ATLASGAL-selected massive clumps in the inner Galaxy

Abstract

Context. The processes leading to the birth of high-mass stars are poorly understood. The key first step to reveal their formation processes is characterising the clumps and cores from which they form. Aims. We define a representative sample of massive clumps in different evolutionary stages selected from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), from which we aim to establish a census of molecular tracers of their evolution. As a first step, we study the shock tracer, SiO, mainly associated with shocks from jets probing accretion processes. In low-mass young stellar objects (YSOs), outflow and jet activity decreases with time during the star formation processes. Recently, a similar scenario was suggested for massive clumps based on SiO observations. Here we analyse observations of the SiO (2−1) and (5−4) lines in a statistically significant sample to constrain the change of SiO abundance and the excitation conditions as a function of evolutionary stage of massive star-forming clumps. Methods. We performed an unbiased spectral line survey covering the 3-mm atmospheric window between 84−117 GHz with the IRAM 30 m telescope of a sample of 430 sources of the ATLASGAL survey, covering various evolutionary stages of massive clumps. A smaller sample of 128 clumps has been observed in the SiO (5−4) transition with the APEX telescope to complement the (2−1) line and probe the excitation conditions of the emitting gas. We derived detection rates to assess the star formation activity of the sample, and we estimated the column density and abundance using both an LTE approximation and non-LTE calculations for a smaller subsample, where both transitions have been observed. Results. We characterise the physical properties of the selected sources, which greatly supersedes the largest samples studied so far, and show that they are representative of different evolutionary stages. We report a high detection rate of >75% of the SiO (2−1) line and a >90% detection rate from the dedicated follow-ups in the (5−4) transition. Up to 25% of the infrared-quiet clumps exhibit high-velocity line wings, suggesting that molecular tracers are more efficient tools to determine the level of star formation activity than infrared colour criteria. We also find infrared-quiet clumps that exhibit only a low-velocity component (FWHM ∼ 5−6 km s −1 ) SiO emission in the (2−1) line. In the current picture, where this is attributed to low-velocity shocks from cloud-cloud collisions, this can be used to pinpoint the youngest, thus, likely prestellar massive structures. Using the optically thin isotopologue ( 29 SiO), we estimate that the (2−1) line is optically thin towards most of the sample. Furthermore, based on the line ratio of the (5−4) to the (2−1) line, our study reveals a trend of changing excitation conditions that lead to brighter emission in the (5−4) line towards more evolved sources. Our models show that a proper treatment of non-LTE effects and beam dilution is necessary to constrain trends in the SiO column den...

The ATLASGAL survey: The sample of young massive cluster progenitors

“…This classification is consistent, but simplified compared to previous works (e.g. Csengeri et al 2014;Giannetti et al 2014;Csengeri et al 2016b). In addition, it quantitatively selects the young clumps based on the luminosArticle number, page 6 of 29 ity of the embedded protostar(s).…”

“…This corresponds well to the average dust temperature also used in various studies of ATLASGAL selected sources (e.g. Csengeri et al 2016b).…”

“…Various samples have been studied from the ATLASGAL survey, mostly based on associations with H II regions and sources from the Red MSX Survey (RMS, Lumsden et al 2013), and their flux densities at 8 µm (GLIMPSE, Benjamin et al 2003), and 24 µm (MIPS-GAL, Carey et al 2009). Focusing on the I st Galactic quadrant, Csengeri et al (2016b) present the largest sample to date of 430 clumps in different evolutionary stages over a wide mass range. The sample studied in Giannetti et al (2014) represents a subsample of this source selection limited to the hundred brightest sources in different evolutionary stages in both the I st and the IV th Galactic quadrant.…”

“…The sample studied in Giannetti et al (2014) represents a subsample of this source selection limited to the hundred brightest sources in different evolutionary stages in both the I st and the IV th Galactic quadrant. This strategic selection of ATLAS-GAL clumps over various evolutionary stages has been targeted by extensive spectroscopic follow-up observations; see details in Csengeri et al (2016b). Other samples using selective star formation tracers of more evolved objects, such as clumps with H II regions, RMS sources, and associations with masers from the Methanol MultiBeam survey, have been discussed in a series of papers by Urquhart et al (2013aUrquhart et al ( ,b, 2014.…”

Molecular Line Emission Towards High-Mass Clumps: The MALT90 Catalogue

“…datasets have allowed detailed studies, in particular, towards the latter phases in evolution where high-mass protostars and H II regions have formed (e.g., Urquhart et al 2013Urquhart et al , 2014bTremblay et al 2015;Csengeri et al 2016).…”