THE FAR-INFRARED LINE SPECTRUM OF THE PROTOSTAR IRAS-16293-2422

Citation
C. Ceccarelli et al., THE FAR-INFRARED LINE SPECTRUM OF THE PROTOSTAR IRAS-16293-2422, Astronomy and astrophysics, 331(1), 1998, pp. 372-382
Citations number
48
Categorie Soggetti
Astronomy & Astrophysics
Journal title
ISSN journal
00046361
Volume
331
Issue
1
Year of publication
1998
Pages
372 - 382
Database
ISI
SICI code
0004-6361(1998)331:1<372:TFLSOT>2.0.ZU;2-W
Abstract
We report mid-IR wavelength observations toward the low mass star form ing region IRAS 16293-2422 between 45 mu m - 197 mu m with the Long Wa velength Spectrometer (LWS) on board ISO, and of the CI(609 mu m) line observed with the JCMT. A map of the CII(157 mu m) line shows that th e region is relatively uncontaminated by Photo-Dissociation Region-lik e emission; there is only weak diffuse CII emission, which results fro m the illumination of the cloud by a faint UV field (G(o) similar to 6 ). The observed CI(609 mu m) line intensity and narrow profile is cons istent with this interpretation. On-source, the LWS detected the OI(63 mu m) and several molecular lines. In this work we report and discuss in detail the lines which dominate the 43 mu m - 197 mu m spectrum, n amely CO, H2O and OH rotational lines and the OI(63 mu m) fine-structu re line. Combining the CO J(up)=14 to 25 observations with previous J( up)=6 measurements, we derive stringent limits on the density (similar to 3 . 10(4)cm(-3)), temperature (similar to 1500 K), and column dens ity (similar to 1.5 . 10(20)cm(-2)) of the emitting gas. We show that this warm gas is associated with the outflow and that a low velocity, C-type shock can account for the characteristics of the CO spectrum. I f the observed H2O and OH lines originate in the same region where the CO lines originate, the H2O and OH abundance derived from the observe d lines is [H2O] / [H-2] similar to 2.10(-5) and [OH] / [H-2] similar to 5.10(-6) respectively. Given the relatively high temperature of the emitting gas, standard chemistry would predict all the gas-phase oxyg en to be in water. The relatively low water abundance we observed may mean either that most of the oxygen is locked into grains or that the time scale required to convert the gas-phase oxygen into water is high er that the outflow time scale, or both. The relatively high abundance of OH with respect to H2O gives support to the latter hypothesis. Fin ally, we speculate that the OI(63 mu m) line emission originates in th e collapsing envelope that surrounds the central object. The successfu l comparison of the observed flux with model predictions of collapsing envelopes gives a mass accretion rate toward the central object great er than or equal to 3.10(-5)M. yr(-1) and an accretion shock radius la rger than three times the protostar radius.