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THE

STAR

FORMATION

NEWSLETTER

An electronic publication dedicated to early stel lar/planetary evolution and molecular clouds
No. 256 -- 10 April 2014 Editor: Bo Reipurth (reipurth@ifa.hawaii.edu)




A: Steve and I had acquired experience in the numerical calculations of accreting massive stars for our pro ject on primordial protostars. Thus, it was natural for me to continue the collaboration on present-day protostars and extend the models that Steve had developed for solar and sub-solar masses. The results were extremely satisfying as, among other things, we identified a new mechanism, deuterium-shell burning, that caused a large swelling of the protostellar radius and a concurrent delay in the contraction of 2-3 M protostars toward the main sequence. I remember with excitement and affection the days when the numerical code would give us troubles in convergence due to the fast increase of the radius in very short time steps. We thought of all possible numerical errors, but the results were in fact real and robust. In this way, we could proceed further with the calculations of more massive stars by varying the accretion flow and exploring different acQ: You first started working on primordial star formation. cretion geometries all the way up to massive ob jects. Was that in connection with your doctoral thesis? Q: This was fol lowed by a widely cited study of the early A: Only partially. My thesis pro ject was on the infrared evolution of intermediate mass stars and a comparison emission from dust grains in galactic molecular clouds, un- with the Herbig Ae/Be stars. der the supervision of Antonella Natta at the University of Rome. The interest on primordial star formation devel- A: Yes, the new results had a direct impact on the underoped later after attending a series of lectures by Joe Silk standing of the location of the intermediate-mass stars in during a Saas Fee school on "Star Formation". Part of his the HR diagram. The puzzle was that the Herbig Ae/Be course was on the first stars and Joe discussed the critical stars occupy a limited portion of the diagram well above role that H2 molecules could play in a gas of pristine com- the main sequence that all the previous models of protoposition with no metals and dust grains. He mentioned stellar evolution had failed to explain. Our-birthline com-5 1 that a small, but not negligible fraction (of order 10-5 - puted at the fiducial rate of 10 M yr did provide -3 10 ) of atomic H could be converted into H2 molecules a gratifying match to the upper envelope of the observed purely by gas phase reactions thanks to the residual ion- distribution of most of the known Herbig Ae/Be stars. ization left over after recombination. This tiny fraction of Few of them actually laid above the birthline, suggestive H2 molecules could help cooling the gas, promoting frag- of a mass accretion rate higher than the fiducial value for mentation and induce the formation of massive, or very more massive stars, an issue that is still with us today. massive stars. I couldn't believe that so little H2 could The other interesting prediction of our models was that have such a large impact on first star formation. So, when stars of mass above 8-10 M should be found on the main I got an ESA Fellowship to continue my research on dust sequence even during the accretion phase, due to the short grains and infrared emission in Mayo Greenberg's Labora- Kelvin-Helmholtz contraction time compared to the accretory in The Netherlands, I decided instead to switch topic tion time, thus excluding a pre-main sequence phase for and explore the unusual (for the time) field of primordial such ob jects. I remember meeting with George Herbig at stars. Thus, I ended up at Cornell University to work the conference on "his" stars in 1993 in Amsterdam. He with Prof. Ed Salpeter, a master of everything including was quite pleased, in his own characteristic style, with our molecules and the interstellar medium. It was there that results.

Francesco Palla

neering calculations of the structure of intermediate mass protostars. What were your main conclusions?

in conversation with Bo Reipurth

I met with Steve Stahler and that it all started. At the end of the pro ject, we realized that the initially atomic gas could be converted into a fully molecular mixture thanks to the three body reactions that take place whenever sufficiently high density (of order 109 cm-3 and more) are achieved during gravitational collapse. As a result, continued fragmentation would allow the formation of low-mass stars even in these unique conditions. Q: In the early 90's, you and Steve Stahler performed pio3

