Chemical tracers in proto-brown dwarfs: CO, ortho-H2CO, para-H2CO, HCO+, CS observations. Issue 1 (15th November 2018)

• Record Type: Journal Article
• Authors: Riaz, B; Thi, W-F; Caselli, P
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 483:Issue 1(2019)
• Page Start: 1139
• Page End: 1157
• Publication Date: 2018-11-15
• Subjects: astrochemistry -- brown dwarfs -- stars: evolution -- stars: formation -- ISM: abundances -- ISM: molecules; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty3111
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We present a study of the CO isotopologues and the high-density tracers H2 CO, HCO +, and CS in Class 0/I proto-brown dwarfs (proto-BDs). We have used the IRAM 30m telescope to observe the 12 CO (2–1), 13 CO (2–1), C 18 O (2–1), C 17 O (2–1), H2 CO (3–2), HCO + (3–2), and CS (5–4) lines in seven proto-BDs. The hydrogen column density for the proto-BDs derived from the CO gas emission is ∼2–15 times lower than that derived from the dust continuum emission, indicating CO depletion from the gas-phase. The mean H2 CO ortho-to-para ratio is ∼3 for the proto-BDs and indicates gas-phase formation for H2 CO. We have investigated the correlations in the molecular abundances between the proto-BDs and protostars. Proto-BDs on average show a factor of ∼2 higher ortho-to-para H2 CO ratio than the protostars. Possible explanations include a difference in the H2 CO formation mechanism, spin-selective photodissociation, self-shielding effects, or different emitting regions for the ortho and para species. There is a tentative trend of a decline in the HCO + and H2 CO abundances with decreasing bolometric luminosity, while the CS and CO abundances show no particular difference between the proto-BDs and protostars. These trends reflect the scaled-down physical structures for the proto-BDs compared to protostars and differences in the peak emitting regions for these species. The C 17 O isotopologue is detected in all of the proto-BDs as well as the more evolved Class Flat/Class II BDs in our sample, and can probe the quiescent gas at both early and late evolutionary stages.