Browsing by Subject "stars: pre-main-sequence"

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New X-Ray-Selected Pre-Main-Sequence Members Of The Serpens Molecular Cloud
(2013-11) Oliveira, Isa; van der Laan, Margriet; Brown, Joanna M.; Oliveira, Isa
The study of young stars no longer surrounded by disks can greatly add to our understanding of how protoplanetary disks evolve and planets form. We have used VLT/FLAMES optical spectroscopy to confirm the youth and membership of 19 new young diskless stars in the Serpens Molecular Cloud, identified at X-ray wavelengths. Spectral types, effective temperatures, and stellar luminosities were determined using optical spectra and optical/near-infrared photometry. Stellar masses and ages were derived based on pre-main-sequence evolutionary tracks. The results yield remarkable similarities for age and mass distribution between the diskless and disk-bearing stellar populations in Serpens. We discuss the important implications these similarities may have on the standard picture of disk evolution.
The Spitzer C2D Survey of Large, Nearby, Interstellar Clouds. XI. Lupus Observed With IRAC and MIPS
(2008-08) Merin, Bruno; Jorgensen, Jes; Spezzi, Loredana; Alcala, Juan M.; Evans, Neal J., II; Harvey, Paul M.; Prusti, Timo; Chapman, Nicholas; Huard, Tracy; van Dishoeck, Ewine F.; Comeron, Fernando; Evans, Neal J., II; Harvey, Paul M.
We present c2d Spitzer IRAC observations of the Lupus I, III, and IV dark clouds and discuss them in combination with optical, near-infrared, and c2d MIPS data. With the Spitzer data, the new sample contains 159 stars, 4 times larger than the previous one. It is dominated by low- and very low mass stars, and it is complete down to M approximate to 0.1 M-circle dot. We find 30%-40% binaries with separations between 100 and 2000 AU with no apparent effect on the disk properties of the members. A large majority of the objects are Class II or III objects, with only 20 (12%) Class I or flat-spectrum sources. The disk sample is complete down to "debris" -like systems in stars as small as M approximate to 0.2 M-circle dot and includes substellar objects with larger IR excesses. The disk fraction in Lupus is 70%-80%, consistent with an age of 1-2 Myr. However, the young population contains 20% optically thick accretion disks and 40% relatively less flared disks. A growing variety of inner disk structures is found for larger inner disk clearings for equal disk masses. Lupus III is themost centrally populated and rich, followed by Lupus I with a filamentary structure and by Lupus IV, where a very high density core with little star Formation activity has been found. We estimate star Formation rates in Lupus of 2-10 M-circle dot Myr(-1) and star Formation efficiencies of a few percent, apparently correlated with the associated cloud masses.
The TEXES Survey For H-2 Emission From Protoplanetary Disks
(2008-12) Bitner, Martin A.; Richter, Matthew J.; Lacy, John H.; Herczeg, Gregory J.; Greathouse, Thomas K.; Jaffe, Daniel T.; Salyk, Colette; Blake, Geoffrey A.; Hollenbach, David J.; Doppmann, Greg W.; Najita, Joan R.; Currie, Thayne; Bitner, Martin A.; Richter, Matthew J.; Lacy, John H.; Herczeg, Gregory J.; Greathouse, Thomas K.; Jaffe, Daniel T.; Salyk, Colette; Blake, Geoffrey A.; Hollenbach, David J.; Doppmann, Greg W.; Najita, Joan R.; Currie, Thayne
We report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H-2 emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC observations of the H-2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H-2 emission from 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. In these cases, we show that accretion shock heating is a plausible excitation mechanism. The detected emission lines are narrow (similar to 10 km s(-1)), centered at the stellar velocity, and spatially unresolved at scales of 0.4 '', which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we derive temperatures greater than or similar to 500 K from similar to 1 M-circle plus of gas. Our upper limits for the nondetections place upper limits on the amount of H-2 gas with T > 500 K of less than a few Earth masses. Such warm gas temperatures are significantly higher than the equilibrium dust temperatures at these radii, suggesting that the gas is decoupled from the dust in the regions that we are studying and that processes such as UV, X-ray, and accretion heating may be important.