.lowell.edu/pub/massey/mcatlas.ps.gz Accepted for publication in the
ApJ
Broad Balmer Wings in BA Hyper/Supergiants Distorted by Diffuse Interstellar Bands: Five Examples in the 30 Doradus Region from the VLT-FLAMES Tarantula Survey
Extremely broad emission wings at HÎČ and Hα have been found in VLT-FLAMES Tarantula Survey data for five very luminous BA supergiants in or near 30 Doradus in the Large Magellanic Cloud. The profiles of both lines are extremely asymmetrical, which we have found to be caused by very broad diffuse interstellar bands (DIBs) in the longward wing of HÎČ and the shortward wing of Hα. These DIBs are well known to interstellar but not to many stellar specialists, so that the asymmetries may be mistaken for intrinsic features. The broad emission wings are generally ascribed to electron scattering, although we note difficulties for that interpretation in some objects. Such profiles are known in some Galactic hyper/supergiants and are also seen in both active and quiescent Luminous Blue Variables (LBVs). No prior or current LBV activity is known in these 30 Dor stars, although a generic relationship to LBVs is not excluded; subject to further observational and theoretical investigation, it is possible that these very luminous supergiants are approaching the LBV stage for the first time. Their locations in the HRD and presumed evolutionary tracks are consistent with that possibility. The available evidence for spectroscopic variations of these objects is reviewed, while recent photometric monitoring does not reveal variability. A search for circumstellar nebulae has been conducted, with an indeterminate result for one of them
Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud
We present a catalog of 1750 massive stars in the Large Magellanic Cloud,
with accurate spectral types compiled from the literature, and a photometric
catalog for a subset of 1268 of these stars, with the goal of exploring their
infrared properties. The photometric catalog consists of stars with infrared
counterparts in the Spitzer SAGE survey database, for which we present uniform
photometry from 0.3-24 microns in the UBVIJHKs+IRAC+MIPS24 bands. The resulting
infrared color-magnitude diagrams illustrate that the supergiant B[e], red
supergiant and luminous blue variable (LBV) stars are among the brightest
infrared point sources in the Large Magellanic Cloud, due to their intrinsic
brightness, and at longer wavelengths, due to dust. We detect infrared excesses
due to free-free emission among ~900 OB stars, which correlate with luminosity
class. We confirm the presence of dust around 10 supergiant B[e] stars, finding
the shape of their spectral energy distributions (SEDs) to be very similar, in
contrast to the variety of SED shapes among the spectrally variable LBVs. The
similar luminosities of B[e] supergiants (log L/Lo>=4) and the rare, dusty
progenitors of the new class of optical transients (e.g. SN 2008S and NGC 300
OT), plus the fact that dust is present in both types of objects, suggests a
common origin for them. We find the infrared colors for Wolf-Rayet stars to be
independent of spectral type and their SEDs to be flatter than what models
predict. The results of this study provide the first comprehensive roadmap for
interpreting luminous, massive, resolved stellar populations in nearby galaxies
at infrared wavelengths.Comment: 57 pages, 19 figures, 5 tables, accepted for publication in the
Astronomical Journal (v3 corrects typos
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