Browsing by Subject "mass-loss rates"

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A Detailed Analysis Of The Dust Formation Zone Of Irc+10216 Derived From Mid-Infrared Bands Of C2H2 And HCN
(2008-01) Fonfria, J. P.; Cernicharo, J.; Richter, Matthew J.; Lacy, John H.; Lacy, John H.
A spectral survey of IRC + 10216 has been carried out in the range 11-14 mu m with a spectral resolution of about 4 km s(-1). We have identified a forest of lines in six bands of C2H2 involving the vibrational states from the ground to 3 nu(5) and in two bands of HCN, involving the vibrational states from the ground up to 2 nu(2). Some of these transitions are observed also in (HCCH)-C-13 and (HCN)-C-13. We have estimated the kinetic, vibrational, and rotational temperatures and the abundances and column densities of C2H2 and HCN between 1R* and 300R* (similar or equal to 1.5 x 10(16) cm) by fitting about 300 of these rovibrational lines. The envelope can be divided into three regions with approximate boundaries at 0.019" (the stellar photosphere), 0.1" (the inner dust formation zone), and 0.4" (outer dust formation zone). Most of the lines might require a large microturbulence broadening. The derived abundances of C2H2 and HCN increase by factors of 10 and 4, respectively, from the innermost envelope outward. The derived column densities for both C2H2 and HCN are similar or equal to 1.6 x 10(19) cm(-2). Vibrational states up to 3000 K above ground are populated, suggesting pumping by near-infrared radiation from the star and innermost envelope. Low rotational levels can be considered under LTE, while those with J > 20-30 are not thermalized. A few lines require special analysis to deal with effects like overlap with lines of other molecules.
A H I Imaging Survey Of Asymptotic Giant Branch Stars
(2013-04) Matthews, L. D.; Le Bertre, T.; Gerard, E.; Johnson, M. C.; Johnson, M. C.
We present an imaging study of a sample of eight asymptotic giant branch stars in the H I 21 cm line. Using observations from the Very Large Array, we have unambiguously detected H I emission associated with the extended circumstellar envelopes of six of the targets. The detected H I masses range from M-H (I) approximate to 0.015-0.055 M-circle dot. The Hi morphologies and kinematics are diverse, but in all cases appear to be significantly influenced by the interaction between the circumstellar envelope and the surrounding medium. Four stars (RX Lep, Y UMa, Y CVn, and V1942 Sgr) are surrounded by detached H I shells ranging from 0.36 to 0.76 pc across. We interpret these shells as resulting from material entrained in a stellar outflow being abruptly slowed at a termination shock where it meets the local medium. RX Lep and TX Psc, two stars with moderately high space velocities (V-space > 56 km s(-1)), exhibit extended gaseous wakes (similar to 0.3 and 0.6 pc in the plane of the sky), trailing their motion through space. The other detected star, R Peg, displays a peculiar >horseshoe-shaped> H I morphology with emission extended on scales up to similar to 1.7 pc; in this case, the circumstellar debris may have been distorted by transverse flows in the local interstellar medium. We briefly discuss our new results in the context of the entire sample of evolved stars that has been imaged in H I to date.
The Spitzer c2d Survey Of Large, Nearby, Interstellar Clouds. X. The Chamaeleon II Pre-Main-Sequence Population As Observed With IRAC And MIPS
(2008-03) Alcala, Juan M.; Spezzi, Loredana; Chapman, Nicholas; Evans, Neal J.; Huard, Tracy L.; Jorgensen, Jes K.; Merin, Bruno; Stapelfeldt, Karl R.; Covino, E.; Frasca, Antonio; Gandolfi, Davide; Oliveira, Isa; Evans, Neal J.
We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 M-circle dot). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M-circle dot pc(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8 '' < theta < 6.0 '' (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Delta t similar to 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M-circle dot into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered.