A Multilevel Study of Ammonia in Star Forming Regions - Part Five - the SAGITTARIUS-B2 Region

Details A Multilevel Study of Ammonia in Star Forming Regions - Part Five - the SAGITTARIUS-B2 Region A Multilevel Study of Ammonia in Star Forming Regions - Part Five - the SAGITTARIUS-B2 Region

Sep 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...276..445h&link_type=abstract

Astronomy and Astrophysics, Vol.276, NO. 2/SEPII, P. 445, 1993

Astronomy and Astrophysics

Astrophysics

61

Galaxy, Center, Interstellar Medium, Abundances, Ism, Individual, Sgrb2, Molecules, Radio Lines, Interstellar, Stars, Formation

Scientific paper

Maps of the Sgr B2 region covering up to 4' have been made in 5 metastable (J = K) and 4 non-metastable (J > K) NH3 inversion transitions with a 40" resolution. Toward Sgr B2 (M) and (N), spectra of 20 additional inversion transitions have been taken arising from energy levels up to E/k=1340 K above ground. From the NH3 data, we obtained rotational and kinetic temperatures (Tkin), cloud sizes, virial masses, NH3 column densities, spatial densities (n(H2)), and NH3 abundances relative to H2 (X(NH3)).
In addition to a cool (Tkin˜20K) component detected at -40km s-1 (presumably a foreground cloud associated with the 3 kpc arm), seven regions containing gas with Tkin≥100 K are identified. All are part of the Sgr B2 molecular complex. (1) A low density (n(H2) ˜1O4 cm-3) molecular cloud at υLSR=5O to 80 km s-1 is shown to be extended over several arcmin in metastable transitions up to at least the (6,6) line. A second cooler component (Trot ˜ 20K) appears to coexist with the warm gas. Together, their area filling factor is almost one. Of all mapped lines from other molecular species, only HC3N emission from the vibrational ground state may arise from the same two gas components. (2) Another warm low density feature, at υLSR ˜ 85 km s-1, is detected in the metastable lines toward Sgr B2 (N) and becomes increasingly prominent as J rises. (3) Toward the peak positions Sgr B2 (M) and (N) the metastable lines with J ≤ 5 show an absorption feature at υLSR ˜ 0 km s-1. (4) A region with υLSR ˜ 65 km s-1 has n(H2) 105 cm-3 and is observed in non-metastable NH3 emission lines. It is centered along a ridge 10" west of the main continuum sources. (5) Three Ori-KL like hot cores are identified, one at υLSR ˜65 km s-1 toward SgrB2 (N) and two, at 55 and 65km s-1 toward Sgr B2(M). Provided that the NH3 inversion lines are thermalized and that the clouds are approximately in virial equilibrium, these are the three most massive hot cores known to exist in the galaxy. With NH3 column densities of order 1021 cm-2, the 55 km s-1 SgrB2 (M) and the 65 km s-1 Sgr B2 (N) cores are particularly outstanding. X(NH3) > 10-5, suggesting that a substantial fraction of the nitrogen is in ammonia. The large number of ultracompact HII (UC HII) regions (≥- 19) indicates that the lifetime of a hot core is only ˜10% of the lifetime of an UC HII region.

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