3D numerical simulations of photodissociated and photoionized disks

Astronomy & Astrophysics A&A 527, A86 (2011) DOI: 10.1051/0004-6361/200913342 c ESO 2011  3D numerical simulations of photodissociated and photoionized disks M. J. Vasconcelos1 , A. H. Cerqueira1 , and A. C. Raga2 1 Laboratório de Astrofísica Teórica e Observacional, DCET, Universidade Estadual de Santa Cruz, Rod. Ilhéus-Itabuna, km 16, Ilhéus, Bahia, Brasil e-mail: , 2 Instituto de Ciencias Nucleares, UNAM, Ap. Postal 70-543, CU, DF 04510, México e-mail: Received 23 September 2009 / Accepted 20 December 2010 ABSTRACT Aims. In this work we study the influence of the UV radiation field of a massive star on the evolution of a disklike mass of gas and dust around a nearby star. This system has similarities with the proplyds seen in Orion. Methods. We study disks with different inclinations and distances from the source, performing fully 3D numerical simulations. We use the YGUAZÚ-A adaptative grid code modified to account for EUV/FUV fluxes and non-spherical mass distributions. We treat H and C photoionization in order to reproduce the ionization fronts and photodissociation regions observed in proplyds. We also incorporate a wind from the ionizing source, in order to investigate the formation of the bow shock observed in several proplyds. We examine density and Hα maps, as well as the mass loss rates in the photoevaporated winds. Results. Our results show that a photoevaporated wind propagates from the disk surface and becomes ionized after an ionization front (IF) seen as a bright peak in the Hα maps. We follow the development of an HI region inside the photoevaporated wind which corresponds to a photodissociated region (PDR) for most of our models, except those without a FUV flux. For disks that are at a distance from the source d ≥ 0.1 pc, the PDR is thick and the IF is detached from the disk surface. In contrast, for disks that are closer to the source, the PDR is thin and not resolved in our simulations. The IF then coincides with the first grid points of the disk that are facing the ionizing photon source. In both cases, the photoevaporated wind shocks (after the IF) with the wind that comes from the ionizing source, and this interaction region is bright in Hα. Conclusions. Our 3D models produce two emission features: a hemispherically shaped structure (associated with the IF) and a detached bow shock where both winds collide. A photodissociated region develops in all of the models exposed to the FUV flux. More importantly, disks with different inclinations with respect to the ionizating source have relatively similar photodissociation regions. If the disk axis is not aligned with the direction of the ionizing photon flux, the IF displays moderate side-to-side asymmetries, in qualitative agreement with images of proplyds, which also show such asymmetries. The mass loss rates are ∼10−7 M yr−1 for face-on disks, and 5 × 10−8 M yr−1 for inclined disks at distances from 0.1 to 0.2 pc from the ionizing photon sources. Key words. H II regions – hydrodynamics – stars: winds, outflows – circumstellar matter 1. Introduction The ultraviolet radiation from a massive star in an environment of low mass star formation produces several interesting phenomena, among them the appearance of the so-called proplyds (an acronym that stands for PROto-PLanetarY Disk; see O’Dell et al. 1993; and O’Dell & Wen 1994). Proplyds are young stellar objects (YSO) with a cometary-shaped ionization front that contain a central, low mass young star surrounded by an accretion disk, a photoevaporated wind, and, in some cases, a monopolar or a bipolar outflow. The proplyds in the Orion nebula (M 42) were first reported as unresolved optical emission features by Laques & Vidal (1979) and then as radio sources by Churchwell et al. (1987). Using the Hubble Space Telescope (HST), O’Dell & Wen (1994) and O’Dell & Wong (1996) reported the discovery of tens of proplyds around the Trapezium cluster. The proplyds present strong optical emission lines such as Hα, [O iii] 5007 Å, [O i] 6300 Å (Bally et al. 2000) and also some ultraviolet (Henney et al. 2002) and IR lines (Takami et al. 2002). With HST high spatial resolution images and spectra, it was possible to see that the emission comes from different parts of the object. Proplyds present a cometary shaped feature, very strong in Hα. Several of them show another bow shaped feature, detached from the cometary structure itself, visible in Hα, [O iii] 5007 Å and in IR lines, lying further from the low mass star than the ionization front (see Fig. 1). In some objects, the disks are seen in emission in [O i] 6300 Å. It is also possible to see that the optical lines are double or even triple peaked. For example, in the central parts of the proplyd 167−317 (LV2), Hα has three components: a low velocity component, peaking at a radial velocity of ∼30 km s−1 , and two high velocity components, centered at ∼ 100 km s−1 and ∼−75 km s−1 (Vasconcelos et al. 2005). Johnstone et al. (1998) explained the observations with a model in which the radiative field and the wind from θ1 Ori C interact with a low mass YSO, generating a photoevaporated wind, an ionization front (IF) and a bow shock. They also proposed the existence of a photodissociated region (PDR), in which the photoevaporated wind is neutral (the H is neutral). They separate their models in two classes that depend on the amount of EUV and FUV flux arriving at the proplyd’s disk surface. The EUV model, valid for a YSO near to or very far from Article published by EDP Sciences Page 1 of 11 A&A 527, A86 (2011) O−STAR Disk axis EUV+FUV Stellar wind H EUV+FUV θ z Bow−shaped interaction region r r vw Stellar Wind 1 Proplyd head 0 (IF) Ionization Front Proplyd tail (IF) Star−disk system Fig. 1. A cartoon of a star-disk system being photoevaporated by the UV radiation field from an O type star. The major features are highlighted: the proplyd head and tail which traces the ionization front from the direct ionization field coming from the O-star and from the diffuse radiative field, respectively, and the interaction region between the photoevaporated flow and the wind from the O-star. θ1 Ori C (d < 1017 cm or d > 1018 cm), presents a very thin PDR. In this case, the EUV flux determines the photoevaporated mass loss rate. On the other hand, for the FUV model, the PDR is thick and can contain a shock front within it. Here, the FUV flux determines the mass loss rate. Störzer & Hollenbach (1999) further improved the models by Johnstone et al. (1998) including results from PDR codes. Some axisymmetric numerical simulations were also done in order to reproduce the characteristics of proplyds. García-Arredondo et al. (2001) carried out axisymmetric simulations of the interaction of the stellar wind from θ1 Ori C with the photoevaporated proplyd flow. They took available observational data to constrain stellar parameters such as the wind density and velocity and the ionization front parameters. Their numerica (...truncated)