185 research outputs found
Are WC9 Wolf-Rayet stars in colliding-wind binaries?
We present results from a spectroscopic search for massive companions to
dust-making Galactic WC9 stars as a step to testing the paradigm that dust
formation in these systems requires colliding winds to produce over densities.
We find evidence for OB companions to the WC9 stars WR 59 and WR 65, but not WR
121 or WR 117. We identify lines of N III-V and possibly N II in the spectrum
of WR 88, one of the few Galactic WC9 stars which do not make circumstellar
dust, and suggest that WR 88 is a transitional WN-WC9 object and less evolved
than the other WC9 stars. On the other hand, the possible identification of a
strong emission line at 4176A in the spectrum of WR 117 with Ne I suggests that
this star is more evolved than other WC9 stars studied.Comment: 4 pages, 3 figures, contribution to "Massive Stars and High-Energy
Emission in OB Associations"; JENAM 2005, held in Liege (Belgium
The Discovery of a Twelfth Wolf-Rayet Star in the Small Magellanic Cloud
We report the discovery of a relatively faint (V=15.5) early-type WN star in
the SMC. The line strength and width of He II lambda 4686 emission is similar
to that of the other SMC WNs, and the presense of N V lambda 4603,19 emission
(coupled with the lack of N III) suggests this star is of spectral type
WN3-4.5, and thus is similar in type to the other SMC WRs. Also like the other
SMC WN stars, an early-type absorption spectrum is weakly present. The absolute
magnitude is comparable to that of other (single) Galactic early-type WNs. The
star is located in the Hodge 53 OB association, which is also the home of two
other SMC WNs. This star, which we designate SMC-WR12, was actually detected at
a high significance level in an earlier interference-filter survey, but the
wrong star was observed as part of a spectroscopic followup, and this case of
mistaken identity resulted in its Wolf-Rayet nature not being recognized until
now.Comment: Accepted by PASP (November 2003 issue
A very large array 3.6 centimeter continuum survey of galactic wolf-rayet stars
We report the results of a survey of radio continuum emission of Galactic Wolf-Rayet (WR) stars north of δ = -46°. The observations were obtained at 8.46 GHz (3.6 cm) using the Very Large Array, with an angular resolution of ∼6″ × 9″ and typical rms noise of ∼0.04 mJy beam-1. Our survey of 34 WR stars resulted in 15 definite, and five probable detections, 13 of these for the first time at radio wavelengths. All detections are unresolved (θ ≲ 5″). Time variations in flux are confirmed in the cases of WR 98a, 104, 105, and 125. WR 79a and WR 89 are also variable in flux, and we suspect they are also nonthermal emitters. Thus, of our sample 20%30% of the detected stars are nonthermal emitters. Average mass-loss rate determinations obtained excluding definite and suspected nonthermal cases give similar values for WN (all subtypes) and WC5-7 stars [M(WN) = (4 ± 3) × 10-5 M⊙ yr-1 and M(WC5-7) = (4 ± 2) × 10-5 M⊙ yr-1], while a lower value was obtained for WC8-9 stars [M(WC8-9) = (2 ± 1) × 10-5 M ⊙ yr-1]. Uncertainties in stellar distances largely contribute to the observed scatter in mass-loss rates. Upper limits to the mass-loss rates were obtained in cases of undetected sources and for sources that probably show additional nonthermal emission.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí
Massive Binary WR112 and Properties of Wolf-Rayet Dust
Some hot, massive, population-I Wolf-Rayet (WR) stars of the carbon subclass
are known to be prolific dust-producers. How dust can form in such a hostile
environment remains a mystery. Here we report the discovery of a relatively
cool, extended, multi-arc dust envelope around the star WR112, most likely
formed by wind-wind collision in a long-period binary system. We derive the
binary orbital parameters, the dust temperature and the dust mass distributions
in the envelope. We find that amorphous carbon is a main constituent of the
dust, in agreement with earlier estimates and theoretical predictions. However,
the characteristic size of the dust grains is estimated to be ~1 micron,
significantly larger than theoretical limits. The dust production rate is
6.1*10^[-7] M_Sun / yr and the total detectable dust mass is found to be about
2.8*10^[-5] M_Sun (for d=4.15 kpc). We also show that, despite the hostile
environment, at least ~20% of the initially-formed dust may reach the
interstellar medium.Comment: 8pp, 3 figures (2 of them in colour). The Astrophysical Journal
Letters (2002) in pres
Wind clumping and the wind-wind collision zone in the Wolf-Rayet binary gamma Velorum
We present XMM-Newton observations of gamma^2 Velorum (WR 11, WC8+O7.5III, P
= 78.53 d), a nearby Wolf-Ray binary system, at its X-ray high and low states.
