199 research outputs found
Revealing evolved massive stars with Spitzer, WISE and SALT
We present the results of optical spectroscopic observations of 54 candidate
evolved massive stars revealed through the detection of mid-infrared nebulae of
various shapes surrounding them with the {\it Spitzer Space Telescope} and {\it
Wide-field Infrared Survey Explorer}. These observations, carried out with the
Southern African Large Telescope (SALT) in 2010-2015, led to the discovery of
about two dozens emission-line stars, of which 15 stars we classify as
candidate luminous blue variables (cLBVs). Spectroscopic and photometric
monitoring revealed significant changes in the spectra and brightness of four
newly identified cLBVs, meaning that they are new members of the class of bona
fide LBVs. We present an updated list of the Galactic bona fide LBVs.
Currently, this list contains eighteen stars, of which more than 70 per cent
are associated with circumstellar nebulae. We also discovered a very rare [WN]
star - the central star of the planetary nebula Abell 48, and a WN3 star in a
close, eccentric binary system with an O6V star in the Large Magellanic Cloud -
the first-ever extragalactic massive star identified via detection of a
circular shell around it. Most of the remaining targets are tentatively
classified as OB, A and M stars.Comment: 8 pages, 3 figures, in proceedings of "SALT Science" conference
(2015
Discovery of a new bona fide luminous blue variable in Norma
We report the results of optical spectroscopy of the candidate evolved
massive star MN44 revealed via detection of a circular shell with the Spitzer
Space Telescope. First spectra taken in 2009 May--June showed the Balmer lines
in emission as well as numerous emission lines of iron, which is typical of
luminous blue variables (LBVs) near the visual maximum. New observations
carried out in 2015 May--September detected significant changes in the
spectrum, indicating that the star became hotter. We found that these changes
are accompanied by significant brightness variability of MN44. In particular,
the I_c-band brightness decreased by \approx 1.6 mag during the last six years
and after reaching its minimum in 2015 June has started to increase. Using
archival data, we also found that the I_c-band brightness increased by \approx
3 mag in \approx 30 yr preceding our observations. MN44 therefore represents
the seventeenth known example of the Galactic bona fide LBVs. We detected a
nitrogen-rich knot to the northwest of the star, which might represent an
interstellar cloudlet interacting with the circumstellar shell. We discuss a
possible association between MN44 and the INTEGRAL transient source of hard
X-ray emission IGR J16327-4940, implying that MN44 might be either a
colliding-wind binary or a high-mass X-ray binary.Comment: 13 pages, 7 figures, accepted for publication in MNRA
WS1: one more new Galactic bona fide luminous blue variable
In this Letter, we report the results of spectroscopic and photometric
monitoring of the candidate luminous blue variable (LBV) WS1, which was
discovered in 2011 through the detection of a mid-infrared circular shell and
follow-up optical spectroscopy of its central star. Our monitoring showed that
WS1 brightened in the B, V and I bands by more than 1 mag during the last three
years, while its spectrum revealed dramatic changes during the same time
period, indicating that the star became much cooler. The light curve of WS1
demonstrates that the brightness of this star has reached maximum in 2013
December and then starts to decline. These findings unambiguously proved the
LBV nature of WS1 and added one more member to the class of Galactic bona fide
LBVs, bringing their number to sixteen (an updated census of these objects is
provided).Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
MN48: a new Galactic bona fide luminous blue variable revealed by Spitzer and SALT
In this paper, we report the results of spectroscopic and photometric
observations of the candidate evolved massive star MN48 disclosed via detection
of a mid-infrared circular shell around it with the Spitzer Space Telescope.
