Browsing by Subject "young stellar objects"

Now showing 1 - 20 of 48

An Analysis Of The Environments Of Fu Orionis Objects With Herschel
(2013-08) Green, Joel D.; Evans, Neal J.; Kospal, Agnes; Herczeg, Gregory; Quanz, Sascha P.; Henning, Thomas; van Kempen, Tim A.; Lee, Jeong-Eun; Dunham, Michael M.; Meeus, Gwendolyn; Bouwman, Jeroen; Chen, Jo-Hsin; Gudel, Manuel; Skinner, Stephen L.; Liebhart, Armin; Merello, Manuel; Green, Joel D.; Evans, Neal J.; Merello, Manuel
We present Herschel-HIFI, SPIRE, and PACS 50-670 mu m imaging and spectroscopy of six FU Orionis-type objects and candidates (FU Orionis, V1735 Cyg, V1515 Cyg, V1057 Cyg, V1331 Cyg, and HBC 722), ranging in outburst date from 1936 to 2010, from the "FOOSH" (FU Orionis Objects Surveyed with Herschel) program, as well as ancillary results from Spitzer Infrared Spectrograph and the Caltech Submillimeter Observatory. In their system properties (L-bol, T-bol, and line emission), we find that FUors are in a variety of evolutionary states. Additionally, some FUors have features of both Class I and II sources: warm continuum consistent with Class II sources, but rotational line emission typical of Class I, far higher than Class II sources of similar mass/luminosity. Combining several classification techniques, we find an evolutionary sequence consistent with previous mid-IR indicators. We detect [O I] in every source at luminosities consistent with Class 0/I protostars, much greater than in Class II disks. We detect transitions of 13CO (J(up) of 5-8) around two sources (V1735 Cyg and HBC 722) but attribute them to nearby protostars. Of the remaining sources, three (FU Ori, V1515 Cyg, and V1331 Cyg) exhibit only low-lying CO, but one (V1057 Cyg) shows CO up to J = 23 -> 22 and evidence for H2O and OH emission, at strengths typical of protostars rather than T Tauri stars. Rotational temperatures for "cool" CO components range from 20 to 81 K, for similar to 10(50) total CO molecules. We detect [C I] and [N II] primarily as diffuse emission.
CO Rovibrational Emission As A Probe Of Inner Disk Structure
(2011-12) Salyk, Colette; Blake, Geoffrey A.; Boogert, Adwin C. A.; Brown, Joanna M.; Salyk, Colette
We present an analysis of CO emission lines from a sample of T Tauri, Herbig Ae/Be, and transitional disks with known inclinations in order to study the structure of inner disk molecular gas. We calculate CO inner radii by fitting line profiles with a simple parameterized model. We find that, for optically thick disks, CO inner radii are strongly correlated with the total system luminosity (stellar plus accretion) and consistent with the dust sublimation radius. Transitional disk inner radii show the same trend with luminosity, but are systematically larger. Using rotation diagram fits, we derive, for classical T Tauri disks, emitting areas consistent with a ring of width similar to 0.15 AU located at the CO inner radius; emitting areas for transitional disks are systematically smaller. We also measure lower rotational temperatures for transitional disks, and disks around Herbig Ae/Be stars, than for those around T Tauri stars. Finally, we find that rotational temperatures are similar to, or slightly lower than, the expected temperature of blackbody grains located at the CO inner radius, in contrast to expectations of thermal decoupling between gas and dust.
A Comparative Astrochemical Study Of The High-Mass Protostellar Objects NGC 7538 IRS 9 And IRS 1
(2012-10) Barentine, John C.; Lacy, John H.; Barentine, John C.; Lacy, John H.
We report the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 and compare our observations to published data on the nearby object NGC 7538 IRS 1. Both objects originated in the same molecular cloud and appear to be at different points in their evolutionary histories, offering an unusual opportunity to study the temporal evolution of envelope chemistry in objects sharing a presumably identical starting composition. Observations were made with the Texas Echelon Cross Echelle Spectrograph, a sensitive, high spectral resolution (R = lambda/Delta lambda similar or equal to 100,000) mid-infrared grating spectrometer. Forty-six individual lines in vibrational modes of the molecules C2H2, CH4, HCN, NH3, and CO were detected, including two isotopologues ((CO)-C-13, (CO)-C-12-O-18) and one combination mode (nu(4) + nu(5) C2H2). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density, and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (similar to 2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9.
