The pre-ZAMS nature of Mol160/IRAS 23385+6053 confirmed by Spitzer

Abstract

Context. The formation of massive stars goes through phases that remain heavily obscured until the object is well along on the main sequence. The identification of massive YSOs in different evolutionary phases is therefore particularly difficult, and requires a statistical approach with large samples of candidate objects to determine the observational signatures of these different phases. Aims. A mandatory activity in this context is the identification and characterisation of all the phases that a massive forming YSO undergoes. It is of particular interest to verify the observability of the phase in which the object is rapidly accreting while not yet igniting the fusion of hydrogen that marks the arrival on the ZAMS. Methods. One of the candidate prototypical objects for this phase is Mol160/IRAS 23385+6053, which has been the subject of detailed studies that confirmed and strengthened the possibility that this massive YSO may be in a pre-hot core stage. We further investigate this issue by means of Spitzer imaging and spectroscopy in the 5−70 μm range. Results. The dense core of Mol160/IRAS 23385+6053, which up to now had only been detected at submillimeter and millimeter wavelengths, with only upper limits below 20 μm, has been revealed for the first time at 24 and 70 μm by Spitzer. These observations confirm the earlier assumptions that this object is dominant at far-IR wavelengths. The complete 24 μm−3.4 mm continuum cannot be fitted with a standard model of a zero-age main-sequence (ZAMS) star embedded in an envelope. A simple greybody fit yields a mass of 220 M . The luminosity is slightly in excess of 3000 L , which is a factor of 5 less than previous estimates when only IRAS fluxes were available between 20 and 100 μm. The source is under-luminous by the same factor with respect to UCHii regions or hot-cores of similar circumstellar mass, and simple models show that this is compatible with an earlier evolutionary stage. Spectroscopy between 5−40 μm shows that the physical conditions are typical of a photo-dissociated or photo-ionized region. The required UV illumination can be provided by some of the other sources revealed at λ ≤ 24 μm in the same star-forming region, that can be plausibly modeled as moderately embedded intermediate-mass ZAMS stars. Conclusions. Our results strengthen the suggestion that the central core in Mol160/IRAS 23385+6053 is a massive YSO actively accreting from its circumstellar envelope and that it has not yet begun hydrogen fusion.

Properties of stellar clusters around high-mass young stars

“…Since the millimeter peak is at the center of symmetry of the semi-circular stellar distribution, we consider this tobe the center of the cluster. This is only for completeness, since we cannot say whether the low density of stars at the millimeter peak is an effect of extreme visual extinction or reflects an intrinsic paucity of NIR-visible forming stars, as the proposed extreme youth of the massive YSO accreting in its depth would seem to suggest (Molinari et al 2008b). …”

Weak and Compact Radio Emission in Early High-Mass Star-Forming Regions. I. Vla Observations

“…Several studies have attempted to understand the molecular condensations where high-mass stars form and attempted to establish an evolutionary sequence for them (e.g., Molinari et al 1996Molinari et al , 1998Molinari et al , 2000Molinari et al , 2008. Based on recent Very Large Array NH 3 (Sánchez-Monge et al 2013c), and ATCA H 2 O maser and centimeter continuum (Sánchez-Monge et al 2013a) observations of a large number of high-mass star-forming candidates, these authors have suggested an evolutionary sequence from quiescent starless cores, with relatively narrow NH 3 lines and low temperatures to protostellar cores that already contain IR point sources, and show larger linewidths and temperatures, as well as the presence of ionized gas.…”

Interstellar Medium and Star Formation Studies with the Square Kilometre Array

“…Along the lines of the well established phases of low-mass star formation (Shu et al, 1987b), recently there have been a number of attempts to carry out a similar evolutionary classification for the higher mass counterparts. These are based on signposts such as masers, near and mid-infrared emission, jets and outflows, shocked gas as well as the presence of radio emission (Molinari et al, 2008;Battersby et al, 2010;Sánchez-Monge et al, 2013;Giannetti et al, 2013). The early phases of massive star forming sites are characterised by the presence of cold clumps detected in millimetre continuum and infrared emission, presence of water or methanol maser and low levels of radio continuum emission (Molinari et al, 2008;Sánchez-Monge et al, 2013).…”

“…These are based on signposts such as masers, near and mid-infrared emission, jets and outflows, shocked gas as well as the presence of radio emission (Molinari et al, 2008;Battersby et al, 2010;Sánchez-Monge et al, 2013;Giannetti et al, 2013). The early phases of massive star forming sites are characterised by the presence of cold clumps detected in millimetre continuum and infrared emission, presence of water or methanol maser and low levels of radio continuum emission (Molinari et al, 2008;Sánchez-Monge et al, 2013). For example, Battersby et al (2010) have examined massive star formation by investigating a number of infrared dark clouds and have described an observational evolutionary sequence comprising of four stages: (i) quiescent clump, (ii) clump with signature of active star formation (maser, green fuzzy 1 , or 24 µm emission), (iii) initiation of ultracompact HII region, and (iv) diffuse red clump 2 , finally leading to the formation of a young stellar stellar cluster.…”