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325 — 15 January 2020 Editor: Bo Reipurth (Reipurth@Ifa.Hawaii.Edu) List of Contents THE STAR FORMATION NEWSLETTER An electronic publication dedicated to early stellar/planetary evolution and molecular clouds No. 325 — 15 January 2020 Editor: Bo Reipurth ([email protected]) List of Contents The Star Formation Newsletter Perspective .................................... 3 Abstracts of Newly Accepted Papers ........... 9 Editor: Bo Reipurth [email protected] Abstracts of Newly Accepted Major Reviews .. 30 Associate Editor: Anna McLeod Dissertation Abstracts ........................ 31 [email protected] New Jobs ..................................... 32 Technical Editor: Hsi-Wei Yen Meetings ..................................... 35 [email protected] Summary of Upcoming Meetings .............. 36 Editorial Board Joao Alves Alan Boss Jerome Bouvier Lee Hartmann Cover Picture Thomas Henning Paul Ho The L1622 cloud in Orion is a fine case of a bright- Jes Jorgensen rimmed cometary cloud. It is located just outside of Charles J. Lada Barnard’s Loop, and points towards the OB stars in Thijs Kouwenhoven the Orion Nebula Cluster. The bright nebulous star Michael R. Meyer near the center of the image is the young multiple Ralph Pudritz system HBC 515. Luis Felipe Rodríguez Subaru 8m image by Bo Reipurth Ewine van Dishoeck Processing by Robert Gendler. Hans Zinnecker The Star Formation Newsletter is a vehicle for fast distribution of information of interest for as- tronomers working on star and planet formation and molecular clouds. You can submit material Submitting your abstracts for the following sections: Abstracts of recently accepted papers (only for papers sent to refereed Latex macros for submitting abstracts journals), Abstracts of recently accepted major re- and dissertation abstracts (by e-mail to views (not standard conference contributions), Dis- [email protected]) are appended to sertation Abstracts (presenting abstracts of new each Call for Abstracts. You can also Ph.D dissertations), Meetings (announcing meet- submit via the Newsletter web inter- ings broadly of interest to the star and planet for- face at http://www2.ifa.hawaii.edu/star- mation and early solar system community), New formation/index.cfm Jobs (advertising jobs specifically aimed towards persons within the areas of the Newsletter), and Short Announcements (where you can inform or re- quest information from the community). Addition- ally, the Newsletter brings short overview articles on objects of special interest, physical processes or theoretical results, the early solar system, as well as occasional interviews. Newsletter Archive www.ifa.hawaii.edu/users/reipurth/newsletter.htm Perspective Protostellar accretion and its impact on the pre-main sequence evolution Emanuele Tognelli 1 Introduction Figure 1: HR diagram for young PMS stars extracted from The detailed computation of what happens to a star at the the literature compared to a set of standard evolutionary beginning of its life, the phases that lead to the formation models and isochrones from the PISA database (Tognelli of a star (protostellar evolution) and the subsequent pre- et al. 2011, 2018). main sequence evolution (PMS) has been a hard task for many years. Starting from the pioneering work by Larson (1969), which analysed the protostellar evolution from the 2 Pre-main sequence stars data gravitational collapse of the parental cloud to the PMS phase, recently such an evolution has been revised thanks From the observational point of view, there are a lot of to the improvements in both the input physics and com- data for young PMS stars (ages ∼ 1 Myr) for solar or putational power. A particularly interesting result is that slightly sub-solar metallicity. Among these objects, some even if the first collapsing phases are difficult to follow, the of them show a still detectable accretion disc, or protoplan- simulations predict the formation of a hydrostatic core: etary disc, and very low accretion rates (see e.g. Hartmann the characteristics of such a hydrostatic core (2nd Larson & Kenyon 1996, Muzerolle et al. 2000, Calvet et al. 2005, core) such as its mass, radius, temperature and density, Muzerolle et al. 2005a,b, Bae et al. 2013, Ingleby et al. appear to be very robust. 