Discovery Of A Low-Mass Bipolar Molecular Outflow From L1014-Irs With The Submillimeter Array
(2005-11) Bourke, Tyler L.; Crapsi, Antonio; Myers, Phillip C.; Evans, Neal J.; Wilner, David J.; Huard, Tracy L.; Jorgensen, Jes K.; Young, Chadwick H.; Evans, Neal J.; Young, Chadwick H.
Using the Submillimeter Array, we report the discovery of a compact low-mass bipolar molecular outflow from L1014-IRS and confirm its association with the L1014 dense core at 200 pc. Consequently, L1014-IRS is the lowest luminosity (L similar to 0.09 L-.), and perhaps the lowest mass, source known to be driving a bipolar molecular outflow, which is one of the smallest known in size (similar to 500 AU), mass (<10(-4) M-.), and energetics (e.g., with a M, force <10(-7) M-. km s(-1) yr(-1)).
Disk And Envelope Structure In Class 0 Protostars. I. The Resolved Massive Disk In Serpens Firs 1
(2009-12) Enoch, Melissa L.; Corder, Stuartt; Dunham, Michael M.; Duchene, Gaspard; Dunham, Michael M.
We present the first results of a program to characterize the disk and envelope structure of typical Class 0 protostars in nearby low-mass star-forming regions. We use Spitzer Infrared Spectrograph (IRS) mid-infrared spectra, high-resolution Combined Array for Research in Millimeter-wave Astronomy (CARMA) 230 GHz continuum imaging, and two-dimensional radiative transfer models to constrain the envelope structure, as well as the size and mass of the circumprotostellar disk in Serpens FIRS 1. The primary envelope parameters (centrifugal radius, outer radius, outflow opening angle, and inclination) are well constrained by the spectral energy distribution (SED), including Spitzer IRAC and MIPS photometry, IRS spectra, and 1.1 mm Bolocam photometry. These together with the excellent uv-coverage (4.5-500 k lambda) of multiple antenna configurations with CARMA allow for a robust separation of the envelope and a resolved disk. The SED of Serpens FIRS 1 is best fit by an envelope with the density profile of a rotating, collapsing spheroid with an inner (centrifugal) radius of approximately 600 AU, and the millimeter data by a large resolved disk with M(disk) similar to 1.0M(circle dot) and R(disk) similar to 300 AU. These results suggest that large, massive disks can be present early in the main accretion phase. Results for the larger, unbiased sample of Class 0 sources in the Perseus, Serpens, and Ophiuchus molecular clouds are needed to determine if relatively massive disks are typical in the Class 0 stage.
Dust, Ice And Gas In Time (DIGIT) Herschel And Spitzer Spectro-Imaging Of SMM3 And SMM4 In Serpens
(2013-10) Dionatos, O.; Jorgensen, J. K.; Green, J. D.; Herczeg, G. J.; Evans, Neal J.; Kristensen, L. E.; Lindberg, J. E.; van Dishoeck, E. F.; Green, Joel D.; Evans, Neal J.
