249 — 12 September 2013 Editor: Bo Reipurth ([email protected]) List of Contents
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THE STAR FORMATION NEWSLETTER An electronic publication dedicated to early stellar/planetary evolution and molecular clouds No. 249 — 12 September 2013 Editor: Bo Reipurth ([email protected]) List of Contents The Star Formation Newsletter My Favorite Object ............................ 3 Perspective ................................... 10 Editor: Bo Reipurth [email protected] Abstracts of Newly Accepted Papers .......... 14 Technical Editor: Eli Bressert Dissertation Abstracts ........................ 49 [email protected] New Jobs ..................................... 51 Technical Assistant: Hsi-Wei Yen Meetings ..................................... 52 [email protected] Other Meetings ............................... 53 Editorial Board Short Announcements ........................ 55 Joao Alves Alan Boss Jerome Bouvier Lee Hartmann Cover Picture Thomas Henning Paul Ho The front cover shows the Orion Nebula Cluster us- Jes Jorgensen ing images obtained with the VISTA infrared sur- Charles J. Lada vey telescope at the ESO Paranal Observatory in Thijs Kouwenhoven Chile. Three images, each with 10 minute expo- Michael R. Meyer sure, were taken through Z, J, and Ks filters and Ralph Pudritz combined. The field is about one degree in width. Luis Felipe Rodr´ıguez Ewine van Dishoeck Image courtesy ESO/J. Emerson/VISTA. Acknowl- Hans Zinnecker edgment: Cambridge Astronomical Survey Unit. 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 for the following sections: Abstracts of recently Submitting your abstracts accepted papers (only for papers sent to refereed journals), Abstracts of recently accepted major re- Latex macros for submitting abstracts views (not standard conference contributions), Dis- and dissertation abstracts (by e-mail to sertation Abstracts (presenting abstracts of new [email protected]) are appended to Ph.D dissertations), Meetings (announcing meet- each Call for Abstracts. You can also ings broadly of interest to the star and planet for- submit via the Newsletter web inter- mation and early solar system community), New face at http://www2.ifa.hawaii.edu/star- Jobs (advertising jobs specifically aimed towards formation/index.cfm 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 2 My Favorite Object LL Ori Will Henney Figure 1: The hyperbolic arcs of the LL objects are pro- duced when a mildly supersonic flow of ionized gas (left) shocks against an obstacle. The obstacle (right) is most likely a supersonic outflow from the enclosed T Tauri star: The classical T Tauri star LL Orionis is the prototype possibly the flow from a proplyd ionization front (above), of the class of LL objects, which show spectacular arcs of or a stellar wind (below). circumstellar emission (see Figs. 1 and 2). The arc around LL Ori was first remarked upon by Gull & Sofia (1979), and six further similar objects were identified by (Bally tion of table) and shocked shells (lower section). & Reipurth 2001), who denoted them LL 1 to LL 7, with Convex Ionization Fronts When an ionization front wraps LL 1 corresponding to LL Ori. They have been intensely around a dense obstacle, then, so long as the confining studied via optical emission line imaging with HST (Bally pressure in the H II region is not too large, it will form et al. 2000; Bally et al. 2006), and more recently with an ionized photoevaporation flow with an approximately high-resolution spectroscopy (Henney et al. 2013). Like D-critical front (e.g., Dyson 1968). The electron den- many other LL objects, LL 1 also harbors a hypersonic sity ne, and therefore the optical emission line bright- jet, HH 888, whose knots are not point-symmetrical about ness, has a sharp peak at the ionization front on a scale the star, but rather show a C-shaped bent morphology, of ≈ (106 cm−3/n) AU (Henney et al. 2005b), superim- curving away from the core of the Orion Nebula. posed on a more gradual decline on a scale of roughly The vast majority of the LL objects detected to date (over 1/10 of the radius of curvature of the front (Bertoldi 1989). 