40 research outputs found
UV Spectroscopy of Metal-Poor Massive Stars in the Small Magellanic Cloud
The Hubble Space Telescope has provided the first clear evidence for weaker
winds of metal-poor massive stars in the Small Magellanic Cloud, confirming
theoretical predictions of the metallicity dependence of mass-loss rates and
wind terminal velocities. For lower luminosity O-type stars however, derived
mass-loss rates are orders of magnitude lower than predicted, and are at
present unexplained.Comment: 4 pages, 3 figures. To appear in 'The Impact of HST on European
Astronomy', Eds., G. De Marchi & F.D. Macchetto, Astrophysics & Space
Science, Springe
30 Doradus - a Template for "Real Starbursts"?
30 Doradus is the closest massive star forming region and the best studied
template of a starburst. In this conference paper we first summarize the
properties of 30 Doradus and its stellar core, R136. We discuss the effects of
insufficient spatial resolution and cluster density profiles on dynamical mass
estimates of super star clusters, and show that their masses can be easily
overestimated by a factor of ten or more. From a very simple model, with
R136-like clusters as representative building blocks, we estimate typical
luminosities of the order 10^11 L_o for starburst galaxies.Comment: To be published in "Starbursts: From 30 Doradus to Lyman Break
Galaxies", eds. R. de Grijs & R.M. Gonzalez Delgad
Properties of Galactic B supergiants
Physical and wind properties of Galactic B supergiants are presented based upon non-LTE line blanketed model atmospheres, including Sher 25 toward the NGC 3603 cluster. We compare Halpha derived wind densities with recent results for SMC B supergiants and generally confirm theoretical expectations for stronger winds amongst Galactic supergiants. Mid B supergiant winds are substantially weaker than predictions from current radiatively driven wind theory, a problem which is exacerbated if winds are already clumped in the Halpha line forming region. We find that the so-called `bistability jump' at B1 (Teff ~ 21kK) from Lamers et al. is rather a more gradual downward trend. CNO elemental abundances, including Sher 25, reveal partially processed material at their surfaces. In general, these are in good agreement with evolutionary predictions for blue supergiants evolving redward accounting for rotational mixing. A few cases, including HD 152236 (zeta^1 Sco), exhibit strongly processed material which is more typical of Luminous Blue Variables. Our derived photospheric [N/O] ratio for Sher~25 agrees with that for its ejecta nebula, although a higher degree of CNO processing would be expected if the nebula originated during a red supergiant phase, as is suspected for the ring nebula ejected by the B supergiant progenitor of SN 1987A, Sk-69 202. Sher 25 has an inferred age of ~5Myr in contrast with ~2Myr for HD 97950, the ionizing cluster of NGC 3603. Sher 25 may be a foreground object or close binary evolution could be responsible for its unusual location in the H-R diagram
A new spectral classification system for the earliest O stars: definition of type O2
High-quality, blue-violet spectroscopic data are collected for 24 stars that have been classified as type O3 and that display the hallmark N IV and N V lines. A new member of the class is presented; it is the second known in the Cyg OB2 association, and only the second in the northern hemisphere. New digital data are also presented for several of the other stars. Although the data are inhomogeneous, the uniform plots by subcategory reveal some interesting new relationships. Several issues concerning the classification of the hottest O-type spectra are discussed, and new digital data are presented for the five original O3 dwarfs in the Carina Nebula, in which the N IV, N V features are very weak or absent. New spectral types O2 and O3.5 are introduced here as steps toward resolving these issues. The relationship between the derived absolute visual magnitudes and the spectroscopic luminosity classes of the O2–O3 stars shows more scatter than at later O types, at least partly because some overluminous dwarfs are unresolved multiple systems, and some close binary systems of relatively low luminosity and mass emulate O3 supergiant spectra. However, it also appears that the behavior of He II λ4686, the primary luminosity criterion at later O types, responds to other phenomena in addition to luminosity at spectral types O2–O3. There is evidence that these spectral types may correspond to an immediate pre-WN phase, with a correspondingly large range of luminosities and masses. A complete census of spectra classified into the original O3 subcategories considered here (not including intermediate O3/WN types or O3 dwarfs without N IV, N V features) totals 45 stars; 34 of them belong to the Large Magellanic Cloud and 20 of the latter to 30 Doradus
