Browsing by Subject "stars: protostars"

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CO2 Ice Toward Low-Luminosity Embedded Protostars: Evidence For Episodic Mass Accretion Via Chemical History
(2012-10) Kim, Hyo Jeeong; Evans, Neal J.; Dunham, Michael M.; Lee, Jeong-Eun; Pontoppidan, Klaus M.; Kim, Hyo Jeeong; Evans, Neal J.
We present Spitzer IRS spectroscopy of CO2 ice bending mode spectra at 15.2 mu m toward 19 young stellar objects (YSOs) with luminosity lower than 1L(circle dot) (3 with luminosity lower than 0.1 L-circle dot). Ice on dust grain surfaces can encode the history of heating because pure CO2 ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1L(circle dot) do not provide the conditions needed to produce pure CO2 ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO2 ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO2 component, traced by the presence of a characteristic band splitting in the 15.2 mu m bending mode. About half of the sources (9 out of 19) in the low-luminosity sample have evidence for pure CO2 ice, and 6 of these have significant double-peaked features, which are very strong evidence of pure CO2 ice. The presence of the pure CO2 ice component indicates that the dust temperature, and hence luminosity of the central star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO2 ice is converted to pure CO2 ice during each high-luminosity phase, explains the presence of pure CO2 ice, the total amount of CO2 ice, and the observed residual (CO)-O-18 gas.
Dust, Ice, and Gas in Time (DIGIT) Herschel Observations of GSS30-IRS1 in Ophiuchus
(2015-03) Je, Hyerin; Lee, Jeong-Eun; Lee, Seokho; Green, Joel D.; Evans, Neal J., II; ee, Jeong-Eun; Green, Joel D.; Evans, Neal J., II
As a part of the "Dust, Ice, and Gas In Time" (DIGIT) key program on Herschel, we observed GSS30-IRS1, a Class I protostar located in Ophiuchus (d = 120 pc), with Herschel/Photodetector Array Camera and Spectrometer. More than 70 lines were detected within a wavelength range from 50 to 200 mu m, including CO, H2O, OH, and two atomic [O I] lines at 63 and 145 mu m. The [C II] line, known as a tracer of externally heated gas by the interstellar radiation field (ISRF), is also detected at 158 mu m. All lines, except [O I] and [C II], are detected only at the central spaxel of 9 ''.4 x 9 ''.4. The [O I] emissions are extended along a NE-SW orientation, and the [C II] line is detected over all spaxels, indicative of an external photodissociation region. The total [C II] intensity around GSS30 reveals that the far-ultraviolet radiation field is in the range of 3 to 20 G(0), where G(0) is in units of the Habing Field, 1.6 x 10(-3) erg cm(-2) s(-1). This enhanced external radiation field heats the envelope of GSS30-IRS1, causing the continuum emission to be extended, unlike the molecular emission. The best-fit continuum model of GSS30-IRS1 with the physical structure including flared disk, envelope, and outflow shows that the internal luminosity is 10 L-circle dot, and the region is externally heated by a radiation field enhanced by a factor of 130 compared to the standard local ISRF.
Evidence For Decay Of Turbulence By MHD Shocks In The ISM via CO Emission
(2015-06) Larson, Rebecca L.; Evans, Neal J.; Green, Joel D.; Yang, Lao-Lun; Larson, Rebecca L.; Evans, Neal J.; Green, Joel D.; Yang, Lao-Lun
We utilize observations of sub-millimeter rotational transitions of CO from a Herschel Cycle 2 open time program ("COPS", PI: J. Green) to identify previously predicted turbulent dissipation by magnetohydrodynamic (MHD) shocks in molecular clouds. We find evidence of the shocks expected for dissipation of MHD turbulence in material not associated with any protostar. Two models fit about equally well: model 1 has a density of 10(3) cm(-3), a shock velocity of 3 km s(-1), and a magnetic field strength of 4 mu G; model 2 has a density of 10(3.5) cm(-3), a shock velocity of 2 km s(-1), and a magnetic field strength of 8 mu G. Timescales for decay of turbulence in this region are comparable to crossing times. Transitions of CO up to J of 8, observed close to active sites of star formation, but not within outflows, can trace turbulent dissipation of shocks stirred by formation processes. Although the transitions are difficult to detect at individual positions, our Herschel-SPIRE survey of protostars provides a grid of spatially distributed spectra within molecular clouds. We averaged all spatial positions away from known outflows near seven protostars. We find significant agreement with predictions of models of turbulent dissipation in slightly denser (10(3.5) cm(-3)) material with a stronger magnetic field (24 mu G) than in the general molecular cloud.
High Resolution Optical And NIR Spectra Of HBC 722
(2015-07) Lee, Jeong-Eun; Park, Sunkyung; Green, Joel D.; Cochran, William D.; Kang, Wonseok; Lee, Sang-Gak; Sung, Hyun-Il; Lee, Jeong-Eun; Green, Joel D.; Cochran, William D.