Context. Mid-and far-infrared observations of the environment around embedded protostars reveal a plethora of high excitation molecular and atomic emission lines. Different mechanisms for the origin of these lines have been proposed, including shocks induced by protostellar jets and radiation or heating by the embedded protostar of its immediate surroundings. Aims. By studying of the most important molecular and atomic coolants, we aim at constraining the physical conditions around the embedded protostars SMM3 and SMM4 in the Serpens molecular cloud core and measuring the CO/H-2 ratio in warm gas. Methods. Spectro-imaging observations from the Spitzer Infrared Spectrograph (IRS) and the Herschel Photodetector Array Camera and Spectrometer (PACS) provide an almost complete wavelength coverage between 5 and 200 mu m. Within this range, emission from all major molecular (H-2, CO, H2O and OH) and many atomic ([OI], [CII], [FeII], [SiII] and [SI]) coolants of excited gas are detected. Emission line maps reveal the morphology of the observed emission and indicate associations between the different species. The excitation conditions for molecular species are assessed through rotational diagrams. Emission lines from major coolants are compared to the results of steady-state C- and J-type shock models. Results. Line emission tends to peak at distances of similar to 10-20 '' from the protostellar sources with all but [CII] peaking at the positions of outflow shocks seen in near-IR and sub-millimeter interferometric observations. The [CII] emission pattern suggests that it is most likely excited from energetic UV radiation originating from the nearby flat-spectrum source SMM6. Excitation analysis indicates that H-2 and CO originate in gas at two distinct rotational temperatures of similar to 300 K and 1000 K, while the excitation temperature for H2O and OH is similar to 100-200 K. The morphological and physical association between CO and H2 suggests a common excitation mechanism, which allows direct comparisons between the two molecules. The CO/H2 abundance ratio varies from similar to 10(-5) in the warmer gas up to similar to 10(-4) in the hotter regions. Shock models indicate that C-shocks can account for the observed line intensities if a beam filling factor and a temperature stratification in the shock front are considered. C-type shocks can best explain the emission from H2O. The existence of J-shocks is suggested by the strong atomic/ionic (except for [CII]) emission and a number of line ratio diagnostics. Dissociative shocks can account for the CO and H-2 emission in a single excitation temperature structure. Conclusions. The bulk of cooling from molecular and atomic lines is associated with gas excited in outflow shocks. The strong association between H2 and CO constrain their abundance ratio in warm gas. Both C-and J-type shocks can account for the observed molecular emission; however, J-shocks are strongly suggested by the atomic emission and provide simpler and more homogeneous solutions for CO and H-2. The variations in the CO/H-2 abundance ratio for gas at different temperatures can be interpreted by their reformation rates in dissociative J-type shocks, or the influence of both C and J shocks.
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.
Embedded Protostars In The Dust, Ice, And Gas In Time (DIGIT) Herschel Key Program: Continuum Seds, And An Inventory Of Characteristic Far-Infrared Lines From Pacs Spectroscopy
(2013-06) Green, Joel D.; Evans, Neal J.; Jorgensen, Jes K.; Herczeg, Gregory J.; Kristensen, Lars E.; Lee, Jeong-Eun; Dionatos, Odysseas; Yildiz, Umut A.; Salyk, Colette; Meeus, Gwendolyn; Bouwman, Jeroen; Visser, Ruud; Bergin, Edwin A.; van Dishoeck, Ewine F.; Rascati, Michelle R.; Karska, Agata; van Kempen, Tim A.; Dunham, Michael M.; Lindberg, Johan E.; Fedele, Davide; Green, Joel D.; Evans, Neal J.; Rascati, Michelle R.
We present 50-210 mu m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 mu m spectral energy distributions, as part of the Dust, Ice, and Gas in Time Key Program. Some sources exhibit up to 75 H2O lines ranging in excitation energy from 100 to 2000 K, 12 transitions of OH, and CO rotational lines ranging from J = 14 -> 13 up to J = 40 -> 39. [O I] is detected in all but one source in the entire sample; among the sources with detectable [O I] are two very low luminosity objects. The mean 63/145 mu m [O I] flux ratio is 17.2 +/- 9.2. The [O I] 63 mu m line correlates with L-bol, but not with the time-averaged outflow rate derived from low-J CO maps. [C II] emission is in general not local to the source. The sample L-bol increased by 1.25 (1.06) and T-bol decreased to 0.96 (0.96) of mean (median) values with the inclusion of the Herschel data. Most CO rotational diagrams are characterized by two optically thin components (< N > = ( 0.70 +/- 1.12) x 10(49) total particles). N-CO correlates strongly with L-bol, but neither T-rot nor N-CO(warm)/N-CO(hot) correlates with L-bol, suggesting that the total excited gas is related to the current source luminosity, but that the excitation is primarily determined by the physics of the interaction (e.g., UV-heating/shocks). Rotational temperatures for H2O (< T-rot > = 194 +/- 85 K) and OH (< T-rot > = 183 +/- 117 K) are generally lower than for CO, and much of the scatter in the observations about the best fit is attributed to differences in excitation conditions and optical depths among the detected lines.