20) are in the Orion Nebula, although a very similar ob- Even within a single nebula, such as Orion, the same ba- ject has recently been found in the Carina Nebula (Smith sic phenomenon explains structures ranging in size from et al. 2010). They provide a fascinating window into the tens of AU (the smallest proplyds) up to tens of thousands complex outflow phenomena (both jets and uncollimated of AU (large scale structures such as the Bright Bar and winds) associated with pre-main sequence stars in dense the champagne flow away from the main ionization front). clusters. At the same time, they can function as “test par- Numerical simulations show that such flows arise natu- ticles”, allowing the internal dynamics of the H II region rally during the evolution of an H II region inside a tur- to be probed. Figure 3 situates the LL objects within the bulent molecular cloud (Mellema et al. 2006; Gritschneder context of the Orion Nebula’s internal flows. et al. 2010; Arthur et al. 2011; Ercolano et al. 2012). It is the largest scale photoevaporation flows of this nature (Zuckerman 1973; O’Dell et al. 1993; Henney et al. 2005a; Taxonomy of bright arcs in H II regions Garc´ıa-D´ıaz et al. 2008) that are believed to provide the external wind that shocks to form the hyperbolic arcs seen Several different phenomena can give rise to sharply de- in the LL objects, whereas the smallest scale photoevap- fined, curved features inside H II regions, which sometimes oration flows from protoplanetary disks (Laques & Vidal leads to confusion in the literature over the classification 1979; O’Dell et al. 1993; Johnstone et al. 1998; Henney & of particular objects. In Table 1, I present a general tax- O’Dell 1999; Ricci et al. 2008) are one candidate for the onomy of such phenomena, which can be broadly divided internal wind (see Fig. 1). into two categories: convex ionization fronts (upper sec- 3 Figure 2: Left panel shows the location of LL Ori in the south-west of the Orion Nebula. The color image is a combination of HST WFPC2 observations (O’Dell & Wong 1996) in three filters: [N II] λ6584 (red), Hα λ6563 (green), and [O III] λ5007 (blue). With the brightness adjusted to bring out LL Ori and other features in the outer nebula, the inner regions of the nebula are saturated and appear white. In this saturated area I have superimposed a negative image of the same Hα mosaic, but with maximum brightness set at 15 times that of the color image. Right panel shows an expanded view of LL Ori itself, indicating the principal components. The hyperbolic bow shock is due to the wind-wind collision between a stellar outflow from the T Tau star and the champagne flow of ionized gas from the core of the Orion Nebula. The colored arrows superimposed on the jet knots show the radial velocity (red or blueshifted) and proper motions (Henney et al. 2013). Jet knot velocities range from 60 to 120 km s−1, whereas velocities in the wings of the bow are only ∼ 20 km s−1. Shocked Shells Whenever a supersonic outflow impinges motions and small radial velocities. The second category on a quiescent or slow-moving medium, or two supersonic can be sub-divided into the classical proplyd bow shocks flows collide, then a double-shock structure will form (Si- (Bally et al. 1998; Garc´ıa-Arredondo et al. 2001; Smith mon & Axford 1966; Avedisova 1971; Dyson & de Vries et al. 2005; Robberto et al. 2005) and the hyperbolic LL 1972). If either of the shocks is strongly radiative, then it arcs (see Fig. 3 and Tab. 1). The principal difference be- will be visible as a bright emission arc. It turns out that, tween the two sub-groups is that in the proplyd case the for shock velocities of 20–60 km s−1 and densities higher outer bow shock is not directly observed since it is in the than a few hundred cm−3, the cooling length behind the fast stellar wind from the O star and is non-radiative. In- shock is very short, so that the shock can be considered stead, the observed emission arcs are due to the termina- isothermal for many purposes. tion shock in the mildly supersonic (M ≃ 3) flow from Such shocked shells can be divided into those driven by the proplyd ionization front. In the case of the hyperbolic non-steady jets, which show high proper motions in the LL arcs, on the other hand, which are generally found plane of the sky and highly blueshifted or redshifted ra- much farther out in the nebula, it is the outer bow shock dial velocities (e.g., Bally et al. 2006; Henney et al. 2007; that dominates the emission from the shell. This is the O’Dell & Henney 2008), and those due to the head-on reason that the LL arcs are much more open than the interaction of two winds or streams, which are approxi- arcs in front of the close-in proplyds, which tend to be mately in a steady state, showing no detectable proper more semi-circular in form. For a low Mach number ex- 4 Figure 3: Schematic diagram of wind-wind interactions in the Orion Nebula (a simpler version of this diagram first appeared in O’Dell et al.