EVOLUTIONARY SIGNATURES IN THE FORMATION OF LOW-MASS PROTOSTARS. II. TOWARD RECONCILING MODELS AND OBSERVATIONS

By M. Dunham, N. Evans II, Susan Terebey, C. Dullemond, C. Young. 2005

205 citations recorded by the source corpus.

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 (Lbol, Tbol, Lbol/Lsmm) 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 (Tbol and Lbol/Lsmm) can be considered reliable indicators of physical stage.

Subfields: Astronomy and Astrophysics, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/e7db172b90dc9a811be68138dd489867cba0c33c

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Citations in the corpus

THE HERSCHEL ORION PROTOSTAR SURVEY: SPECTRAL ENERGY DISTRIBUTIONS AND FITS USING A GRID OF PROTOSTELLAR MODELS (2016, background): “The luminosity of protostars, which can be dominated by accretion, is observed to span more than three orders of magnitude, yet the underlying physics of this luminosity range is also not understood (Dunham et al. 2010; Offner & McKee 2011).”
EPISODIC ACCRETION AT EARLY STAGES OF EVOLUTION OF LOW-MASS STARS AND BROWN DWARFS: A SOLUTION FOR THE OBSERVED LUMINOSITY SPREAD IN H–R DIAGRAMS? (2009, methodology): “We find that assuming initial accretion rates ∼ 10−6M yr−1, as traditionally used in protostellar evolutionary models (Myers et al. 1998; Young & Evans 2005), produces too small a luminosity scatter in HR diagrams after a few Myr to explain the observed spread.”
A CORRELATION BETWEEN SURFACE DENSITIES OF YOUNG STELLAR OBJECTS AND GAS IN EIGHT NEARBY MOLECULAR CLOUDS (2011, background): “For example, the protostar phase may last longer in high density regions, or protostars in these regions may be less prone to oscillate between Class II and Class I SEDs (Dunham et al. 2010).”
VARIABLE PROTOSTELLAR ACCRETION WITH EPISODIC BURSTS (2015, methodology): “Our adopted definition of physical stages was extensively investigated in Dunham et al. (2010).”
MAMBO Mapping Of Spitzer c2d Small Clouds And Cores (2008, background): “Sources in a given class (i.e., with similar bolometric temperature) are believed to be in a similar evolutionary stage (Chen et al. 1995; Chen et al. 1997; see Young & Evans 2005, for illustrative examples).”
Accretion onto Pre-Main-Sequence Stars (2016, result): “The existence of a luminosity problem was supported by the later c2d Spitzer Legacy Program results (Evans et al. 2009, Dunham et al. 2010).”
PROPERTIES OF THE YOUNGEST PROTOSTARS IN PERSEUS, SERPENS, AND OPHIUCHUS (2008, methodology): “Neither models with a constant mass accretion rate (Young & Evans 2005), nor those with an exponentially decreasing rate (Myers et al. 1998) fit the observed sources.”
Identifying the Low-Luminosity Population of Embedded Protostars in the c2d Observations of Clouds and Cores (2007, background): “The division between Class 0/I objects in Tbol is from Chen et al. (1995), while the division between Class 0/I and Class I/II objects in Lbol/Lsmm is from Young & Evans (2005).”
THE SPITZER c2d LEGACY RESULTS: STAR-FORMATION RATES AND EFFICIENCIES; EVOLUTION AND LIFETIMES (2008, methodology): “For Class I sources, most of the observed Lbol fall well below the predictions by Young & Evans (2005), suggesting that accretion occurs at a rate even lower than that of the Shu picture, which is the smallest among star formation models.”
YOUNG STELLAR OBJECTS IN THE GOULD BELT (2015, background): “While Lsmm/Lbol is generally a better discriminator between Class 0 and Class I objects than Tbol, and is less sensitive to source geometry (e.g., Young & Evans 2005; Dunham et al. 2010, 2014), it cannot be calculated without 350 µm photometry.”
RESOLVING THE LUMINOSITY PROBLEM IN LOW-MASS STAR FORMATION (2011, background): “…Class 0 sources drive strong outflows implying higher average mass accretion rates than expected from their current luminosities (André et al. 1999; Dunham et al. 2006, 2010a; Lee et al. 2010) and Watson et al. (2007) showed a mismatch between the accretion rates onto the disk and protostar of…”
The Spitzer c2d Survey of Nearby Dense Cores. II. Discovery of a Low-Luminosity Object in the “Evolved Starless Core” L1521F (2006, methodology): “A full explanation of the model parameters is given by Young & Evans (2005), and here we briefly describe the specifics of the model used for L1521F-IRS.”
Star Formation in the Milky Way and Nearby Galaxies (2012, unknown): “In this picture, the offset of the turnover implies that about 0.2 to 0.3 of the core mass winds up in the star, consistent with simulations that include envelope clearing by outflows (e.g., Dunham et al. 2010).”
Protoplanetary Disks and Their Evolution (2011, background): “These statistics can be matched by models of episodic mass accretion from self-gravitating disks (Dunham et al. 2010).”
Interpreting Spectral Energy Distributions from Young Stellar Objects. I. A Grid of 200,000 YSO Model SEDs (2006, methodology): “Envelope accretion rate We sampled values of Ṁenv/M⋆ from an envelope function that is constant for t⋆ < 10 4 yr (Shu 1977; Terebey, Shu, & Cassen 1984), decreases between 105 yr and 106 yr (Foster & Chevalier 1993; Foster 1994; Hartmann 2001) and goes to zero around 106 yr (Young & Evans 2005).”