We present the results of high resolution (R >= 30,000) optical and near-IR spectroscopic monitoring observations of HBC 722, a recent FU Orionis object that underwent an accretion burst in 2010. We observed HBC 722 in the optical/near-IR with the Bohyunsan Optical Echelle Spectrograph, Hobby-Eberly Telescope-HRS, and Immersion Grating Infrared Spectrograph, at various points in the outburst. We found atomic lines with strongly blueshifted absorption features or P Cygni profiles, both evidence of a wind driven by the accretion. Some lines show a broad double-peaked absorption feature, evidence of disk rotation. However, the wind-driven and disk-driven spectroscopic features are anti-correlated in time; the disk features became strong as the wind features disappeared. This anti-correlation might indicate that the rebuilding of the inner disk was interrupted by the wind pressure during the first 2 years. The half-width at half-depth of the double-peaked profiles decreases with wavelength, indicative of the Keplerian rotation; the optical spectra with the disk feature are fitted by a G5 template stellar spectrum convolved with a rotation velocity of 70 km s(-1) while the near-IR disk features are fitted by a K5 template stellar spectrum convolved with a rotation velocity of 50 km s(-1). Therefore, the optical and near-IR spectra seem to trace the disk at 39 and 76 R-circle dot, respectively. We fit a power-law temperature distribution in the disk, finding an index of 0.8, comparable to optically thick accretion disk models.
Infrared and Radio Observations of a Small Group of Protostellar Objects in the Molecular Core, L1251-C
(2015-05) Kim, Jungha; Lee, Jeong-Eun; Choi, Minho; Bourke, Tyler L.; Evans, Neal J., II; Di Francesco, James; Cieza, Lucas A.; Dunham, Miranda M.; Kang, Miju; Lee, Jeong-Eun; Evans, Neal J., II
We present a multi-wavelength observational study of a low-mass star-forming region, L1251-C, with observational results at wavelengths from the near-infrared to the millimeter. Spitzer Space Telescope observations confirmed that IRAS 22343+7501 is a small group of protostellar objects. The extended emission in the east-west direction with its intensity peak at the center of L1251A has been detected at 350 and 850 mu m with the Caltech Submillimeter Observatory and James Clerk Maxwell telescopes, tracing dense envelope material around L1251A. The single-dish data from the Korean VLBI Network and TRAO telescopes show inconsistencies between the intensity peaks of several molecular emission lines and that of the continuum emission, suggesting complex distributions of molecular abundances around L1251A. The Submillimeter Array interferometer data, however, show intensity peaks of CO 2-1 and (CO)-C-13 2-1 located at the position of IRS 1, which is both the brightest source in the Infrared Array Camera image and the weakest source in the 1.3 mm dust-continuum map. IRS 1 is the strongest candidate for the driving source of the newly detected compact CO 2-1 outflow. Over the entire region (14' x 14') of L125l-C, 3 Class I and 16 Class II sources have been detected, including three young stellar objects (YSOs) in L1251A. A comparison between the average projected distance among the 19 YSOs in L1251-C and that among the 3 YSOs in L1251A suggests that L1251-C is an example of low-mass cluster Formation where protostellar objects form in a small group.
The Luminosities Of Protostars In The Spitzer c2d And Gould Belt Legacy Clouds
(2013-04) Dunham, Michael M.; Arce, Hector G.; Allen, Lori E.; Evans, Neal J.; Broekhoven-Fiene, Hannah; Chapman, Nicholas L.; Cieza, Lucas A.; Gutermuth, Robert A.; Harvey, Paul M.; Hatchell, Jennifer; Huard, Tracy L.; Kirk, Jason M.; Matthews, Brenda C.; Merin, Bruno; Miller, Jennifer F.; Peterson, Dawn E.; Spezzi, Loredana; Evans, Neal J.
Motivated by the long-standing >luminosity problem> in low-mass star formation whereby protostars are underluminous compared to theoretical expectations, we identify 230 protostars in 18 molecular clouds observed by two Spitzer Space Telescope Legacy surveys of nearby star-forming regions. We compile complete spectral energy distributions, calculate L-bol for each source, and study the protostellar luminosity distribution. This distribution extends over three orders of magnitude, from 0.01 L-circle dot to 69 L-circle dot, and has a mean and median of 4.3 L-circle dot and 1.3 L-circle dot, respectively. The distributions are very similar for Class 0 and Class I sources except for an excess of low luminosity (L-bol <= 0.5 L-circle dot) Class I sources compared to Class 0. 100 out of the 230 protostars (43%) lack any available data in the far-infrared and submillimeter (70 mu m < lambda < 850 mu m) and have L-bol underestimated by factors of 2.5 on average, and up to factors of 8-10 in extreme cases. Correcting these underestimates for each source individually once additional data becomes available will likely increase both the mean and median of the sample by 35%-40%. We discuss and compare our results to several recent theoretical studies of protostellar luminosities and show that our new results do not invalidate the conclusions of any of these studies. As these studies demonstrate that there is more than one plausible accretion scenario that can match observations, future attention is clearly needed. The better statistics provided by our increased data set should aid such future work.