Evolutionary Signatures In The Formation Of Low-Mass Protostars. II. Toward Reconciling Models And Observations
(2010-02) Dunham, Michael M.; Evans, Neal J.; Terebey, Susan; Dullemond, Cornelis P.; Young, Chadwick H.; Dunham, Michael M.; Evans, Neal J.
A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars. We use as our starting point the evolutionary model following the inside-out collapse of a singular isothermal sphere as presented by Young & Evans. We calculate the radiative transfer of the collapsing core throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures (L(bol), T(bol), L(bol)/L(smm)) to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions; we include (1) the opacity from scattering in the radiative transfer, (2) a circumstellar disk directly in the two-dimensional radiative transfer, (3) a two-dimensional envelope structure, taking into account the effects of rotation, (4) mass-loss and the opening of outflow cavities, and (5) a simple treatment of episodic mass accretion. We find that scattering, two-dimensional geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. On the other hand, we find that a cycle of episodic mass accretion similar to that predicted by recent theoretical work can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function and stellar initial mass function. Finally, the combination of outflow cavities and episodic mass accretion reduces the connection between observational class and physical stage to the point where neither of the two commonly used observational signatures (T(bol) and L(bol)/L(smm)) can be considered reliable indicators of physical stage.
The Gould Belt "Misfits" Survey: The Real Solar Neighborhood Protostars
(2015-06) Heiderman, Amanda; Evans, Neal J.; Evans, Neal J.
We present an HCO+ J = 3. 2 survey of Class 0+I and Flat SED young stellar objects (YSOs) found in the Gould Belt clouds by surveys with Spitzer. Our goal is to provide a uniform Stage 0+I source indicator for these embedded protostar candidates. We made single point HCO+J = 3 -> 2 measurements toward the source positions at the CSO and APEX of 546 YSOs (89% of the Class 0+I +Flat SED sample). Using the criteria from van Kempen et al., we classify sources as Stage 0+I or bona fide protostars and find that 84% of detected sources meet the criteria. We recommend a timescale for the evolution of Stage 0+I (embedded protostars) of 0.54 Myr. We find significant correlations of HCO+integrated intensity with a and Tbol but not with Lbol. The detection fraction increases smoothly as a function of a and Lbol, while decreasing smoothly with Tbol. Using the Stage 0+I sources tightens the relation between protostars and high extinction regions of the cloud; 89% of Stage I sources lie in regions with A(V) > 8 mag. Class 0+I and Flat SED YSOs that are not detected in HCO+ have, on average, a factor of similar to 2 higher Tbol and a factor of similar to 5 lower L-bol than YSOs with HCO+ detections. We find less YSO contamination, defined as the number of undetected YSOs divided by the total number surveyed, for sources with T-bol less than or similar to 600 K and L-bol greater than or similar to 1 L-circle dot. The contamination percentage is > 90% at A(V) < 4 mag and decreases as A(V) increases.
The Gould's Belt Very Large Array Survey. I. The Ophiuchus Complex
(2013-09) Dzib, Sergio A.; Loinard, Laurent; Mioduszewski, Amy J.; Rodriguez, Luis F.; Ortiz-Leon, Gisela N.; Pech, Gerardo; Rivera, Juana L.; Torres, Rosa M.; Boden, Andrew F.; Hartmann, Lee; Evans, Neal J.; Briceno, Cesar; Tobin, John; Evans, Neal J.
We present large-scale (similar to 2000 arcmin(2)), deep (similar to 20 mu Jy), high-resolution (similar to 1 '') radio observations of theOphiuchus star-forming complex obtained with the Karl G. Jansky Very Large Array at lambda = 4 and 6 cm. In total, 189 sources were detected, 56 of them associated with known young stellar sources, and 4 with known extragalactic objects; the other 129 remain unclassified, but most of them are most probably background quasars. The vast majority of the young stars detected at radio wavelengths have spectral types K or M, although we also detect four objects of A/F/B types and two brown dwarf candidates. At least half of these young stars are non-thermal (gyrosynchrotron) sources, with active coronas characterized by high levels of variability, negative spectral indices, and (in some cases) significant circular polarization. As expected, there is a clear tendency for the fraction of non-thermal sources to increase from the younger (Class 0/I or flat spectrum) to the more evolved (Class III or weak line T Tauri) stars. The young stars detected both in X-rays and at radio wavelengths broadly follow a Gudel-Benz relation, but with a different normalization than the most radioactive types of stars. Finally, we detect a similar to 70 mJy compact extragalactic source near the center of the Ophiuchus core, which should be used as gain calibrator for any future radio observations of this region.