Follow-up optical spectroscopy of MN48 with the Southern African Large
Telescope (SALT) carried out in 2011--2015 revealed significant changes in the
spectrum of this star, which are typical of luminous blue variables (LBVs). The
LBV status of MN48 was further supported by photometric monitoring which shows
that in 2009--2011 this star has brightened by approx 0.9 and 1 mag in the V
and I_c bands, respectively, then faded by approx 1.1 and 1.6 mag during the
next four years, and apparently started to brighten again recently. The
detected changes in the spectrum and brightness of MN48 make this star the 18th
known Galactic bona fide LBV and increase the percentage of LBVs associated
with circumstellar nebulae to more than 70 per cent. We discuss the possible
birth place of MN48 and suggest that this star might have been ejected either
from a putative star cluster embedded in the HII region IRAS 16455-4531 or the
young massive star cluster Westerlund 1.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Nuclei of dwarf spheroidal galaxies KKs3 and ESO269-66 and their counterparts in our Galaxy
We present the analysis of medium-resolution spectra obtained at the Southern
African Large Telescope (SALT) for nuclear globular clusters (GCs) in two dwarf
spheroidal galaxies (dSphs). The galaxies have similar star formation
histories, but they are situated in completely different environments.
ESO269-66 is a close neighbour of the giant S0 NGC5128. KKs3 is one of the few
truly isolated dSphs within 10 Mpc. We estimate the helium abundance ,
Gyr, dex, and abundances of C, N,
Mg, Ca, Ti, and Cr for the nuclei of ESO269-66 and KKs3. Our surface photometry
results using HST images yield the half-light radius of the cluster in KKs3,
pc. We demonstrate the similarities of medium-resolution
spectra, ages, chemical compositions, and structure for GCs in ESO269-66 and
KKs3 and for several massive Galactic GCs with dex. All
Galactic GCs posses Extended Blue Horizontal Branches and multiple stellar
populations. Five of the selected Galactic objects are iron-complex GCs. Our
results indicate that the sample GCs observed now in different environments had
similar conditions of their formation 1 Gyr after the Big Bang.Comment: 14 pages, 8 figures, 10 tables. Table 1 and two references were
corrected. Accepted for publication in MNRA
Optical spectroscopy of the blue supergiant Sk-69 279 and its circumstellar shell with SALT
We report the results of optical spectroscopy of the blue supergiant Sk-69
279 and its circular shell in the Large Magellanic Cloud (LMC) with the
Southern African Large Telescope (SALT). We classify Sk-69 279 as an O9.2 Iaf
star and analyse its spectrum by using the stellar atmosphere code CMFGEN,
obtaining a stellar temperature of \approx 30 kK, a luminosity of
log(L_*/Lsun)=5.54, a mass-loss rate of log(\dot{M}/(Msun/yr))=-5.26, and a
wind velocity of 800 km/s. We found also that Sk-69 279 possesses an extended
atmosphere with an effective temperature of \approx 24 kK and that its surface
helium and nitrogen abundances are enhanced, respectively, by factors of
\approx 2 and 20--30. This suggests that either Sk-69 279 was initially a
(single) fast-rotating (\ga 400 km/s) star, which only recently evolved off the
main sequence, or that it is a product of close binary evolution. The long-slit
spectroscopy of the shell around Sk-69 279 revealed that its nitrogen abundance
is enhanced by the same factor as the stellar atmosphere, which implies that
the shell is composed mostly of the CNO processed material lost by the star.
Our findings support previous propositions that some massive stars can produce
compact circumstellar shells and, presumably, appear as luminous blue variables
while they are still on the main sequence or have only recently left it.Comment: 15 pages, 12 figures, accepted for publication in MNRA
Discovery of a new Wolf-Rayet star using SAGE-LMC
We report the first-ever discovery of an extragalactic Wolf-Rayet (WR)star
with Spitzer. A new WR star in the Large Magellanic Cloud (LMC) was revealed
via detection of its circumstellar shell using 24 {\mu}m images obtained in the
framework of the Spitzer Survey of the Large Magellanic Cloud (SAGE-LMC).
