Looking for high-mass young stellar objects: H$_{\sf 2}$O and OH masers in ammonia cores

Abstract

Context. The earliest stages of high-mass star formation have yet to be characterised well, because high-angular resolution observations are required to infer the properties of the molecular gas hosting the newly formed stars. Aims. We search for high-mass molecular cores in a large sample of 15 high-mass star-forming regions that are observed at highangular resolution, extending a pilot survey based on a smaller number of objects. Methods. The sample was chosen from surveys of H 2 O and OH masers to favour the earliest phases of high-mass star formation. Each source was first observed with the 32-m single-dish Medicina antenna in the (1, 1) and (2, 2) inversion transitions at 1.3 cm of ammonia, which is an excellent tracer of dense gas. High-resolution maps in the NH 3 (2, 2) and (3, 3) lines and the 1.3 cm continuum were obtained successively with the VLA interferometer. Results. We detect continuum emission in almost all the observed star-forming regions, which corresponds to extended and UCHii regions created by young stellar objects with typical luminosities of ∼10 4 L . However, only in three cases do we find a projected overlap between Hii regions and H 2 O and OH maser spots. On the other hand, the VLA images detect eight ammonia cores closely associated with the maser sources. The ammonia cores have sizes of ∼10 4 AU, and high masses (up to 10 4 M ), and are very dense (from ∼10 6 to a few ×10 9 cm −3 ). The typical relative NH 3 abundance is ≤10 −7 , in agreement with previous measurements in highmass star-forming regions.Conclusions. The statistical analysis of the distribution between H 2 O and OH masers, NH 3 cores, and Hii regions confirms that the earliest stages of high-mass star formation are characterised by high-density molecular cores with temperatures of on average ≥30 K, either without a detectable ionised region or associated with a hypercompact Hii region.

Filamentary structure and Keplerian rotation in the high-mass star-forming region G35.03+0.35 imaged with ALMA

“…The corrected mass is M / sin 2 i. The estimated range of values of M is consistent with the mass of a main-sequence star of spectral type B1 (10.7 M ): i.e., the spectral type estimated for CM2, which is the centimeter continuum source associated with core A, from the 1.3 cm continuum emission assuming that it is optically thin (Codella et al 2010) and following the tables of Davies et al (2011) and Mottram et al (2011). As already mentioned in Sect.…”

“…This value is greater than the stellar luminosity of 6.3 × 10 3 L estimated for the centimeter continuum source CM2 that is associated with core A. This stellar luminosity estimate has been obtained from the 1.3 cm emission, assuming that it is optically thin (Codella et al 2010) and following the tables of Davies et al (2011) and Mottram et al (2011), which tabulate the ZAMS stellar parameters (temperature, luminosity, and radius) and the spectral type as a function of mass and Lyman photon flux. These authors used the calculations of Martins et al (2005) However, the SED was computed by integrating the emission over a region of ∼2 , which also includes the emission of the UC Hii region CM1.…”

“…These authors used the calculations of Martins et al (2005) However, the SED was computed by integrating the emission over a region of ∼2 , which also includes the emission of the UC Hii region CM1. According to Codella et al (2010), the 1.3 cm flux density of CM1 would be consistent with a single ionizing star of spectral type B0.5, corresponding to a stellar luminosity of 1.1 × 10 4 L . Therefore, the stellar luminosity of the whole star-forming region would be ∼1.7 × 10 4 L , which is consistent with the value obtained by integrating the SED.…”

“…The G35.03 region has been studied in several molecular high-density tracers, such as NH 3 (Harju et al 1993;Codella et al 2010;, CS (Anglada et al 1996;Bronfman et al 1996;Larionov et al 1999), H 13 CO + , and CH 3 OH (Cyganowski et al 2009). Interferometric centimeter continuum observations have revealed the presence of at least five compact sources see Fig.…”

“…10). Cyganowski et al (2011) classify the centimeter source CM2 as a hypercompact (HC) Hii region, with its 1.3 cm flux density being consistent with a single ionizing star of spectral type B1 based on the assumption of optically thin emission (Codella et al 2010). This makes G35.03 one of the few EGOs associated with an HC Hii region (Cyganowski et al 2008).…”