Francesco Palla

Disks and outflows around intermediate-mass stars and protostars

In order to study the existence and evolution of circumstellar disks around intermediate-mass stars (M 3M), we have obtained single-dish and interferometric continuum images at 2.6 mm and 1.3 mm of the intermediate-mass protostar NGC 7129 FIRS 2 and of the Herbig Be stars LkH 234 and HD 200775. These objects are representative of the dierent stages of the pre-main sequence evolution with ages ranging from a few 10 3 to 8 10 6 years. Single-dish and interferometric observations of the outflows associated with these sources are also presented. In NGC 7129 FIRS 2, two millimeter sources are required to t the interferometric 1.3 mm continuum emission. Only the most intense of these millimeter objects, FIRS 2 { MM1, seems to be associated with the CO outflow. The second and weaker source, FIRS 2 { MM2, does not present any sign of stellar activity. The single-dish map of the CO outflow presents an unusual morphology with the blue and red lobes separated by an angle of 82 .T he COJ =1 ! 0 interferometric image shows that this unusual morphology is the result of the superposition of two outflows, one of them associated with FIRS 2 { MM1 (the blue lobe in the single-dish map) and the other (the red lobe) with a new infrared source (FIRS 2 { IR) which is not detected in the millimeter continuum images. The interferometric 1.3 mm continuum image of NGC 7129 FIRS 1 reveals that LkH 234 is a member of a cluster of embedded objects. Two millimeter clumps are detected in this far-infrared source. The strongest is spatially coincident with the mid-infrared companion of LkH 234, IRS 6. A new millimeter clump, FIRS 1 { MM1, is detected at an oset ( 3:23 00 ,3 .0 00 )f rom LkH 234. We have not detected any compact source towards LkH 234 with a limit for the mass of a circumstellar disk, MD < 0.1 M. The comparison of the interferometric CO J =1 ! 0 and continuum images reveals that IRS 6 very likely drives the energetic molecular outflow detected towards NGC 7129 FIRS 1 and the (SII) jet. The extremely young object FIRS 1 { MM1 (it has not been detected in the near-and mid-infrared) turns out to be the driving source of the H2 jet. There is no evidence for the existence of a bipolar outflow associated with LkH 234. We have not detected 1.3 mm continuum emission towards HD 200775. Our observations imply a 3- upper limit of < 0.002 M for the mass of a circumstellar disk. This is the lowest upper limit obtained so far in a Herbig Be star. Thus our observations provide new important information on three protostars (IRS 6, FIRS 1 { MM1 and FIRS 2 { MM1), one infrared star (FIRS 2 { IR) and two Herbig Be stars. The luminosities of the protostars are consistent with being intermediate-mass objects (M 3:5{4:5 M). They are surrounded by thick envelopes with masses ranging between2 3:5 M and drive energetic outflows. Circumstellar disks and bipolar outflows are not detected toward the Herbig Be stars. We have obtained an upper limit for the disk/stellar mass ratio, MD=M ,o f< 0.02 in LkH 234 and of < 0.0002 in HD 200775. Our limit in HD 200775 implies that in evolved Herbig Be stars the MD=M ratio is more than two orders of magnitude lower than in T Tauri and Herbig Ae stars. We propose that in massive stars (M 5 M )b oth the dispersal of the outer disk and the energetic mass-loss, occur early in the stellar evolution before the star becomes visible. Some mechanisms for the dispersal of the outer disk are discussed.

The history of mass dispersal around Herbig Ae/Be stars

We present a systematic study of the material surrounding intermediate-mass stars. Our sample includes 34 Herbig Ae/Be (HAEBE) stars of dierent ages and luminosities. This is a quite complete representation of the whole class of HAEBE stars and consequently, our conclusions should have a solid statistical meaning. In addition, we have observed 2 intermediate-mass protostars and included published data on 15 protostellar objects in order to determine the evolution of the circumstellar material in the early stages of stellar evolution. All the HAEBE stars have been classied according with the three Types already dened in Fuente et al. (1998): Type I stars are immersed in a dense clump and have associated bipolar outflows, their ages are0.1 Myr; Type II stars are still immersed in the molecular cloud though not in a dense clump, their ages are between af ew 0.1 to af ew Myr; Type III stars have completely dispersed the surrounding material and are located in a cavity of the molecular cloud, their ages are >1 Myr. Our observations are used to reconstruct the evolution of the circumstellar material around intermediate-mass stars and investigate the mass dispersal mechanisms at the dierent stages of the stellar evolution. Our results can be summarized as follows: intermediate-mass stars disperse90% of the mass of the parent clump during the protostellar phase. During this phase, the energetic outflows sweep out the gas and dust forming a biconical cavity while the equatorial material is infalling to feed the circumstellar disk and eventually the protostar. In this way, the density structure of the parent clump remains well described by a density law n/ r with 2 0:08 pc) cavities in the molecular cloud, producing a dramatic change in the morphology of the region. This dierence is easily understood if photodissociation plays an important role in the mass dispersal around these objects.

Post-T Tauri Stars: A False Problem

We consider the problem of the apparent lack of old T Tauri stars in nearby star-forming regions in the framework of the standard model of low-mass star formation. We argue that the similarity between molecular cloud lifetimes and the ambipolar diffusion timescale implies that star formation does not take place instantaneously, nor at a constant rate. Thus, models based on the assumption of a constant star formation rate overestimate the predicted number of stars of ages greater than 2-5 Myr. We argue that the probability of finding a large population of old stars in a star-forming region is intrinsically very small. It is therefore unlikely that the dispersed X-ray sources found by ROSAT at large distances from molecular cloud complexes can be identified with such a population. We conclude that the post-T Tauri problem is by and large nonexistent.

New observations of the pulsating PMS star V351 Ori

We present new precise photoelectric observations of the Pre{Main-Sequence Scuti star, V351 Ori. These new data show that V351 Ori pulsates in a mixture of several radial modes (at least four). The comparison between observations and detailed pulsational models, allows us to provide independent constraints on the mass and luminosity of the star. The predicted distance is 210 pc, indicating that V351 Ori is much closer than the Orion star forming region. With an inferred mass of 1.8 M and an uncertain evolutionary stage, V351 Ori represents an excellent candidate for future asteroseismological studies that will assess whether it is a young PMS star (6 Myr) or an evolved object (1 Gyr) leaving the main-sequence.

Thermal coupling of the CMBR and the primordial gas in the post-recombination epoch (z ≤ 1000)

The thermal evolution of the Universe after the recombination era is followed by studying in detail the coupling of the radiation field with the primordial gas, due to the presence of a minor, but finite, amount of trace molecules. It is shown that, under some circumstances, the formation of H 2 and LiH molecules can lead to an appreciable heating of the gas, that would otherwise cool adiabatically as the Universe expands. We also present results for the expected distortions of the CMBR, due to the formation of H 2 in highly excited states and the subsequent radiative cascade to the ground state.