An HST/NICMOS view of the prototypical giant HII region NGC604 in M33
We present the first high-spatial resolution near-infrared (NIR) imaging of
NGC 604, obtained with the NICMOS camera aboard the Hubble Space Telescope
(HST). These NICMOS broadband images reveal new NIR point sources, clusters,
and diffuse structures. We found an excellent spatial correlation between the
8.4 GHz radio continuum and the 2.2mu-m nebular emission. Moreover, massive
young stellar object candidates appear aligned with these radio peaks,
reinforcing the idea that those areas are star-forming regions. Three different
scaled OB associations are recognized in the NICMOS images. The brightest NIR
sources in our images have properties that suggest that they are red supergiant
stars, of which one of them was previously known. This preliminary analysis of
the NICMOS images shows the complexity of the stellar content of the NGC 604
nebula.Comment: Paper presented in the Workshop "Young massive star clusters: initial
conditions and environments" (Granada, Spain - Sept 2007). Astrophysics &
Space Science in press, 7 pages, 4 figure
VLT/X-shooter spectroscopy of massive young stellar objects in the 30 Doradus region of the Large Magellanic Cloud
The process of massive star (M ≥ 8 M⊙) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud. We took advantage of the spectral resolution and wavelength coverage of X-shooter (300−2500 nm), which is mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect Hα, β, [O I] 630.0 nm, Ca II, infrared triplet [Fe II] 1643.5 nm, fluorescent Fe II 1687.8 nm, H2 2121.8 nm, Brγ, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of ten candidates as bona fide MYSOs. We compared our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We computed lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Brγ luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features, such as fluorescent Fe II and CO bandheads, which is consistent with the Galactic rate (40%). Most of our MYSOs show outflow features
VLT/X-shooter spectroscopy of massive young stellar objects in the 30 Doradus region of the Large Magellanic Cloud
Erratum: "Rotation, spectral variability, magnetic geometry and magnetosphere of the Of?p star CPD -28° 2561"
Este documento es una errata de "Rotation, spectral variability, magnetic geometry and magnetosphere of the Of?p star CPD -28° 2561" (ver documento relacionado).Instituto de Astrofísica de La Plat
The R136 star cluster dissected with Hubble Space Telescope/STIS. I:Far-ultraviolet spectroscopic census and the origin of He II λ1640 in young star clusters
Low-mass pre--main-sequence stars in the Magellanic Clouds
[Abridged] The stellar Initial Mass Function (IMF) suggests that sub-solar
stars form in very large numbers. Most attractive places for catching low-mass
star formation in the act are young stellar clusters and associations, still
(half-)embedded in star-forming regions. The low-mass stars in such regions are
still in their pre--main-sequence (PMS) evolutionary phase. The peculiar nature
of these objects and the contamination of their samples by the evolved
populations of the Galactic disk impose demanding observational techniques for
the detection of complete numbers of PMS stars in the Milky Way. The Magellanic
Clouds, the companion galaxies to our own, demonstrate an exceptional star
formation activity. The low extinction and stellar field contamination in
star-forming regions of these galaxies imply a more efficient detection of
low-mass PMS stars than in the Milky Way, but their distance from us make the
application of special detection techniques unfeasible. Nonetheless, imaging
with the Hubble Space Telescope yield the discovery of solar and sub-solar PMS
stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of
such objects are identified as the low-mass stellar content of their
star-forming regions, changing completely our picture of young stellar systems
outside the Milky Way, and extending the extragalactic stellar IMF below the
persisting threshold of a few solar masses. This review presents the recent
developments in the investigation of PMS stars in the Magellanic Clouds, with
special focus on the limitations by single-epoch photometry that can only be
circumvented by the detailed study of the observable behavior of these stars in
the color-magnitude diagram. The achieved characterization of the low-mass PMS
stars in the Magellanic Clouds allowed thus a more comprehensive understanding
of the star formation process in our neighboring galaxies.Comment: Review paper, 26 pages (in LaTeX style for Springer journals), 4
figures. Accepted for publication in Space Science Review