Browsing by Subject "t-tauri stars"
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Item The 69 μm forsterite band in spectra of protoplanetary disks. Results from the Herschel DIGIT programme(2013-05) Sturm, B.; Bouwman, J.; Henning, T.; Evans, N. J.; Waters, Lbfm; van Dishoeck, E. F.; Green, J. D.; Olofsson, J.; Meeus, G.; Maaskant, K.; Dominik, C.; Augereau, J. C.; Mulders, G. D.; Acke, B.; Merin, B.; Herczeg, G. J.; Digit Team; Evans, Neal J.; Green, J. D.Context. We have analysed far-infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 mu m emission band of the crystalline silicate forsterite. Aims. This work aims at providing an overview of the 69 mu m forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods. The 69 mu m forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 mu m band to the forsterite bands at shorter wavelengths. Results. A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 mu m emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 mu m bands are found to be warm (similar to 100-200 K) and iron-poor (less than similar to 2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3-4% of iron. Conclusions. Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 mu m bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 mu m band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result.Item Accretion Onto Planetary Mass Companions of Low-Mass Young Stars(2014-03) Zhou, Yifan; Herczeg, Gregory J.; Kraus, Adam L.; Metchev, Stanimir; Cruz, Kelle L.; Kraus, Adam L.Measurements of accretion rates onto planetary mass objects may distinguish between different planet Formation mechanisms, which predict different accretion histories. In this Letter, we use Hubble Space Telescope (HST)/WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC 06214-00210 b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10(-9)-10(-11) M-circle dot yr(-1) for these three objects. Their accretion rates are an order of magnitude higher than expected from the correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large separation from the central star demonstrate the presence of massive disks around these objects. Models for the Formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of Ha luminosity over accretion luminosity for objects with low accretion rates suggest that searches for Ha emission may be an efficient way to find accreting planets.Item 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, ManuelWe 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.Item 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, ColetteWe 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.Item Detection of Water Vapor in the Terrestrial Planet Forming Region of a Transition Disk(2015-09) Salyk, Colette; Lacy, John H.; Richter, Matthew J.; Zhang, Ke; Blake, Geoffrey A.; Pontoppidan, Klaus M.; Lacy, John H.We report a detection of water vapor in the protoplanetary disk around DoAr 44 with the Texas Echelon Cross Echelle Spectrograph-a visitor instrument on the Gemini north telescope. The DoAr 44 disk consists of an optically thick inner ring and outer disk, separated by a dust-cleared 36 AU gap, and has therefore been termed "pre-transitional." To date, this is the only disk with a large inner gap known to harbor detectable quantities of warm ( T similar to 450 K) water vapor. In this work, we detect and spectrally resolve three mid-infrared pure rotational emission lines of water vapor from this source, and use the shapes of the emission lines to constrain the location of the water vapor. We find that the emission originates near 0.3 AU-the inner disk region. This characteristic region coincides with that inferred for both optically thick and thin thermal infrared dust emission, as well as rovibrational CO emission. The presence of water in the dust-depleted region implies substantial columns of hydrogen (> 1022 cm(-2)) as the water vapor would otherwise be destroyed by photodissociation. Combined with the dust modeling, this column implies a gas/small-dust ratio in the optically thin dusty region of. 1000. These results demonstrate that DoAr 44 has maintained similar physical and chemical conditions to classical protoplanetary disks in its terrestrial-planet forming regions, in spite of having formed a large gap.Item DIGIT Survey Of Far-Infrared Lines From Protoplanetary Disks I. Oi , Cii , Oh, H2O, And CH+(2013-11) Fedele, Davide; Bruderer, Simon; van Dishoeck, Ewine F.; Carr, J.; Herczeg, G. J.; Salyk, C.; Evans, Neal J.; Bouwman, J.; Meeus, Gwendolyn; Henning, Thomas; Green, Joel; Najita, J. R.; Gudel, M.; Evans, Neal J.; Green, JoelWe present far-infrared (50-200 mu m) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [OI], [CII], CO, OH, H2O, CH+. The most common feature is the [OI] 63 mu m line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 10(13) < N-OH < 10(16) cm(-2), emitting radii 15 < r < 100 AU and excitation temperatures 100 < T-ex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n greater than or similar to 10(10) cm(-3)) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H2O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H2O abundance ratios across the disk than do T Tauri disks, from near-to far-infrared wavelengths. Far-infrared CH+ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (T-ex similar to 100 K) and compact (r similar to 60 AU) emission in the case of HD 100546. Off-source [OI] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [CII] emission is spatially extended in all sources where the line is detected. This suggests that not all [CII] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([OI] 63 mu m, [OI] 145 mu m, [CII]) are analyzed with PDR models and require high gas density (n greater than or similar to 10(5) cm(-3)) and high UV fluxes (G(o) similar to 10(3)-10(7)), consistent with a disk origin for the oxygen lines for most of the sources.Item DIGIT Survey Of Far-Infrared Lines From Protoplanetary Disks I. Oi , Cii , Oh, H2O, And CH+(2013-11) Fedele, Davide; Bruderer, Simon; van Dishoeck, Ewine F.; Carr, J.; Herczeg, G. J.; Salyk, C.; Evans, Neal J.; Bouwman, J.; Meeus, Gwendolyn; Henning, Thomas; Green, Joel; Najita, J. R.; Gudel, M.; Evans, Neal J.; Green, JoelWe present far-infrared (50-200 mu m) spectroscopic observations of young pre-main-sequence stars taken with Herschel/PACS as part of the DIGIT key project. The sample includes 16 Herbig AeBe and 4 T Tauri sources observed in SED mode covering the entire spectral range. An additional 6 Herbig AeBe and 4 T Tauri systems have been observed in SED mode with a limited spectral coverage. Multiple atomic fine structure and molecular lines are detected at the source position: [OI], [CII], CO, OH, H2O, CH+. The most common feature is the [OI] 63 mu m line detected in almost all of the sources, followed by OH. In contrast with CO, OH is detected toward both Herbig AeBe groups (flared and non-flared sources). An isothermal LTE slab model fit to the OH lines indicates column densities of 10(13) < N-OH < 10(16) cm(-2), emitting radii 15 < r < 100 AU and excitation temperatures 100 < T-ex < 400 K. We used the non-LTE code RADEX to verify the LTE assumption. High gas densities (n greater than or similar to 10(10) cm(-3)) are needed to reproduce the observations. The OH emission thus comes from a warm layer in the disk at intermediate stellar distances. Warm H2O emission is detected through multiple lines toward the T Tauri systems AS 205, DG Tau, S CrA and RNO 90 and three Herbig AeBe systems HD 104237, HD 142527, HD 163296 (through line stacking). Overall, Herbig AeBe sources have higher OH/H2O abundance ratios across the disk than do T Tauri disks, from near-to far-infrared wavelengths. Far-infrared CH+ emission is detected toward HD 100546 and HD 97048. The slab model suggests moderate excitation (T-ex similar to 100 K) and compact (r similar to 60 AU) emission in the case of HD 100546. Off-source [OI] emission is detected toward DG Tau, whose origin is likely the outflow associated with this source. The [CII] emission is spatially extended in all sources where the line is detected. This suggests that not all [CII] emission is associated with the disk and that there is a substantial contribution from diffuse material around the young stars. The flux ratios of the atomic fine structure lines ([OI] 63 mu m, [OI] 145 mu m, [CII]) are analyzed with PDR models and require high gas density (n greater than or similar to 10(5) cm(-3)) and high UV fluxes (G(o) similar to 10(3)-10(7)), consistent with a disk origin for the oxygen lines for most of the sources.Item Dusty Disks Around Central Stars Of Planetary Nebulae(2014-06) Clayton, Geoffrey C.; De Marco, Orsola; Nordhaus, Jason; Green, Joel; Rauch, Thomas; Werner, Klaus; Chu, You-Hua; Green, Joel D.Only a few percent of cool, old white dwarfs (WDs) have infrared excesses interpreted as originating in small hot disks due to the infall and destruction of single asteroids that come within the star's Roche limit. Infrared excesses at 24 mu m were also found to derive from the immediate vicinity of younger, hot WDs, most of which are still central stars of planetary nebulae (CSPNe). The incidence of CSPNe with this excess is 18%. The Helix CSPN, with a 24 mu m excess, has been suggested to have a disk formed from collisions of Kuiper belt-like objects (KBOs). In this paper, we have analyzed an additional sample of CSPNe to look for similar infrared excesses. These CSPNe are all members of the PG 1159 class and were chosen because their immediate progenitors are known to often have dusty environments consistent with large dusty disks. We find that, overall, PG 1159 stars do not present such disks more often than other CSPNe, although the statistics (five objects) are poor. We then consider the entire sample of CSPNe with infrared excesses and compare it to the infrared properties of old WDs, as well as