Astrochemistry and the Birth of Massive Stars

The process that leads to the formation of a high-mass star is not clearly understood but does occur extremely fast, ~ 104-105 yrs, and according to some evolutionary models, it depends on the final mass of the star (e.g., Bernasconi & Maeder 1996). The contraction of a core from a low to a very high density ( 107 cm−3) occurs very quickly and the star will reach the Zero Age Main Sequence while still embedded (Palla & Stahler 1993). This, together with the fact that massive stars tend to form in association, makes the determination of the early stages of the evolution of a massive star via observations of Spectral Energy Distributions (SEDs) a rather difficult task. 

The earliest phases of massive star formation can however be traced using sub mm observations of molecules in hot molecular cores. Hot molecular cores are small (10−2 – 10−1 pc), dense ( > 107 cm−3), relatively warm ( AV > 102 K), optically thick (Av>102 mag), and transient (6105 yr) objects, where massive stars are forming. Observations of molecules in these objects probe both temperature and density conditions around the forming star, as well as the physical structures that are typically associated with massive star formation (e.g. outflows, masers, rotation).

Spectral surveys have revealed a very rich chemistry around hot molecular cores (see review by Herbst & van Dishoeck (2009); Garay & Lizano (1999)), which include high abundances of small saturated molecules (e.g. H2O, NH3, H2S, CH3OH) as well as large organic species (CH3CN, CH2CHCN, CH2(OH)CHO), CH3CH2CN, C2H5OH). In particular we can use high resolution observations of these large 'complex' molecules to probe the hottest, most dense regions close to where the star is forming (Figure 1, Calcutt et al. 2013). 

Hot molecular cores

Figure 1: Methyl formate (HCOOCH3, blue contours), glycolaldehyde (CH2(OH)CHO, white contours), and methyl cyanide (CH3CN, colour) which trace the most compact region of hot molecular cores, where massive stars form.

Page last modified on 16 jan 14 14:52 by Amira K F Val Baker