The Gould's Belt Very Large Array Survey. II. The Serpens Region
(2015-05) Ortiz-Leon, Gisela N.; Loinard, Laurent; Mioduszewski, Amy J.; Dzib, Sergio A.; Rodriguez, Luis F.; Pech, Gerardo; Rivera, Juana L.; Torres, Rosa M.; Boden, Andrew F.; Hartmann, Lee; Evans, Neal J.; Briceno, Cesar; Tobin, John; Kounkel, Marina A.; Gonazlez-Lopezlira, R. A.; Evans, Neal J.
We present deep (similar to 17 mu Jy) radio continuum observations of the Serpens molecular cloud, the Serpens south cluster, and the W40 region obtained using the Very Large Array in its A configuration. We detect a total of 146 sources, 29 of which are young stellar objects (YSOs), 2 of which are BV stars, and 5 more of which are associated with phenomena related to YSOs. Based on their radio variability and spectral index, we propose that about 16 of the remaining 110 unclassified sources are also YSOs. For approximately 65% of the known YSOs detected here as radio sources, the emission is most likely non-thermal and related to stellar coronal activity. As also recently observed in Ophiuchus, our sample of YSOs with X-ray counterparts lies below the fiducial Gudel & Benz relation. Finally, we analyze the proper motions of nine sources in the W40 region. This allows us to better constrain the membership of the radio sources in the region.
The Gould's Belt Very Large Array Survey. III. The Orion Region
(2014-07) Kounkel, Marina; Hartmann, Lee; Loinard, Laurent; Mioduszewski, Amy J.; Dzib, Sergio A.; Ortiz-Leon, Gisela N.; Rodriguez, Luis F.; Pech, Gerardo; Rivera, Juana L.; Torres, Rosa M.; Boden, Andrew F.; Evans, Neal J.; Briceno, Cesar; Tobin, John; Evans, Neal J.
We present results from a high-sensitivity (60 mu Jy), large-scale (2.26 deg(2)) survey obtained with the Karl G. Jansky Very Large Array as part of the Gould's Belt Survey program. We detected 374 and 354 sources at 4.5 and 7.5 GHz, respectively. Of these, 148 are associated with previously known young stellar objects (YSOs). Another 86 sources previously unclassified at either optical or infrared wavelengths exhibit radio properties that are consistent with those of young stars. The overall properties of our sources at radio wavelengths such as their variability and radio to X-ray luminosity relation are consistent with previous results from the Gould's Belt Survey. Our detections provide target lists for follow-up Very Long Baseline Array radio observations to determine their distances as YSOs are located in regions of high nebulosity and extinction, making it difficult to measure optical parallaxes.
The Gould's Belt Very Large Array Survey. IV. The Taurus-Auriga Complex
(2015-03) Dzib, Sergio A.; Loinard, Laurent; Rodriguez, Luis F.; Mioduszewski, Amy J.; Ortiz-Leon, Gisela N.; Kounkel, Marina A.; Pech, Gerardo; Rivera, Juana L.; Torres, Rosa M.; Boden, Andrew F.; Hartmann, Lee; Evans, Neal J.; Briceno, Cesar; Tobin, John; Evans, Neal J.
