Browsing by Subject "auriga molecular cloud"
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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 Far-Infrared Observations Of The Very Low Luminosity Embedded Source L1521F-IRS In The Taurus Star-Forming Region(2009-05) Terebey, Susan; Fich, Michael; Noriega-Crespo, Alberto; Padgett, Deborah L.; Fukagawa, Misato; Audard, Marc; Brooke, Tyler; Carey, Sean; Evans, Neal J.; Guedel, Manuel; Hines, Dean; Huard, Tracy; Knapp, Gillian R.; McCabe, Caer-Eve; Menard, Francois; Monin, Jean-Louis; Rebull, Luisa; Evans, Neal J.We investigate the environment of the very low luminosity object L1521F-IRS using data from the Taurus Spitzer Legacy Survey. The MIPS 160 mu m image shows both extended emission from the Taurus cloud and emission from multiple cold cores over a 1 degrees x 2 degrees region. Analysis shows that the cloud dust temperature is 14.2 +/- 0.4 K and the extinction ratio is A(160)/A(K) = 0.010 +/- 0.001 up to A(V) similar to 4 mag. We find kappa(160) = 0.23 +/- 0.046 cm(2) g(-1) for the specific opacity of the gas-dust mixture. Therefore, for dust in the Taurus cloud we find that the 160 mu m opacity is significantly higher than that measured for the diffuse interstellar medium, but not too different from dense cores, even at modest extinction values. Furthermore, the 160 mu m image shows features that do not appear in the IRAS 100 mu m image. We identify six regions as cold cores, i.e., colder than 14.2 K, all of which have counterparts in extinction maps or C(18)O maps. Three of the six cores contain embedded young stellar objects, which demonstrates the cores are sites of current star formation. We compare the effects of L1521F-IRS on its natal core and find there is no evidence for dust heating at 160 or 100 mu m by the embedded source. From the infrared luminosity L(TIR) = 0.024 L(circle dot) we find L(bol_int) = 0.034 -0.046 L(circle dot), thus confirming the source's low luminosity. Comparison of L1521F-IRS with theoretical simulations for the very early phases of star formation appears to rule out the first core collapse phase. The evolutionary Stateappears similar to or younger than the class 0 phase, and the estimated mass is likely to be substellar.Item 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.Item Properties Of The Youngest Protostars In Perseus, Serpens, And Ophiuchus(2009-02) Enoch, Melissa L.; Evans, Neal J.; Sargent, Anneila I.; Glenn, Jason; Evans, Neal J.We present an unbiased census of deeply embedded protostars in Perseus, Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam continuum and Spitzer Legacy surveys. We identify protostellar candidates based on their mid-infrared (mid-IR) properties, correlate their positions with 1.1 mm core positions from Enoch et al. in 2006 and 2007, and Young et al. in 2006, and construct well-sampled spectral energy distributions using our extensive wavelength coverage (lambda = 1.25-1100 mu m). Source classification based on the bolometric temperature yields a total of 39 Class 0 and 89 Class I sources in the three-cloud sample. We compare to protostellar evolutionary models using the bolometric temperature-luminosity diagram, finding a population of low-luminosity Class I sources that are inconsistent with constant or monotonically decreasing mass accretion rates. This result argues strongly for episodic accretion during the Class I phase, with more than 50% of sources in a "sub-Shu" (dM/dt < 10(-6)M(circle dot) yr(-1)) accretion state. Average spectra are compared to protostellar radiative transfer models, which match the observed spectra fairly well in Stage 0, but predict too much near-IR and too little mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I sources is used to estimate the lifetime of the Class 0 phase; the three-cloud average yields a Class 0 lifetime of 1.7 +/- 0.3 x 10(5) yr, ruling out an extremely rapid early accretion phase. Correcting photometry for extinction results in a somewhat shorter lifetime (1.1 x 10(5) yr). In Ophiuchus, however, we find very few Class 0 sources (N(Class0)/N(Class I) similar to 0.1-0.2), similar to previous studies of that cloud. The observations suggest a consistent picture of nearly constant average accretion rate through the entire embedded phase, with accretion becoming episodic by at least the Class I stage, and possibly earlier.Item The Spitzer C2D Legacy Results: Star-Formation Rates and Efficiencies; Evolution and Lifetimes(2009-04) Evans, Neal J., II; Dunham, Michael M.; Jorgensen, Jes K.; Enoch, Melissa L.; Merin, Bruno; van Dishoeck, Ewine F.; Alcala, Juan M.; Myers, Philip C.; Stapelfeldt, Karl R.; Huard, Tracy L.; Allen, Lori E.; Harvey, Paul M.; van Kempen, Tim; Blake, Geoffrey A.; Koerner, David W.; Mundy, Lee G.; Padgett, Deborah L.; Sargent, Anneila I.; Evans, Neal J., II; Dunham, Michael M.; Harvey, Paul M.The c2d Spitzer Legacy project obtained images and photometry with both IRAC and MIPS