Revealing The Millimeter Environment Of The New FU Orionis Candidate HBC722 With The Submillimeter Array
(2012-08) Dunham, Michael M.; Arce, Hector G.; Bourke, Tyler L.; Chen, Xuepeng; van Kempen, Tim A.; Green, Joel D.; Green, Joel D.
We present 230 GHz Submillimeter Array continuum and molecular line observations of the newly discovered FU Orionis candidate HBC722. We report the detection of seven 1.3 mm continuum sources in the vicinity of HBC722, none of which corresponds to HBC722 itself. We compile infrared and submillimeter continuum photometry of each source from previous studies and conclude that three are Class 0 embedded protostars, one is a Class I embedded protostar, one is a Class I/II transition object, and two are either starless cores or very young, very low luminosity protostars or first hydrostatic cores. We detect a northwest-southeast outflow, consistent with the previous detection of such an outflow in low-resolution, single-dish observations, and note that its axis may be precessing. We show that this outflow is centered on and driven by one of the nearby Class 0 sources rather than HBC722, and find no conclusive evidence that HBC722 itself is driving an outflow. The non-detection of HBC722 in the 1.3 mm continuum observations suggests an upper limit of 0.02 M-circle dot for the mass of the circumstellar disk. This limit is consistent with typical T Tauri disks and with a disk that provides sufficient mass to power the burst.
SOFIA/EXES Observations of Water Absorption in the Protostar AFGL 2591 At High Spectral Resolution
(2015-04) Indriolo, Nick; Neufeld, D. A.; DeWitt, C. N.; Richter, Matthew J.; Boogert, A. C. A.; Harper, G. M.; Jaffe, D. T.; Kulas, K. R.; McKelvey, M. E.; Ryde, N.; Vacca, W.; Jaffe, D. T.
We present high spectral resolution (similar to 3 km s(-1)) observations of the nu(2) ro-vibrational band of H2O in the 6.086-6.135 mu m range toward the massive protostar AFGL 2591 using the Echelon-Cross-Echelle Spectrograph (EXES) on the Stratospheric Observatory for Infrared Astronomy (SOFIA). Ten absorption features are detected in total, with seven caused by transitions in the nu(2) band of H2O, two by transitions in the first vibrationally excited nu(2) band of H2O, and one by a transition in the nu(2) band of (H2O)-O-18. Among the detected transitions is the nu(2) 1(1,1)-0(0,0) line that probes the lowest-lying rotational level of para-H2O. The stronger transitions appear to be optically thick, but reach maximum absorption at a depth of about 25%, suggesting that the background source is only partially covered by the absorbing gas or that the absorption arises within the 6 mu m emitting photosphere. Assuming a covering fraction of 25%, the H2O column density and rotational temperature that best fit the observed absorption lines are N(H2O) = (1.3 +/- 0.3) x 10(19) cm(-2) and T = 640 +/- 80 K.
The Spitzer c2d Survey Of Nearby Dense Cores. X. Star Formation In L673 And Cb188
(2010-12) Tsitali, Anastasia E.; Bourke, Tyler L.; Peterson, Dawn E.; Myers, Phillip C.; Dunham, Michael M.; Evans, Neal J.; Huard, Tracy L.; Dunham, Michael M.; Evans, Neal J.
L673 and CB188 are two low-mass clouds isolated from large star-forming regions that were observed as part of the Spitzer Legacy Project "From Molecular Clouds to Planet Forming disks" (c2d). We identified and characterized all the young stellar objects (YSOs) of these two regions and modeled their spectral energy distributions (SEDs) to examine whether their physical properties are consistent with values predicted from the theoretical models and with the YSO properties in the c2d survey of larger clouds. Overall, 30 YSO candidates were identified by the c2d photometric criteria, 27 in L673 and 3 in CB188. We confirm the YSO nature of 29 of them and remove a false Class III candidate in L673. We further present the discovery of two new YSO candidates, one Class 0 and another possible Class I candidate in L673, therefore bringing the total number of YSO candidates to 31. Multiple sites of star formation are present within L673, closely resembling other well-studied c2d clouds containing small groups such as B59 and L1251B, whereas CB188 seems to consist of only one isolated globule-like core. We measure a star formation efficiency (SFE) of 4.6%, which resembles the SFE of the larger c2d clouds. From the SED modeling of our YSO sample we obtain envelope masses for Class I and Flat spectrum sources of 0.01-1.0 M-circle dot. The majority of Class II YSOs show disk accretion rates from 3.3 x 10(-10) to 3 x 10(-8) M-circle dot yr(-1) and disk masses that peak at 10(-4) to 10(-3) M-circle dot. Finally, we examined the possibility of thermal fragmentation in L673 as the main star-forming process. We find that the mean density of the regions where significant YSO clustering occurs is of the order of similar to 10(5) cm(-3) using 850 mu m observations and measure a Jeans Length that is greater than the near-neighbor YSO separations by approximately a factor of 3-4. We therefore suggest that other processes, such as turbulence and shock waves, may have had a significant effect on the cloud's filamentary structure and YSO clustering.