cooler post-asymptotic giant branch (AGB) stars. We conclude with the suggestion that the infrared properties of CSPNe more plausibly derive from AGB-formed disks rather than disks formed via the collision of KBOs, although the latter scenario cannot be ruled out. Finally, there seems to be an association between CSPNe with a 24 mu m excess and confirmed or possible binarity of the central star.Item 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.Item Emission From Water Vapor And Absorption From Other Gases At 5-7.5 Mu M In Spitzer-Irs Spectra Of Protoplanetary Disks(2014-09) Sargent, B. A.; Forrest, W.; Watson, D. M.; D'Alessio, P.; Calvet, N.; Furlan, E.; Kim, K. H.; Green, J.; Pontoppidan, K.; Richter, I.; Tayrien, C.; Green, J.We present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming region showing emission in Spitzer Space Telescope Infrared Spectrograph 5-7.5 mu m spectra from water vapor and absorption from other gases in these stars' protoplanetary disks. Seven stars' spectra show an emission feature at 6.6 mu m due to the nu(2) = 1-0 bending mode of water vapor, with the shape of the spectrum suggesting water vapor temperatures >500 K, though some of these spectra also show indications of an absorption band, likely from another molecule. This water vapor emission contrasts with the absorption from warm water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other 6 of the 13 stars have spectra showing a strong absorption band, peaking in strength at 5.6-5.7 mu m, which for some is consistent with gaseous formaldehyde (H2CO) and for others is consistent with gaseous formic acid (HCOOH). There are indications that some of these six stars may also have weak water vapor emission. Modeling of these stars' spectra suggests these gases are present in the inner few AU of their host disks, consistent with recent studies of infrared spectra showing gas in protoplanetary disks.Item 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.Item First Detection Of Near-Infrared Line Emission From Organics In Young Circumstellar Disks(2012-03) Mandell, Avi M.; Bast, Jeanette; van Dishoeck, Ewine F.; Blake, Geoffrey A.; Salyk, Colette; Mumma, Michael J.; Villanueva, Geronimo; Salyk, ColetteWe present an analysis of high-resolution spectroscopy of several bright T Tauri stars using the CRIRES spectrograph on the Very Large Telescope and NIRSPEC spectrograph on the Keck Telescope, revealing the first detections of emission from HCN and C2H2 in circumstellar disks at near-infrared wavelengths. Using advanced data reduction techniques, we achieve a dynamic range with respect to the disk continuum of similar to 500 at 3 mu m, revealing multiple emission features of H2O, OH, HCN, and C2H2. We also present stringent upper limits for two other molecules thought to be abundant in the inner disk, CH4 and NH3. Line profiles for the different detected molecules are broad but centrally peaked in most cases, even for disks with previously determined inclinations of greater than 20 degrees, suggesting that the emission has both a Keplerian and non-Keplerian component as observed previously for CO emission. We apply two different modeling strategies to constrain the molecular abundances and temperatures: we use a simplified single-temperature local thermal equilibrium (LTE) slab model with a Gaussian line profile to make line identifications and determine a best-fit temperature and initial abundance ratios, and we compare these values with constraints derived from a detailed disk radiative transfer model assuming LTE excitation but utilizing a realistic temperature and density structure. Abundance ratios from both sets of models are consistent with each other and consistent with expected values from theoretical chemical models, and analysis of the line shapes suggests that the molecular emission originates from within a narrow region in the inner disk (R < 1 AU).Item 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.Item H2O and OH Gas in the Terrestrial Planet-Forming Zones of Protoplanetary Disks(2008-03) Salyk, Colette; Pontoppidan, Klaus M.; Blake, Geoffrey A.; Lahuis, Fred; van Dishoeck, Ewine F.; Evans, Neal J., II; Evans, Neal J., IIWe present detections of numerous 10-20 mu m H(2)O emission lines from two protoplanetary disks around the T Tauri stars AS 205A and DR Tau, obtained using the InfraRed Spectrograph on the Spitzer Space Telescope. Followup 3-5 mu m Keck NIRSPEC data confirm the presence of abundant water and spectrally resolve the lines. We also detect the P4.5 (2.934 mu m) and P9.5 (3.179 mu m) doublets of OH and (12)CO/(13)CO nu = 1 -> 0 emission in both sources. Line shapes and LTE models suggest that the emission from all three molecules originates between similar to 0.5 and 5 AU, and so will provide a new window for understanding the chemical environment during terrestrial planet Formation. LTE models also imply significant columns of H(2)O and OH in the inner