2014). Such residual accretion discs show clear footprints of a previous accretion phase. The ideal approach would be to follow the whole proto- stellar and then stellar evolution in hydrodynamical simu- It is interesting to note that once placed into a colour- lation, to treat simultaneously and in detail the structure magnitude or HR diagram, young PMS stars are located and evolution of both the protostar and the surrounding in a region corresponding to bright and extended (hence material that falls on it. However, such an approach is at cold) objects. As an example, Figure 1 shows a sample of the moment extremely difficult and not feasible. To over- young PMS stars available in the literature. come this problem, some simplifications can be adopted. Such stars are fully formed, in the sense that the mea- A common approach consists in starting the protostel- sured accretion rates are extremely small, thus, they have lar evolution after the formation of the 2nd Larson core, already reached their final mass. Consequently, they can when the central protostar is assumed to be hydrostatic. be considered as stars that are evolving as constant mass The matter surrounding the star can be treated both as a structures. Moreover, given the large luminosity and radii, spherical structure that radially falls on the star (spherical 2 the thermal or Kelvin-Helmholtz time (τKH ∝ GM /LR) accretion, e.g. Stahler et al. 1980) or a disc like struc- are relatively small (of the order of 103-104 yr), thus it can ture that provides accreting matter via accretion streams be assumed that they have lost memory (or they will) of (disc accretion, e.g. Hartmann et al. 1997). In this short what happened during their protostellar phase. review, I will present some of the main characteristics of accreting models in these two geometries (spherical or disc The location in the colour-magnitude (CM) or Hertzprung- accretion) and some related problems, focusing on the for- Russell (HR) diagram of such objects poses a first con- straint on the models. Stellar models must reproduce the mation of low-mass (M < 1 M⊙) solar metallicity stars. More details can be found in the review by Larson (2003) position of these stars. In particular, young stellar mod- and Hartmann et al. (2016). els (ages of the order of ∼ 1 Myr) must be located in the region of bright, cold and extended objects. 3 3 Cloud collapse and protostellar ac- cretion Stars form in a region (cloud) that contracts and gives birth to denser cores that eventually form the protostar and then the star. In this process, the mass of a protostar progressively increases as the matter in the cloud falls onto a central denser object. The cloud collapse is a complex hydrodynamical problem, a challenging task in the stellar formation field. Depending on the characteristics of the cloud and on the configuration assumed in the compu- tations (chemical composition, magnetic fields, rotation, geometry...) the collapse occurs faster or slower, but the general evolution can be summarised as outlined below (for more details see Larson 1969, 1972 and the review by Larson 2003): • Isothermal collapse: during the first collapse, the cloud does not warm up as it contracts. This is true until a central density of about 10−13 g cm−3 is reached. When this occurs, the matter in the inner Figure 2: Comparison between standard isochrones (thin region of the cloud is dense enough to partially trap solid lines) and the loci of the end of protostellar accreting radiation. sequence for two different initial temperatures of the cloud • Formation of the first Larson core: the energy is par- (10 K thick solid and 20 K thick dashed line). Circles mark tially trapped inside the denser region of the cloud the position of 0.25, 0.5, 1, 1.5, 2, 3, and 5 M⊙ models. preventing a further collapse of this region. A first Figure adapted from Larson (1972). temporarily hydrostatic core forms (with a mass of about 0.01 M⊙ and a radius of several AU). Outside of it the matter is still falling on the core. There is the computations. Larson (1969) remarked the fact that a transition region (shock front) close to the surface at some stage the cloud gives birth to a hydrostatic central of the core where the matter settles and passes from object (2nd Larson core) that can be considered the first supersonic to subsonic. protostellar core. The characteristics of this model (i.e. mass, radius, density and central temperature) appeared • Second collapse: the hydrostatic core contracts be- to be not much sensitive to the adopted initial conditions cause it can partially radiate energy at the surface. of the cloud or to the adopted input physics (Masunaga & So, although its mass is increasing due to accretion Inutsuka 2000, Machida et al. 2008, Tomida et al. 2013, of mass, its radius shrinks. The contraction of the Vaytet et al. 2013). Reasonable intervals for the mass, ra- core leads to a temperature rise, until T ∼ 2000 K dius and temperature of such a core are the following: the is reached, and molecular hydrogen dissociates. The mass ranges from 1 - 20 MJ, the radius from 0.5 - 10 R⊙ contraction energy does not warm the core but it is and the central temperature from 2-6×104 K. In Larson used to dissociate H2. The density and pressure of (1969, 1972), the author followed the subsequent evolu- the core increase. tion until the star reaches the Hayashi track, finding that low mass stars (M < 1 M⊙) attain, after the protostellar Formation of the 2nd Larson core: • after the total accretion, characteristics similar to that of standard evo- dissociation of H2 and the increase of density and lution along the Hayashi track.
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