Browsing by Subject "self-similar collapse"
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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 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.