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

Triggered star formation on the borders of the Galactic HII region RCW 120

To investigate the process of star formation triggered by the expansion of an HII region, we present a multi-wavelength analysis of the Galactic HII region RCW 120 and its surroundings. The collect and collapse model predicts that the layer of gas and dust accumulated between the ionization and shock fronts during the expansion of the HII region collapses and forms dense fragments, giving rise to potential sites of massive-star formation. The aim of our study is to look for such massive fragments and massive young stars on the borders of RCW 120. We mapped the RCW 120 region in the cold dust continuum emission at 1.2 mm to search for these fragments. We supplemented this study with the available near- (2MASS) and mid-IR (GLIMPSE) data to locate the IR sources observed towards this region and to analyse their properties. We then compared the observational results with the predictions of Hosokawa & Inutsuka's model (2005, 2006). At 1.2 mm we detected eight fragments towards this region, five located on its borders. The largest fragment has a mass of about 370 Msun. Class I and Class II young stellar objects are detected all over the region, with some observed far from the ionization front. This result emphasises the possible importance distant interactions between the radiation, escaping from the ionized region, and the surrounding medium

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

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

Photometric distances to young stars in the inner Galactic disk. I. The L = 314o^{o} direction

The spiral structure of the Milky Way is nowadays receiving renewed attention thanks to the combined efforts of observational campaigns in different wavelength regimes, from the optical to the radio. We start in the paper the exploration of a number of key sectors in the inner Milky Way, where the spiral structure is still poorly known. We search for density enhancements of young stars that might plausibly be associated with spiral structure. To this aim we collect sufficiently wide-field UBVI photometry. The intensive usage of U-band photometry ensures robust determination of reddening and hence distance for stars of spectral type earlier than A0, which are well-known spiral arm tracers. The fields we use are large enough to include in most cases well-studied Galactic clusters, which we use as bench-marks to assess the quality and standardisation of the data, and to validate our method. We focus in this paper on the line of sight to the Galactic longitude l= 314$^o$. First of all, we made exhaustive tests to show that our data-set is in the standard system, and calibrated our method using the two open clusters NGC 5617 and Pismis 19 which happen to be in the field, and for which we found estimates of the basic parameters in full agreement with the literature. We then applied the method to the general field stars and detected signatures of three different groups of stars, evenly distributed across the field of view, at 1.5$^{+0.5}_{-0.2}$, 2.5$^{+0.3}_{-0.5}$, and 5.1$^{+1.5}_{-1.1}$ kpc, respectively. These distances are compatible with the location of the nowadays commonly accepted description of the Carina-Sagittarius and Scutum-Crux arms, at heliocentric distance of $\sim$ 2 an 5 kpc,Comment: 23 pages, 13 eps figure, in press in Astronomy and Astrophysic

Effect of the angle of attack of a rectangular wing on the heat transfer enhancement in channel flow at low Reynolds number

Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions. One method involves the use of vortex generators to produce streamwise and transverse vortices superimposed to the main flow. This study presents numerical computation results of laminar convection heat transfer in a rectangular channel whose bottom wall is equipped with one row of rectangular wing vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible flow. Three-dimensional numerical simulations are performed to study the effect of the angle of attack α of the wing on heat transfer and pressure drop. Different values are taken into consideration within the range 0° &lt; α &lt; 30°. For all of these geometrical configurations the Reynolds number is maintained to Re = 456. To assess the effect of the angle of attack on the heat transfer enhancement, Nusselt number and the friction factor are studied on both local and global perspectives. Also, the location of the generated vortices within the channel is studied, as well as their effect on the heat transfer enhancement throughout the channel for all α values. Based on both local and global analysis, our results show that the angle of attack α has a direct impact on the heat transfer enhancement. By increasing its value, it leads to better enhancement until an optimal value is reached, beyond which the thermal performances decrease