instruments for five large, nearby molecular clouds. Three of the clouds were also mapped in dust continuum emission at 1.1 mm, and optical spectroscopy has been obtained for some clouds. This paper combines inFormation drawn from studies of individual clouds into a combined and updated statistical analysis of star-Formation rates and efficiencies, numbers and lifetimes for spectral energy distribution (SED) classes, and clustering properties. Current star-Formation efficiencies range from 3% to 6%; if star Formation continues at current rates for 10 Myr, efficiencies could reach 15-30%. Star-Formation rates and rates per unit area vary from cloud to cloud; taken together, the five clouds are producing about 260 M(circle dot) of stars per Myr. The star-Formation surface density is more than an order of magnitude larger than would be predicted from the Kennicutt relation used in extragalactic studies, reflecting the fact that those relations apply to larger scales, where more diffuse matter is included in the gas surface density. Measured against the dense gas probed by the maps of dust continuum emission, the efficiencies are much higher, with stellar masses similar to masses of dense gas, and the current stock of dense cores would be exhausted in 1.8 Myr on average. Nonetheless, star Formation is still slow compared to that expected in a free-fall time, even in the dense cores. The derived lifetime for the Class I phase is 0.54 Myr, considerably longer than some estimates. Similarly, the lifetime for the Class 0 SED class, 0.16 Myr, with the notable exception of the Ophiuchus cloud, is longer than early estimates. If photometry is corrected for estimated extinction before calculating class indicators, the lifetimes drop to 0.44 Myr for Class I and to 0.10 for Class 0. These lifetimes assume a continuous flow through the Class II phase and should be considered median lifetimes or half-lives. Star Formation is highly concentrated to regions of high extinction, and the youngest objects are very strongly associated with dense cores. The great majority (90%) of young stars lie within loose clusters with at least 35 members and a stellar density of 1 M(circle dot) pc(-3). Accretion at the sound speed from an isothermal sphere over the lifetime derived for the Class I phase could build a star of about 0.25 M(circle dot), given an efficiency of 0.3. Building larger mass stars by using higher mass accretion rates could be problematic, as our data confirm and aggravate the "luminosity problem" for protostars. At a given T(bol), the values for L(bol) are mostly less than predicted by standard infall models and scatter over several orders of magnitude. These results strongly suggest that accretion is time variable, with prolonged periods of very low accretion. Based on a very simple model and this sample of sources, half the mass of a star would be accreted during only 7% of the Class I lifetime, as represented by the eight most luminous objects.Item 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.Item The Spitzer Infrared Spectrograph Survey of T Tauri Stars in Taurus(2011-07) Furlan, E.; Luhman, K. L.; Espaillat, C.; D'Alessio, P.; Adame, L.; Manoj, P.; Kim, K. H.; Watson, D. M.; Forrest, W. J.; McClure, M. K.; Calvet, N.; Sargent, B. A.; Green, Joel D.; Fischer, W. J.; Green, Joel D.We present 161 Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars and young brown dwarfs in the Taurus star-forming region. All of the targets were selected based on their infrared excess and are therefore surrounded by protoplanetary disks; they form the complete sample of all available IRS spectra of T Tauri stars with infrared excesses in Taurus. We also present the IRS spectra of seven Class 0/I objects in Taurus to complete the sample of available IRS spectra of protostars in Taurus. We use spectral indices that are not significantly affected by extinction to distinguish between envelope-and disk-dominated objects. Together with data from the literature, we construct spectral energy distributions for all objects in our sample. With spectral indices derived from the IRS spectra we infer disk properties such as dust settling and the presence of inner disk holes and gaps. We find a transitional disk frequency, which is based on objects with unusually large 13-31 mu m spectral indices indicative of a wall surrounding an inner disk hole, of about 3%, and a frequency of about 20% for objects with unusually large 10 mu m features, which could indicate disk gaps. The shape and strength of the 10 mu m silicate emission feature suggests weaker 10 mu m emission and more processed dust for very low mass objects and brown dwarfs (spectral types M6-M9). These objects also display weaker infrared excess emission from their disks, but do not appear to have more settled disks than their higher-mass counterparts. We find no difference for the spectral indices and properties of the dust between single and multiple systems.