Abstract. This paper presents a study of the accretion properties of 19 very low mass objects (M ∼ 0.01−0.1 M ) in the regions Chamaeleon I and ρ Oph. For 8 objects we obtained high resolution Hα profiles and determined mass accretion rateṀ ac and accretion luminosity L ac . Paβ is detected in emission in 7 of the 10 ρ Oph objects, but only in one in Cha I. Using objects for which we have both a determination of L ac from Hα and a Paβ detection, we show that the correlation between the Paβ luminosity and luminosity L ac , found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass ∼0.03 M ; L(Paβ) can be used to measure L ac also in the substellar regime. The results were less conclusive for Brγ, which was detected only in 2 objects, neither of which had an Hα estimate ofṀ ac . Using the relation between L(Paβ) and L ac we determined the accretion rate for all the objects in our sample (including those with no Hα spectrum), more than doubling the number of substellar objects with knownṀ ac . When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lowerṀ ac for lower M , although with a large spread. Some of the spread is probably due to an age effect; our very young objects in ρ Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Hα measured at 10% peak intensity is not only a qualitative indicator of the accreting nature of very low mass objects, but can be used to obtain a quantitative, although not very accurate, estimate ofṀ ac over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss in their optical spectra.

We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ∼0.03 to ∼1.2 M , but mostly with 0.1 M < M < 0.5 M . Our aim is twofold: firstly, to 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 (L acc ), and in turn the accretion rate (Ṁ acc ), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (L line ) of many emission lines of H , He , and Ca ii, observed simultaneously in the range from ∼330 nm to 2500 nm. The luminosity of all the lines is well correlated with L acc . We provide empirical relationships between L acc and the luminosity of 39 emission lines, which have a lower dispersion than relationships previously reported in the literature. Our measurements extend the Paβ and Brγ relationships to L acc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies of measuring L acc andṀ acc yield significantly different results: Hα line profile modelling may underestimateṀ acc by 0.6 to 0.8 dex with respect toṀ acc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships betweenṀ acc and other YSOs properties reported in the literature. We derivedṀ acc in the range 2 × 10 −12 -4 × 10 −8 M yr −1 and conclude thatṀ acc ∝ M 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 inṀ acc , confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.

Abstract.A spectroscopic survey of a small area at the center of the Lupus 3 star-forming core has revealed four new mid-tolate M-type members, including a M7.5 brown dwarf. One of the new members, classified as M5, displays prominent forbidden lines and strong Hα emission (EW(Hα) = 410 Å), in addition to other permitted lines, and its luminosity is far below that of other members of the region with similar or later spectral types. We estimate a mass accretion rate rate of ∼1.4 × 10 −9 M yr −1 for this object, although with uncertainties that do not exclude values as low as 10 −10 M yr −1 . Based on the Hα/[SII] ratio, the detection of HeI, and the CaII infrared triplet, we argue that most of the Hα emission is produced near the surface of the object, probably in accretion columns or at the base of jets, rather than in a low-density extended region. The strong emission-line spectrum superimposed on an unusually faint photospheric continuum thus seems to be a real, intrinsic feature rather than a result of the viewing geometry caused by an edge-on disk blocking the light from the central object. Other Lupus 3 late-type members also display noticeable underluminosity, all of them having EW(Hα) > 100 Å as a result of the faint underlying continuum. We tentatively interpret these findings as evidence for the pre-main sequence evolution of objects with very low (possibly substellar) initial masses being significantly modified by accretion.

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

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. Tycho's 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.