Fernando Comeron

The binary progenitor of Tycho Brahe's 1572 supernova

The brightness of type Ia supernovae, and their homogeneity as a class, makes them powerful tools in cosmology, yet little is known about the progenitor systems of these explosions. They are thought to arise when a white dwarf accretes matter from a companion star, is compressed and undergoes a thermonuclear explosion. Unless the companion star is another white dwarf (in which case it should be destroyed by the mass-transfer process itself), it should survive and show distinguishing properties. Tychos supernova is one of only two type Ia supernovae observed in our Galaxy, and so provides an opportunity to address observationally the identification of the surviving companion. Here we report a survey of the central region of its remnant, around the position of the explosion, which excludes red giants as the mass donor of the exploding white dwarf. We found a type G0–G2 star, similar to our Sun in surface temperature and luminosity (but lower surface gravity), moving at more than three times the mean velocity of the stars at that distance, which appears to be the surviving companion of the supernova.

X-shooter spectroscopy of young stellar objects - IV. Accretion in low-mass stars and substellar objects in Lupus

We present X-Shooter/VLT observations of a sample of 36 accreting low-mass stellar and sub-stellar objects (YSOs) in the Lupus star forming region, spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun < Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (Lacc), and from it the accretion rate (Macc), is derived by modelling the excess emission, from the UV to the near-IR, as the continuum emission of a slab of hydrogen. The flux and luminosity (Ll) of a large number of emission lines of H, He, CaII, etc., observed simultaneously in the range from ~330nm to 2500nm, were computed. The luminosity of all the lines is well correlated with Lacc. We provide empirical relationships between Lacc and the luminosity of 39 emission lines, which have a lower dispersion as compared to previous relationships in the literature. Our measurements extend the Pab and Brg relationships to Lacc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies to measure Lacc and Macc yield significantly different results: Ha line profile modelling may underestimate Macc by 0.6 to 0.8dex with respect to Macc derived from continuum-excess measures. Such differences may explain the likely spurious bi-modal relationships between Macc and other YSOs properties reported in the literature. We derive Macc in the range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in Macc.

Triggered massive-star formation on the borders of Galactic H ii regions ? II. Evidence for the collect and collapse process around RCW 79

We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFIJHKS images of the Galactic H ii 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 H ii 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 H ii 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 H ii region RCW 79 has been triggered by its expansion, most probably by the collect and collapse process.

Exploring brown dwarf disks in ρ Ophiuchi

This paper discusses evidence for and properties of disks associated to brown dwarfs in the star-forming region Oph. We selected nine objects from the ISOCAM survey of Bontemps et al. (2001) that have detections in the two mid- infrared bands (6.7 and 14.3m), relatively low extinction and low luminosity. We present low-resolution near-infrared spectra in the J, H and K bands, and determine for each source spectral type, extinction, eective temperature and luminosity by comparing the spectra to those of field dwarfs and to the most recent model stellar atmospheres. The results indicate that eight objects have spectral types M6-M7.5, eective temperature of 2600-2700 K, one has a later spectral type (M8.5) and lower temperature (about 2400 K). The derived extinctions range between AV 2 and 8 mag. The location of the objects on the HR diagram, in spite of the uncertainties of the evolutionary tracks for young objects of substellar mass, indicates that all the objects are very young and have masses below about 0.08 M. The coolest object in our sample has mass in the range 8-12 MJ (0.008-0.012 M). In all cases, the mid-infrared excess is consistent with the predictions of models of disks irradiated by the central object, showing that circumstellar disks are commonly associated to young brown dwarfs and planetary-mass objects. Finally, we discuss possible variations of the disk geometry among dierent objects, as well as the possibility of using these data to discriminate between various formation scenarios.

The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed with IRAC and MIPS

We present c2d Spitzer/IRAC observations of the Lupus I, III and IV dark clouds and discuss them in combination with optical and near-infrared and c2d MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times larger than the previous one. It is dominated by low- and very-low mass stars and it is complete down to M ≈ 0.1M⊙. We find 30-40% binaries with separations between 100 to 2000 AU with no apparent effect in the disk properties of the members. A large majority of the objects are Class II or Class III objects, with only 20 (12%) of Class I or Flat spectrum sources. The disk sample is complete down to “debris”-like systems in stars as small as M ≈ 0.2 M⊙ and includes sub-stellar objects with larger IR excesses. The disk fraction in Lupus is 70 – 80%, consistent with an age of 1 – 2 Myr. However, the young population contains 20% optically thick accretion disks and 40% relatively less flared disks. A growing variety of inner disk structures is found for larger inner disk clearings for

