354 research outputs found
Evidence for quasi-chemically homogeneous evolution of massive stars up to solar metallicity
Long soft gamma ray bursts (LGRBs) are usually associated with the death of
the most massive stars. A large amount of core angular momentum in the phases
preceding the explosion is required to form LGRBs. A very high initial
rotational velocity can provide this angular momentum. Such a velocity strongly
influences the way the star evolves: it is chemically homogeneously mixed and
evolves directly towards the blue part of the HR diagram from the main
sequence. We have shown that chemically homogeneous evolution (CHE) takes place
in the SMC, at low metallicity. We want to see if there is a metallicity
threshold above which such an evolution does not exist. We perform a
spectroscopic analysis of H-rich early-type WN stars in the LMC and the Galaxy.
We use the code CMFGEN to determine the fundamental properties and the surface
composition of the target stars. We then place the stars in the HR diagram and
determine their evolution. We show that both the LMC and Galactic WNh stars we
selected cannot be explained by standard stellar evolution. They are located on
the left of the main sequence but show surface abundances typical of CN
equilibrium. In addition, they still contain a large amount of hydrogen. They
are thus core-H burning objects. Their properties are consistent with CHE. We
determine the metallicity of the Galactic stars from their position and
Galactic metallicity gradients, and conclude that they have 0.6<Z<1.0. A
moderate coupling between the core and the envelope is required to explain that
stellar winds do not extract to much angular momentum to prevent a blueward
evolution. In view of the findings that some long gamma ray bursts appear in
solar environments, CHE may be a viable way to form them over a wide range of
metallicities.Comment: 10 pages, 10 figures. Accepted in Astronomy and Astrophysic
Star formation in RCW 108: triggered or spontaneous?
We present visible, near IR, and mm-wave observations of RCW 108, a molecular
cloud complex in the AraOB1 association that is being eroded by the energetic
radiation of two O-type stars in the nearby cluster NGC 6193. The western part
of the RCW108 cloud contains an embedded compact HII region, IRAS 16362-4845,
ionized by an aggregate of early-type stars. We notice a lack of stars later
than A0 in the aggregate and speculate that this might be a consequence of its
extreme youth. We examine the distribution of stars displaying IR excesses
projected across the molecular cloud. While many of them are located in the
densest area of the molecular cloud near IRAS16362-4845, we also find a group
concentrating towards the edge of the cloud that faces NGC 6193, as well as
some other stars beyond the edge of the molecular cloud. The intense ionizing
radiation field by the O stars in NGC6193 is a clear candidate trigger of star
formation in the molecular cloud, and we suggest that the existence and
arrangement of stars in this region of the cloud supports a scenario in which
their formation may be a consequence of this. However, IR excess stars are also
present in some areas of the opposite side of the cloud, where no obvious
candidate external trigger is identified. The existence of such tracers of
recent star formation scattered across the more massive molecular cloud
associated with IRAS 16362-4845, and the low star formation efficiency that we
derive, indicate that it is in a state to still form stars. This is in contrast
to the less massive cloud close to NGC 6193, which seems to be more evolved and
mostly already recycled into stars, and whose internal kinematics show hints of
having been perturbed by the presence of the massive stars formed out of it.Comment: Accepted for publication in Astronomy and Astrophysics, 25 pages,
paper with original figures available at
http://www.eso.org/~fcomeron/rcw108.ps.g
Triggered massive-star formation on the borders of Galactic HII regions. II. Evidence for the collect and collapse process around RCW 79
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of
the Galactic HII region RCW 79. The millimetre continuum data reveal the
presence of massive fragments located in a dust emission ring surrounding the
ionized gas. The two most massive fragments are diametrically opposite each
other in the ring. The near-IR data, centred on the compact HII region located
at the south-eastern border of RCW 79, show the presence of an IR-bright
cluster containing massive stars along with young stellar objects with near-IR
excesses. A bright near- and mid-IR source is detected towards maser emissions,
1.2 pc north-east of the compact HII region centre. Additional information,
extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of
the bright IR sources observed towards RCW 79. Twelve luminous Class I sources
are identified towards the most massive millimetre fragments. All these facts
strongly indicate that the massive-star formation observed at the border of the
HII region RCW 79 has been triggered by its expansion, most probably by the
collect and collapse process.Comment: 14 pages, 15 figures. Accepted for publication in A&A. The images
have been highly compressed for astro-ph. A version of this paper with
higher-resolution figures is available at
http://www.oamp.fr/matiere/rcw79.pd
The molecular complex associated with the Galactic HII region Sh2-90: a possible site of triggered star formation
We investigate the star formation activity in the molecular complex
associated with the Galactic HII region Sh2-90, using radio-continuum maps
obtained at 1280 MHz and 610 MHz, Herschel Hi-GAL observations at 70 -- 500
microns, and deep near-infrared observation at JHK bands, along with Spitzer
observations. Sh2-90 presents a bubble morphology in the mid-IR (size ~ 0.9 pc
x 1.6 pc). Radio observations suggest it is an evolved HII region with an
electron density ~ 144 cm^-3, emission measure ~ 6.7 x 10^4 cm^-6 pc and a
ionized mass ~ 55 Msun. From Hi-GAL observations it is found that the HII
region is part of an elongated extended molecular cloud (size ~ 5.6 pc x 9.7
pc, H_2 column density >= 3 x 10^21 cm^-2 and dust temperature 18 -- 27 K) of
total mass >= 1 x 10^4 Msun. We identify the ionizing cluster of Sh2-90, the
main exciting star being an O8--O9 V star. Five cold dust clumps (mass ~ 8 --
95 Msun), four mid-IR blobs around B stars, and a compact HII region are found
at the edge of the bubble.The velocity information derived from CO (J=3-2) data
cubes suggests that most of them are associated with the Sh2-90 region. 129
YSOs are identified (Class I, Class II, and near-IR excess sources). The
majority of the YSOs are low mass (<= 3 Msun) sources and they are distributed
mostly in the regions of high column density. Four candidate Class 0/I MYSOs
have been found; they will possibly evolve to stars of mass >= 15 Msun. We
suggest multi-generation star formation is present in the complex. From the
evidences of interaction, the time scales involved and the evolutionary status
of stellar/protostellar sources, we argue that the star formation at the
immediate border/edges of Sh2-90 might have been triggered by the expanding HII
region. However, several young sources in this complex are probably formed by
some other processes.Comment: 22 pages, 22 figures, accepted for publication in Astronomy and
Astrophysic
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