We present a multi-epoch radio study of the Taurus-Auriga star-forming complex made with the Karl G. Jansky Very Large Array at frequencies of 4.5 GHz and 7.5 GHz. We detect a total of 610 sources, 59 of which are related to young stellar objects (YSOs) and 18 to field stars. The properties of 56% of the young stars are compatible with non-thermal radio emission. We also show that the radio emission of more evolved YSOs tends to be more non-thermal in origin and, in general, that their radio properties are compatible with those found in other star-forming regions. By comparing our results with previously reported X-ray observations, we notice that YSOs in Taurus-Auriga follow a Giidel Benz relation with k = 0.03, as we previously suggested for other regions of star formation. In general, YSOs in Taurus-Auriga and in all the previous studied regions seem to follow this relation with a dispersion of similar to 1 dex. Finally, we propose that most of the remaining sources are related with extragalactic objects but provide a list of 46 unidentified radio sources whose radio properties are compatible with a YSO nature.
Herschel Key Program, "Dust, Ice, and Gas in Time" (DIGIT): the Origin of Molecular and Atomic Emission in Low-Mass Protostars in Taurus
(2014-10) Lee, Jeong-Eun; Lee, Jinhee; Lee, Seokho; Evans, Neal J., II; Green, Joel D.; Lee, Jeong-Eun; Evans, Neal J., II; Green, Joel D.
Six low-mass embedded sources (L1489, L1551-IRS5, TMR1, TMC1-A, L1527, and TMC1) in Taurus have been observed with Herschel-PACS to cover the full spectrum from 50 to 210 mu m as part of the Herschel key program, "Dust, Ice, and Gas In Time." The relatively low intensity of the interstellar radiation field surrounding Taurus minimizes contamination of the [C II] emission associated with the sources by diffuse emission from the cloud surface, allowing study of the [C II] emission from the source. In several sources, the [C II] emission is distributed along the outflow, as is the [O I] emission. The atomic line luminosities correlate well with each other, as do the molecular lines, but the atomic and molecular lines correlate poorly. The relative contribution of CO to the total gas cooling is constant at similar to 30%, while the cooling fraction by H2O varies from source to source, suggesting different shock properties resulting in different photodissociation levels of H2O. The gas with a power-law temperature distribution with a moderately high density can reproduce the observed CO fluxes, indicative of CO close to LTE. However, H2O is mostly subthermally excited. L1551-IRS5 is the most luminous source (L-bol = 24.5 L-circle dot) and the [O I] 63.1 mu m line accounts for more than 70% of its FIR line luminosity, suggesting complete photodissociation of H2O by a J shock. In L1551-IRS5, the central velocity shifts of the [O I] line, which exceed the wavelength calibration uncertainty (similar to 70 km s(-1)) of PACS, are consistent with the known redshifted and blueshifted outflow direction.
Identifying the Low-Luminosity Population of Embedded Protostars in the c2d Observations of Clouds and Cores
(2008-11) Dunham, Miranda M.; Crapsi, Antonio; Evans, Neal J., II; Bourke, Tyler L.; Huard, Tracy L.; Myers, Philip C.; Kauffmann, Jens; Dunham, Miranda M.; Crapsi, Antonio; Evans, Neal J., II
We present the results of a search for all embedded protostars with internal luminosities <= 1.0 L(circle dot) in the full sample of nearby, low-mass star-forming regions surveyed by the Spitzer Space Telescope Legacy Project "From Molecular Cores to Planet Forming Disks'' (c2d). The internal luminosity of a source, L(int), is the luminosity of the central source and excludes luminosity arising from external heating. On average, the Spitzer c2d data are sensitive to embedded protostars with L(int) >= 4 x 10(-3)(d/140 pc)(2) L(circle dot), a factor of 25 better than the sensitivity of the Infrared Astronomical Satellite (IRAS) to such objects. We present a set of selection criteria used to identify candidates from the Spitzer data and examine complementary data to decide whether each candidate is truly an embedded protostar. We find a tight correlation between the 70 mu m flux and internal luminosity of a protostar, an empirical result based on both observations and detailed two-dimensional radiative transfer models of protostars. We identify 50 embedded protostars with L(int) <= 1: 0 L(circle dot); 15 have L(int) <= 0: 1 L(circle dot). The intrinsic distribution of source luminosities increases to lower luminosities. While we find sources down to the above sensitivity limit, indicating that the distribution may extend to luminosities lower than probed by these observations, we are able to rule out a continued rise in the distribution below L(int) = 0.1 L(circle dot). Between 75% and 85% of cores classified as starless prior to being observed by Spitzer remain starless to our luminosity sensitivity; the remaining 15%-25% harbor low-luminosity, embedded protostars. We compile complete spectral energy distributions for all 50 objects and calculate standard evolutionary signatures (L(bol), T(bol), and L(bol)/L(smm)) and argue that these objects are inconsistent with the simplest picture of star Formation, wherein mass accretes from the core onto the protostar at a constant rate.