disk atmospheres, suggesting physical transport of volatile ices either vertically or radially, while the significant radial extent of the emission stresses the importance of a more complete understanding of nonthermal excitation processes.Item 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., IIWe 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.Item 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.Item 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.Item Nature Of Transition Circumstellar Disks. I. The Ophiuchus Molecular Cloud(2010-04) Cieza, Lucas A.; Schreiber, Matthias R.; Romero, Grisela A.; Mora, Marcelo D.; Merin, Bruno; Swift, Jonathan J.; Orellana, Mariana; Williams, Jonathan P.; Harvey, Paul M.; Evans, Neal J.; Harvey, Paul M.; Evans, Neal J.We have obtained millimeter-wavelength photometry, high-resolution optical spectroscopy, and adaptive optics near-infrared imaging for a sample of 26 Spitzer-selected transition circumstellar disks. All of our targets are located in the Ophiuchus molecular cloud (d similar to 125 pc) and have spectral energy distributions (SEDs) suggesting the presence of inner opacity holes. We use these ground-based data to estimate the disk mass, multiplicity, and accretion rate for each object in our sample in order to investigate the mechanisms potentially responsible for their inner holes. We find that transition disks are a heterogeneous group of objects, with disk masses ranging from <0.6 to 40 M(JUP) and accretion rates ranging from <10(-11) to 10(-7) M(circle dot) yr(-1), but most tend to have much lower masses and accretion rates than "full disks" (i.e., disks without opacity holes). Eight of our targets have stellar companions: six of them are binaries and the other two are triple systems. In four cases, the stellar companions are close enough to suspect they are responsible for the inferred inner holes. We find that nine of our 26 targets have low disk mass (<2.5 M(JUP)) and negligible accretion (<10(-11) M(circle dot) yr(-1)), and are thus consistent with photoevaporating (or photoevaporated) disks. Four of these nine non-accreting objects have fractional disk luminosities <10(-3) and could already be in a debris disk stage. Seventeen of our transition disks are accreting. Thirteen of these accreting objects are consistent with grain growth. The remaining four accreting objects have SEDs suggesting the presence of sharp inner holes, and thus are excellent candidates for harboring giant planets.Item 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, IsaThe 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.Item The Physical Structure Of Protoplanetary Disks: The Serpens Cluster Compared With Other Regions(2013-01) Oliveira, Isa; Merin, Bruno; Pontoppidan, Klaus M.; van Dishoeck, Ewine F.; Oliveira, IsaSpectral energy distributions are presented for 94 young stars surrounded by disks in the Serpens Molecular Cloud, based on photometry and Spitzer/IRS spectra. Most of the stars have spectroscopically determined spectral types. Taking a distance to the cloud of 415 pc rather than 259 pc, the distribution of ages is shifted to lower values, in the 1-3Myr range, with a tail up to 10 Myr. The mass distribution spans 0.2-1.2 M-circle dot, with median mass of 0.7 M-circle dot. The distribution of fractional disk luminosities in Serpens resembles that of the young Taurus Molecular Cloud, with most disks consistent with optically thick, passively irradiated disks in a variety of disk geometries (L-disk/L-star similar to 0.1). In contrast, the distributions for the older Upper Scorpius and. Chamaeleontis clusters are dominated by optically thin lower luminosity disks (L-disk/L-star similar to 0.02). This evolution in fractional disk luminosities is concurrent with that of disk fractions: with time disks become fainter and the disk fractions decrease. The actively accreting and non-accreting stars (based on H alpha data) in Serpens show very similar distributions in fractional disk luminosities, differing only in the brighter tail dominated by strongly accreting stars. In contrast with a sample of Herbig Ae/Be stars, the T Tauri stars in Serpens do not have a clear separation in fractional disk luminosities for different disk geometries: both flared and flat disks present wider, overlapping distributions. This result is consistent with previous suggestions of a faster evolution for disks around Herbig Ae/Be stars. Furthermore, the results for the mineralogy of the dust in the disk surface (grain sizes, temperatures and crystallinity fractions, as derived from Spitzer/IRS spectra) do not show any correlation to either stellar and disk characteristics or mean cluster age in the 1-10 Myr range probed here. A possible explanation for the lack of correlation is that the processes affecting the dust within disks have short timescales, happening repeatedly, making it difficult to distinguish long-lasting evolutionary effects.