At high state, emission from a hot collisional plasma dominates from about 1 to
8 keV. At low state, photons between 1 and 4 keV are absorbed. The hot plasma
is identified with the shock zone between the winds of the primary Wolf-Rayet
star and the secondary O giant. The absorption at low state is interpreted as
photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to
measure the absorbing column density and to derive a mass loss rate 8x10^{-6}
M_sun/yr for the WC8 star. This mass loss rate, in conjunction with a previous
Wolf-Rayet wind model, provides evidence for a clumped WR wind. A clumping
factor of 16 is required. The X-ray spectra below 1 keV (12 Ang) show no
absorption and are essentially similar in both states. There is a rather clear
separation in that emission from a plasma hotter than 5 MK is heavily absorbed
in low state while the cooler plasma is not. This cool plasma must come from a
much more extended region than the hot material. The Neon abundance in the
X-ray emitting material is 2.5 times the solar value. The unexpected detection
of CV (25.3 Ang) and CVI (31.6 Ang) radiative recombination continua at both
phases indicates the presence of a cool (~40,000 K) recombination region
located far out in the binary system.Comment: 16 page
A Morphological Diagnostic for Dynamical Evolution of Wolf-Rayet Bubbles
We have observed H-alpha and [OIII] emission from eight of the most well
defined Wolf-Rayet ring nebulae in the Galaxy. We find that in many cases the
outermost edge of the [OIII] emission leads the H-alpha emission. We suggest
that these offsets, when present, are due to the shock from the Wolf-Rayet
bubble expanding into the circumstellar envelope. Thus, the details of the WR
bubble morphology at H-alpha and [OIII] can then be used to better understand
the physical condition and evolutionary stage of the nebulae around Wolf-Rayet
stars, as well as place constraints on the nature of the stellar progenitor and
its mass loss history.Comment: 11 pages, LaTex, 8 figures, accepted for publication in AJ, November
200
A very large array 3.6 centimeter continuum survey of galactic wolf-rayet stars
We report the results of a survey of radio continuum emission of Galactic Wolf-Rayet (WR) stars north of δ = -46°. The observations were obtained at 8.46 GHz (3.6 cm) using the Very Large Array, with an angular resolution of ∼6″ × 9″ and typical rms noise of ∼0.04 mJy beam-1. Our survey of 34 WR stars resulted in 15 definite, and five probable detections, 13 of these for the first time at radio wavelengths. All detections are unresolved (θ ≲ 5″). Time variations in flux are confirmed in the cases of WR 98a, 104, 105, and 125. WR 79a and WR 89 are also variable in flux, and we suspect they are also nonthermal emitters. Thus, of our sample 20%30% of the detected stars are nonthermal emitters. Average mass-loss rate determinations obtained excluding definite and suspected nonthermal cases give similar values for WN (all subtypes) and WC5-7 stars [M(WN) = (4 ± 3) × 10-5 M⊙ yr-1 and M(WC5-7) = (4 ± 2) × 10-5 M⊙ yr-1], while a lower value was obtained for WC8-9 stars [M(WC8-9) = (2 ± 1) × 10-5 M ⊙ yr-1]. Uncertainties in stellar distances largely contribute to the observed scatter in mass-loss rates. Upper limits to the mass-loss rates were obtained in cases of undetected sources and for sources that probably show additional nonthermal emission.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí
The highly polarized open cluster Trumpler 27
We have carried out multicolor linear polarimetry (UBVRI) of the brightest
stars in the area of the open cluster Trumpler 27. Our data show a high level
of polarization in the stellar light with a considerable dispersion, from to . The polarization vectors of the cluster members appear to be
aligned. Foreground polarization was estimated from the data of some non-member
objects, for which two different components were resolved: the first one
associated with a dust cloud close to the Sun producing
and degrees, and a second component, the main source of
polarization for the cluster members, originated in another dust cloud, which
polarizes the light in the direction of degrees. From a detailed
analysis, we found that the two components have associated values for the first one, and for the other. Due the
difference in the orientation of both polarization vectors, almost 90 degrees
(180 degrees at the Stokes representation), the first cloud (
degrees) depolarize the light strongly polarized by the second one ( degrees).Comment: 12 Pages, 6 Figures, 2 tables (9 Pages), accepted for publication in
A
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
Dynamical Masses for the Large Magellanic Cloud Massive Binary System [L72] LH 54-425
We present results from an optical spectroscopic investigation of the massive
binary system [L72] LH~54-425 in the LH 54 OB association in the Large
Magellanic Cloud. We revise the ephemeris of [L72] LH 54-425 and find an
orbital period of 2.247409 +/- 0.000010 days. We find spectral types of O3 V
for the primary and O5 V for the secondary. We made a combined solution of the
radial velocities and previously published V-band photometry to determine the
inclination for two system configurations, i = 52 degrees for the configuration
of the secondary star being more tidally distorted and i = 55 degrees for the
primary as the more tidally distorted star. We argue that the latter case is
more probable, and this solution yields masses and radii of M_1 = 47 +/- 2
M_Sun and R_1 = 11.4 +/- 0.1 R_Sun for the primary, and M_2 = 28 +/- 1 M_Sun
and R_2 = 8.1 +/- 0.1 R_Sun for the secondary. Our analysis places LH 54-425
amongst the most massive stars known. Based on the position of the two stars
plotted on a theoretical HR diagram, we find the age of the system to be about
1.5 Myr.Comment: 21 pages, 6 figures. Accepted in ApJ. To appear vol. 683, Aug. 10t
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