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

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High-Resolution Far-Infrared Studies Of Intermediate-Mass Pre-Main-Sequence Objects
(1998-12) Di Francesco, James; Evans, Neal J.; Harvey, Paul M.; Mundy, Lee G.; Butner, Harold M.; Di Francesco, James; Evans, Neal J.; Harvey, Paul M.
We have obtained high-resolution far-infrared maps of nine regions with 10 Herbig Ae/Be stars (intermediate-mass pre-main-sequence stars). Similar maps were obtained for 10 embedded IRAS sources with S-v(100 mu m) > S-v(60 mu m) and L similar to 200 L., which are possible evolutionary precursors of Herbig Ae/Be stars. Single far-infrared sources were found in most maps. The embedded sources have positions in agreement with those of the IRAS PSC, but some of the Herbig Ae/Be stars are offset significantly from the position of peak far-infrared emission. For all objects where it was possible to obtain 100 mu m flux densities, they are consistent with those observed by IRAS, but derived 50 mu m Aux densities are larger than expected. The far-infrared maps reveal that objects in at least 17 of 19 emission regions are significantly extended at the 30 "-40 " resolution of the Kuiper Airborne Observatory at 100 mu m. Only sources associated with AB Aur and possibly IRAS 05338-0624 have unresolved far-infrared emission. Detailed analyses of the flux densities and positions from our maps suggest the far-infrared emission in regions with Herbig Ae/Be stars may not immediately surround these stars in all cases. Instead, far-infrared emission from these objects may originate from dust heated externally by the Herbig stars, or from dust heated internally by other sources. For other objects arguably surrounded by far-infrared emission, the Herbig stars or embedded IRAS objects have similar mean deconvolved sizes (i.e., 0.10-0.15 pc), but possibly have different mean deconvolved shapes (i.e., aspect ratios). Thus, far-infrared emission here may originate from flattened dust envelopes; the appearance of a far-infrared object as either a Herbig Ae/Be star or an embedded IRAS source may be merely a matter of viewing orientation.
Mid-Infrared Photometry And Spectra Of Three High-Mass Protostellar Candidates At IRAS 18151-1208 And IRAS 20343+4129
(2008-02) Campbell, M. F.; Sridharan, T. K.; Beuther, H.; Lacy, John H.; Hora, J. L.; Zhu, Q.; Kassis, M.; Saito, M.; De Buizer, J. M.; Fung, S. H.; Johnson, L. C.; Lacy, John H.
We present arcsecond-scale mid-IR photometry ( in the 10.5 mu m N band and at 24.8 mu m), and low-resolution spectra in the N band ( R similar or equal to 100) of a candidate high-mass protostellar object ( HMPO) in IRAS 18151-1208 and of two HMPO candidates in IRAS 20343+4129, IRS 1 and IRS 3. In addition, we present high-resolution mid-IR spectra (R similar or equal to 80, 000) of the two HMPO candidates in IRAS 20343+4129. These data are fitted with simple models to estimate the masses of gas and dust associated with the mid-IR-emitting clumps, the column densities of overlying absorbing dust and gas, the luminosities of the HMPO candidates, and the likely spectral type of the HMPO candidate for which [Ne II] 12.8 mu m emission was detected ( IRAS 20343+4129 IRS 3). We suggest that IRAS 18151-1208 is a pre-ultracompact H II region HMPO, IRAS 20343+4129 IRS 1 is an embedded young stellar object with the luminosity of a B3 star, and IRAS 20343+4129 IRS 3 is a B2 ZAMS star that has formed an ultracompact H II region and disrupted its natal envelope.
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.
The Young Population Of The Chamaeleon II Dark Cloud
(2008-06) Spezzi, Loredana; Alcala, Juan M.; Covino, Elvira; Frasca, Antonio; Gandolfi, Davide; Oliveira, Isa; Chapman, Nicholas; Evans, Neal J.; Huard, Tracy L.; Jorgensen, Jes K.; Merin, Bruno; Stapelfeldt, Karl R.; Evans, Neal J.
We discuss the results of the optical spectroscopic follow-up of pre-main-sequence (PMS) objects and candidates selected in the Chamaeleon II dark cloud based on data from the Spitzer Legacy survey "From Molecular Cores to Planet Forming Disks" (c2d) and from previous surveys. Our sample includes both objects with infrared excess selected according to c2d criteria and referred to as young stellar objects and other cloud members and candidates selected from complementary optical and near-infrared data. We characterize the sample of objects by deriving their physical parameters. The vast majority of objects have masses M <= 1 M(circle dot) and ages < 6 Myr. Several of the PMS objects and candidates lie very close to or below the hydrogen-burning limit. A first estimate of the slope of the initial mass function in Cha II is consistent with that of other T associations. The star formation efficiency in the cloud (1%-4%) is consistent with our own estimates for Taurus and Lupus, but significantly lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds may reflect a different history of star formation. We also find that the Cha II cloud is turning some 8 M(circle dot) into stars every megayear, which is less than the star formation rate in the other c2d clouds. However, the star formation rate is not steady and evidence is found that the star formation in Cha II might have occurred very rapidly. The H alpha emission of the Cha II PMS objects, as well as possible correlations between their stellar and disk properties, is also investigated.