Subsequent spectroscopic bservations with the Gemini South resolved the central
star in two components, one of which is a WN3b+abs star, while the second one
is a B0V star. We consider the lopsided brightness distribution over the
circumstellar shell as an indication that the WR star is a runaway and use this
interpretation to identify a possible parent cluster of the star.Comment: To be published in the proceedings of the meeting 'Four decades of
research on massive stars' in honor of Tony Moffat, 11-15 July 2011,
Saint-Michel-des-Saints, Quebe
Two circumstellar nebulae discovered with the Wide-field Infrared Survey Explorer and their massive central stars
We report the discovery of two mid-infrared nebulae in the northern
hemisphere with the Wide-field Infrared Survey Explorer and the results of
optical spectroscopy of their central stars, BD+60 2668 (composed of two
components, separated from each other by \approx 3 arcsec) and ALS 19653, with
the Calar Alto 3.5-m telescope and the Southern African Large Telescope (SALT),
respectively. We classify the components of BD+60 2668 as stars of spectral
types B0.5 II and B1.5 III. ALS 19653 is indicated in the SIMBAD data base as a
planetary nebula, while our observations show that it is a massive B0.5 Ib
star, possibly in a binary system. Using the stellar atmosphere code FASTWIND,
we derived fundamental parameters of the three stars as well as their surface
element abundances, implying that all of them are either on the main sequence
or only recently left it. This provides further evidence that massive stars can
produce circumstellar nebulae while they are still relatively unevolved. We
also report the detection of optical counterparts to the mid-infrared nebulae
and a second, more extended optical nebula around ALS 19653, and present the
results of SALT spectroscopy of both nebulae associated with this star. The
possible origin of the nebulae is discussed.Comment: 24 pages, 13 figures, 6 tables, accepted for publication in A
Modelling interstellar structures around Vela X-1
We report the discovery of filamentary structures stretched behind the
bow-shock-producing high-mass X-ray binary Vela X-1 using the SuperCOSMOS
H-alpha Survey and present the results of optical spectroscopy of the bow shock
carried out with the Southern African Large Telescope (SALT). The geometry of
the detected structures suggests that Vela X-1 has encountered a wedge-like
layer of enhanced density on its way and that the shocked material of the layer
partially outlines a wake downstream of Vela X-1. To substantiate this
suggestion, we carried out 3D magnetohydrodynamic simulations of interaction
between Vela X-1 and the layer for three limiting cases. Namely, we run
simulations in which (i) the stellar wind and the interstellar medium (ISM)
were treated as pure hydrodynamic flows, (ii) a homogeneous magnetic field was
added to the ISM, while the stellar wind was assumed to be unmagnetized, and
(iii) the stellar wind was assumed to possess a helical magnetic field, while
there was no magnetic field in the ISM. We found that although the first two
simulations can provide a rough agreement with the observations, only the third
one allowed us to reproduce not only the wake behind Vela X-1, but also the
general geometry of the bow shock ahead of it.Comment: 12 pages, 7 figures 7, accepted for publication in MNRA
A massive white-dwarf merger product before final collapse
Gravitational wave emission can lead to the coalescence of close pairs of
compact objects orbiting each other. For the case of neutron stars such mergers
may yield masses above the Tolman-Oppenheimer-Volkoff limit, leading to the
formation of black holes. For the case of white dwarfs the merger product may
exceed the Chandrasekhar limit, leading either to a thermonuclear explosion as
Type Ia supernova, or to a collapse forming a neutron star. If a Type Ia
supernova explosion is avoided, the merger of two massive white dwarfs is
expected to form a hydrogen- and helium-free circumstellar nebula with a hot
and luminous, rapidly rotating and highly magnetized central star for several
10,000 yr before its final collapse. Here we report the discovery of a hot star
with an emission line dominated spectrum in the centre of a circular
mid-infrared nebula. Both the star and the nebula appear to be free of hydrogen
and helium. Our tailored stellar atmosphere and wind models indicate a stellar
surface temperature of about 200,000 K, and a record outflow velocity of 16,000
km/s. This extreme velocity, together with the derived mass outflow rate, imply
rapid stellar rotation and a strong magnetic field aiding the wind
acceleration. The Gaia distance of the star leads to a luminosity of 10^{4.5}
Lsun, which matches models of the post-merger evolution of super-Chandrasekhar
mass white dwarfs. The high stellar temperature and the nebular size argue for
a short remaining lifetime of the star, which will produce a bright optical and
high-energy transient upon collapse. Our observations indicate that
super-Chandrasekhar mass white dwarf mergers can indeed avoid a thermonuclear
explosion as Type Ia supernova, and provide empirical evidence for magnetic
field generation in stellar mergers.Comment: 14 pages, 3 figures, original version of the manuscript submitted to
Nature; accepted version is published at
https://www.nature.com/articles/s41586-019-1216-1 and is accessible at
https://rdcu.be/bDo6
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