Q: You and Steve have compared your evolutionary tracks with observations of clusters and associations, such as the Orion Nebula Cluster and the Taurus-Auriga region, and have found evidence that star formation is accelerating. Is this a universal phenomenon? A: Ah, this is controversial! But we still maintain that the interpretation of the pattern of accelerating star formation, in spite of all the simplifying assumptions of our






analysis and the uncertainties with the evolutionary tracks and mo dels, is an intrinsic property of the way molecular cloud and dense cores turn the gas into stars. So, if it works, it should be a universal feature. I had stumbled on it while thinking of a solution to the post-T Tauri stars (or lack thereof ) based on the fact that star formation is not a rapid pro cess and also that it do esn't o ccur at a constant rate during the cloud lifetime. Then, the paucity of old T Tauri stars in young asso ciations and clusters would not be an observational problem, but just an intrinsic fact common to all such regions. Afterwards, I have tried to find independent evidence of the observed large spread of the stellar distributions in the HR diagram from a variety of indicators, mainly spectroscopic (i.e. variation of surface gravity, lithium abundance etc.). The discussion that was spurred after our suggestion is still lively to day, although the advo cates of a rapid mo de of star formation appear to have a louder voice. However, recent numerical mo dels that follow the gravitational contraction of molecular clouds under their own gravity have corroborated our empirical finding, showing that the star formation rate indeed increases with time. Q: Ten years ago, you and Isabel le Baraffe predicted that brown dwarfs might pulsate. What is the latest status? A: Isabelle and I were very excited at this result, mainly for two reasons. First, unlike traditional pulsation instabilities excited in the outer layers of the stars by opacity effects (the so-called "kappa" mechanism), those asso ciated with brown dwarfs are due to the high sensitivity of the energy generation rate of deuterium burning in the core to small temperature and density perturbations (the "epsilon" mechanism). The latter has never been observed in any astronomical ob ject. We thought that the ephemeral, but critical, D-burning phase of young BDs would provide a splendid opportunity to search for this pro cess. Second, if present, the observed frequencies would have offered a unique way to probe the interior of these peculiar ob jects and to test evolutionary mo dels. Unfortunately, given the linear nature of our study, we could not determine the amplitude of the predicted pulsations, but we just computed the expected perio ds of a few hours. Now, dedicated optical and infrared photometric searches for the perio dic variations have been carried out (in particular by Anne Marie Co dy and Lynne Hillenbrand in Ori) without a positive result down to stringent limits in amplitudes (a few mmag). However, Ori might not be the ideal case where to lo ok at, while for example Cha I with its younger age is better suited for such studies. Q: Lately you have focused on lithium depletion in YSOs. A: Being a nuclear precise way of gaug the lithium depletio tify brown dwarfs is clo ck, lithium burning offers a very ing stellar ages. The application of n boundary to unambiguously idena beautiful demonstration of its po4

tential. I have been involved in the determination of the LDB in pre-main sequence clusters in the age range of 2540 Myr, finding in some cases a very go o d agreement with the iso chronal ages, but also departures. When applied to individual stars in younger clusters and star forming regions, the derived abundances in low-mass stars (in the mass range 0.2-0.4 M ) have yielded large nuclear ages consistent with the iso chronal ones, thus providing support for the co-existence of a tail of older stars in a predominantly younger population. This data will be greatly expanded by the results of the Gaia-ESO survey that will accumulate high quality spectra, including the lithium feature, on an extended sample of young clusters and asso ciations. Q: Although your work is mainly theoretical, you have had a life-long passion for observations of masers. A: Apart from the very interesting physics that is involved with astrophysical masers, my interest in water masers was mainly driven by their property of being excellent signposts of massive star formation throughout the Galaxy. Now, in the early 1990s the Arcetri group was deeply involved with water maser studies using the Medicina 32m radiotelescope and we had noticed that all the bright IRAS sources selected using the Wo o d & Churchwell color criteria were inevitably asso ciated with strong H2 O masers. Part of our group had developed a keen interest in the search for young massive stars, but it was difficult at the time to select the appropriate sources. The water masers identified by us formed the basis of a rich sample that was later exploited by Riccardo Cesaroni, Sergio Molinari and others to make big progress in the study of the initial conditions of massive star formation. Q: As director of Arcetri Observatory, how do you see the current situation in Italy affecting the life of the institute? A: Well, since 2012 I'm no longer director of the Observatory, but I've been following the status of the research in our National Institute for Astrophysics. If I'm allowed a personal reflection on my experience as director, I would say that the most gratifying result of all the effort required to run an institute is the possibility to hire young people with a passion in astronomy and give them the same (or even better) chances that I had at a similar age. This is now almost impossible, both at the lo cal and general level. While the amount of funding might have increased in recent years, mostly for dedicated large-scale observational/instrumental pro jects, the number of positions has shrunk to a number absolutely insufficient for the younger generation to be able to compete for permanent jobs. At the same time, the number of short-term contracts has increased abnormally, creating a huge pressure on the institutions. I am afraid that this situation will not improve in the next years unless a radical change in the political attitude at the national and European levels will o ccur.