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.
Infrared Studies Of Pre-Main-Sequence Intermediate-Mass Stars - Lkh-Alpha-198
(1992-06) Natta, Antonella; Palla, Francisco; Butner, Harold M.; Evans, Neal J.; Harvey, Paul M.; Evans, Neal J.; Harvey, Paul M.
Far-infrared scans of LkH-alpha 198 at 50 and 100-mu-m along several directions reveal an extended, roughly spherical source with an average FWHM of 33" or 2 x 10(4) AU at 100-mu-m and up to 18" or 1.5 x 10(4) AU at 50-mu-m. By comparing the observed properties at optical, infrared, and millimeter wavelengths with the predictions of radiation transfer models, we estimate a luminosity of 250 L. at a distance of 600 pc. The optical depth at 100-mu-m is 0.004-0.006, implying a mass of approximately 2 M. within a radius of 10(4) AU. The dust configuration is characterized by a power-law distribution n(r) approximately r-alpha, with alpha-approximately 0.5, in most of the directions around the star and over large distances. The inner radius of the dusty envelope is probably rather small, r(i) less-than-or-similar-to 300 AU (0".5), and it may be as small as the dust destruction front: in LkH-alpha 198 there is no evidence of a large region devoid of grains. The fit to the observed spectral energy distribution (SED) from optical to millimeter wavelengths favors a model that includes a disk and a spherical envelope. The former is required in order to provide a good match to the SED at near-infrared wavelengths. The disk mass is 0.27 M., i.e., approximately 7% of the stellar mass. Alternatively, the SED can be fitted by a spherical dust envelope only, if there is a component of very small grains and polycyclic aromatic hydrocarbons. Within the current scenario of star formation, mostly developed for solar-mass stars, it is not easy to account for the flat density profile of the dusty envelope we observe around LkH-alpha 198.
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.
MAMBO Mapping Of Spitzer c2d Small Clouds And Cores
(2008-09) Kauffmann, J.; Bertoldi, F.; Bourke, T. L.; Evans, Neal J.; Lee, C. W.; Evans, Neal J.
Aims. To study the structure of nearby (<500 pc) dense starless and star-forming cores with the particular goal to identify and understand evolutionary trends in core properties, and to explore the nature of Very Low Luminosity Objects (<= 0.1 L(circle dot); VeLLOs). Methods. Using the MAMBO bolometer array, we create maps unusually sensitive to faint (few mJy per beam) extended (approximate to 5 ') thermal dust continuum emission at 1.2 mm wavelength. Complementary information on embedded stars is obtained from Spitzer, IRAS, and 2MASS. Results. Our maps are very rich in structure, and we characterize extended emission features (>subcores>) and compact intensity peaks in our data separately to pay attention to this complexity. We derive, e. g., sizes, masses, and aspect ratios for the subcores, as well as column densities and related properties for the peaks. Combination with archival infrared data then enables the derivation of bolometric luminosities and temperatures, as well as envelope masses, for the young embedded stars. Conclusions. Starless and star-forming cores occupy the same parameter space in many core properties; a picture of dense core evolution in which any dense core begins to actively form stars once it exceeds some fixed limit in, e. g., mass, density, or both, is inconsistent with our data. A concept of necessary conditions for star formation appears to provide a better description: dense cores fulfilling certain conditions can form stars, but they do not need to, respectively have not done so yet. Comparison of various evolutionary indicators for young stellar objects in our sample (e. g., bolometric temperatures) reveals inconsistencies between some of them, possibly suggesting a revision of some of these indicators. Finally, we challenge the notion that VeLLOs form in cores not expected to actively form stars, and we present a first systematic study revealing evidence for structural differences between starless and candidate VeLLO cores.