Triggered star formation on the borders of the Galactic Hii region RCW 82

Context. To investigate the process of star formation triggered by th e expansion of an H region, we present a multi-wavelength analysis of the Galactic H region RCW 120 and its surroundings. The collect and collaps e model predicts that the layer of gas and dust accumulated between the ionization and shock fronts during the expansio n of the H region collapses and forms dense fragments, giving rise to p otential sites of massive-star formation. Aims. The aim of our study is to look for such massive fragments and m assive young stars on the borders of RCW 120. Methods. We mapped the RCW 120 region in the cold dust continuum emissi on at 1.2 mm to search for these fragments. We supplemented this study with the available near(2MASS) and mid-IR (GLIM PSE) data to locate the IR sources observed towards this regi on and to analyse their properties. We then compared the observational resul ts with the predictions of Hosokawa & Inutsuka’s model (2005 , 2006). Results. At 1.2 mm we detected eight fragments towards this region, fiv e located on its borders. The largest fragment has a mass of ab out 370 M⊙. Class I and Class II young stellar objects are detected all o ver the region, with some observed far from the ionization fr ont. This result emphasises the possible importance of distant interactions between the radiation, escaping from the ioni zed region, and the surrounding

On the massive star contents of Cygnus OB2

We present a near-infrared spectroscopic survey of a large area centered on the Cygnus OB2 association aimed at constraining its massive star contents. Our goal is to establish a nearly complete list of O-type members of the association, both to examine recent claims based on starcounts that suggest a richer content than previously thought, and to provide a suitable database for further studies of the entire high-mass end of one of the richest associations of the Galaxy. The target selection is based on the JHK photometry published in the 2MASS all-sky survey. We identify 46 new early-type candidates, most of them expected to be O-type stars, plus 16 new stars with emission in Br and often in other lines as well, characteristic of evolved massive stars undergoing intense mass loss. We also present spectra of three luminous stars with CO overtone emission, one of them having also intense H2 emission and being associated with compact nebulosity. By considering our ndings, those of other authors, and plausible completeness corrections, we estimate the number of O-type stars or stars having evolved from a O-type progenitor to be 90{100, slightly below, but compatible with, most recent starcounts estimates by Knodlseder (2000, A&A, 360, 539). These results support the notion that Cygnus OB2 may be considered as a young globular cluster. The lists of new members that we provide, in particular those with emission lines, should be a useful resource for future investigations of Cygnus OB2 itself, as well as of very massive stellar evolution by providing a nearby, abundant sample of stars sharing a common environment.

Intense accretion and mass loss of a very low mass young stellar object

We present visible and near-infrared photometry and spectroscopy of LS-RCrA 1, a faint, very late-type object (M 6.5-M 7) seen in the direction of the R Coronae Australis star forming complex. While its emission spectrum shows prominent features of accretion and mass loss typical of young stellar objects, its underlying continuum and photometric properties are puzzling when trying to derive a mass and age based on pre-main sequence evolutionary tracks: the object appears to be far too faint for a young member of the R Coronae Australis complex of its spectral type. We speculate that this may be due to either its evolution along pre-main sequence tracks being substantially altered by the intense accretion, or to a combination of partial blocking and scattering of the light of the object by a nearly edge-on circumstellar disk. The rich emission line spectrum superimposed on the stellar continuum is well explained by an intense accretion process: the H α , CaII infrared triplet, and HeI 6678 lines show equivalent widths typical of very active classical T Tauri stars. The near-infrared observations show anomalously weak spectral features and no significant excess emission in the K band, which we tentatively interpret as indicating line filling due to emission in a magnetic accretion funnel flow. At the same time, numerous, strong forbidden optical lines ([OI], [NII] and [SII]) and H 2 emission at 2.12 μ m suggest that the object is simultaneously undergoing mass loss, providing another example that shows that mass loss and accretion are closely related processes. Such an intense accretion and mass loss activity is observed for the first time in a young stellar object in the transition region between low mass stars and brown dwarfs, and provides a valuable observational test on the effects of accretion on the evolution of objects with such low masses.

Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud ⋆

We present the ROSAT All-Sky Survey data in a 126 deg 2 area in and around the CrA star forming region. With low-resolution spectroscopy of unidentied ROSAT sources we could nd 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be 10 Myrs old, by comparing their location in the H R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new o-cloud weak-line T Tauri stars may have formed in similar cloudlets, which have dispersed re- cently. High-resolution spectra of our new T Tauri stars show that they have signicantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion { south relative to the galatic plane { of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered Send oprint requests to :R. Neuh¨ ? Partly based on observations collected at the 1.52 m and 3.5 m telescopes of the European Southern Observatory, Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and on ob- servations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope. the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.

Triggered massive-star formation on the borders of Galactic HII regions

Various physical processes are believed to trigger star formation on the borders of Galactic HII regions. Among these, the collect & collapse process is particularly attractive as it allows the formation of massive objects (single stars or clusters). In order to identify specific cases of this way of triggering star formation we are carrying out a multi-wavelength study of Galactic HII regions that exhibit signposts of massive-star formation at their borders. Hereby, we present two typical examples of such sources and discuss the results in the framework of the collect and collapse process, which seems to be at work as the major triggering agent in these two cases.