The molecular emission of the irradiated star forming core ahead of HH 80N

📝 Abstract

We present a Berkeley-Illinois-Maryland Association (BIMA) Array molecular survey of the star forming core ahead of HH 80N, the optically obscured northern counterpart of the Herbig-Haro objects HH 80/81. Continuum emission at 1.4 mm and 8 micron is detected at the center of the core, which confirms the presence of an embedded very young stellar object in the core. All detected molecular species arise in a ring-like structure, which is most clearly traced by CS(2-1) emission. This molecular ring suggests that strong molecular depletion occurs in the inner part of the core (at a radius of ~0.1 pc and densities higher than ~5 x 10^4 cm^{-3}). Despite of the overall morphology and kinematic similarity between the different species, there is significant molecular differentiation along the ring-like structure. The analysis of the chemistry along the core shows that part of this differentiation may be caused by the UV irradiation of the nearby HH 80N object, that illuminates the part of the core facing HH 80N, which results in an abundance enhancement of some of the detected species.

💡 Analysis

We present a Berkeley-Illinois-Maryland Association (BIMA) Array molecular survey of the star forming core ahead of HH 80N, the optically obscured northern counterpart of the Herbig-Haro objects HH 80/81. Continuum emission at 1.4 mm and 8 micron is detected at the center of the core, which confirms the presence of an embedded very young stellar object in the core. All detected molecular species arise in a ring-like structure, which is most clearly traced by CS(2-1) emission. This molecular ring suggests that strong molecular depletion occurs in the inner part of the core (at a radius of ~0.1 pc and densities higher than ~5 x 10^4 cm^{-3}). Despite of the overall morphology and kinematic similarity between the different species, there is significant molecular differentiation along the ring-like structure. The analysis of the chemistry along the core shows that part of this differentiation may be caused by the UV irradiation of the nearby HH 80N object, that illuminates the part of the core facing HH 80N, which results in an abundance enhancement of some of the detected species.

📄 Content

arXiv:0901.4034v1 [astro-ph.SR] 26 Jan 2009 The molecular emission of the irradiated star forming core ahead of HH 80N Josep M. Masqu´e1, Josep M. Girart2, Maria T. Beltr´an1, Robert Estalella1 and Serena Viti3 ABSTRACT We present a Berkeley-Illinois-Maryland Association (BIMA) Array molec- ular survey of the star forming core ahead of HH 80N, the optically obscured northern counterpart of the Herbig-Haro objects HH 80/81. Continuum emis- sion at 1.4 mm and 8 µm is detected at the center of the core, which confirms the presence of an embedded very young stellar object in the core. All detected molecular species arise in a ring-like structure, which is most clearly traced by CS (2–1) emission. This molecular ring suggests that strong molecular deple- tion occurs in the inner part of the core (at a radius of ≃0.1 pc and densities higher than ∼5 × 104 cm−3). Despite of the overall morphology and kinematic similarity between the different species, there is significant molecular differenti- ation along the ring-like structure. The analysis of the chemistry along the core shows that part of this differentiation may be caused by the UV irradiation of the nearby HH 80N object, that illuminates the part of the core facing HH 80N, which results in an abundance enhancement of some of the detected species. Subject headings: ISM: individual (HH 80N) — ISM: molecules — radio lines: ISM — stars: formation — ISM: abundances — 1. Introduction IRAS 18162−2048 is a high mass protostar, with a luminosity of ∼2 × 104 L⊙, powering the most luminous and largest Herbig-Haro (HH) system, HH 80/81/80N, associated with a 1Departament d’Astronomia i Meteorologia, Universitat de Barcelona, Mart´ı i Franques 1, 08028 Barcelona, Catalunya, Spain 2Institut de Ciencies de l’Espai, (CSIC-IEEC), Campus UAB, Facultat de Ci`encies, Torre C5 - parell 2, 08193 Bellaterra, Catalunya, Spain 3Department of Physics and Astronomy, University College London, London, WC1E 6BT, UK – 2 – very highly collimated radio jet (Rodr´ıguez et al. 1980; Reipurth & Graham 1988; Mart´ı et al. 1993). This source is located in the GGD 27 region, at a distance of 1.7 kpc in Sagittarius (Rodr´ıguez et al. 1980). A far-IR spectroscopic study of the HH 80/81/80N system shows that the FUV field radiated by the ionized material in the shock recombination region is able to induce the formation of a photodissociation region (PDR) in the surrounding medium of the HH objects and the jet flow (Molinari et al. 2001). The HH 80N object has not been detected in the optical neither at near infrared wavelengths, which is likely a consequence of the extinction due to a molecular cloud in the foreground (Mart´ı et al. 1993). A dense clump of 20 M⊙was found ahead of HH 80N, firstly detected in ammonia (Girart et al. 1994) and afterward detected in other species, such as CS and HCO+ (Girart et al. 1998). BIMA array observations (with 10′′ of angular resolution) carried out by Girart et al. (2001) (hereafter GEVWH) showed that the CS (2–1) emission traces a ring-like structure, with a radius of 0.24 pc, seen edge on. This observed morphology is most likely the result of a strong CS depletion in the inner region of the core (GEVWH). The analysis of the CS kinematics suggests that this structure is contracting with an infall velocity of 0.6 km s−1. Assuming a gas temperature of 17 K, equivalent to the ammonia rotational temperature (Girart et al. 1994) the sound speed is ∼0.3 km s−1, implying supersonic collapse. In addition, the CO (1–0) emission reveals a bipolar outflow that implies the presence of an embedded protostellar object within the HH 80N core (GEVWH). In recent years, observations have found that a number of HH objects have associated molecular condensations. Typically, these clumps are cool (∼10–20 K), dense (≳3 × 104 cm−3), small (sizes of ≲0.1 pc and masses of ≲1 M⊙), starless and show little or no evidence of dynamical interaction with the stellar jet (e.g. HH 1/2: Torrelles et al. 1994; Viti et al. 2006; HH 34: Anglada et al. 1995). They are possibly of the same type as the transient small clumps found by Morata et al. (2005) in L673. The high molecular abundances of some species (e.g., CH3OH, NH3, HCO+) found in these clumps (Girart et al. 2005; Viti et al. 2006) suggest a chemical alteration induced by the UV radiation incoming from the HH object (Taylor & Williams 1996; Viti & Williams 1999; Viti et al. 2003). However, BIMA observations towards the HH 2 region reveal that despite the generally quiescent nature of the molecular condensations ahead of the HH objects, complex dynamical and radiative interactions occur in this region (Girart et al. 2005). The dense core ahead of HH 80N seems to be one of these examples of irradiated cores (Girart et al. 1994, 1998), but it has two peculiarities that make this region an interesting target to study. First, the HH 80N core is significantly larger and more massive than those described above, and shows star formation signatures (GEVW

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