Browsing by Subject "class-i"

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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.
Spectroscopic Confirmation Of Young Planetary-Mass Companions On Wide Orbits
(2014-03) Bowler, Brendan P.; Liu, Michael C.; Kraus, Adam L.; Mann, Andrew W.; Kraus, Adam L.; Mann, Andrew W.
We present moderate-resolution ( R similar to 4000-5000) near-infrared integral field spectroscopy of the young (1-5 Myr) 6-14 M-Jup companions ROXs 42B b and FW Tau b obtained with Keck/OSIRIS and Gemini-North/NIFS. The spectrum of ROXs 42B b exhibits clear signs of low surface gravity common to young L dwarfs, confirming its extreme youth, cool temperature, and low mass. Overall, it closely resembles the free-floating 4-7 M-Jup L-type Taurus member 2MASS J04373705+2331080. The companion to FW Tau AB is more enigmatic. Our optical and near-infrared spectra show strong evidence of outflow activity and disk accretion in the form of line emission from [S II], [O I], H alpha, Ca II, [Fe II], Pa beta, and H-2. The molecular hydrogen emission is spatially resolved as a single lobe that stretches approximate to 0 ''.1 (15 AU). Although the extended emission is not kinematically resolved in our data, its morphology resembles shock-excited H-2 jets primarily seen in young Class 0 and Class I sources. The near-infrared continuum of FW Tau b is mostly flat and lacks the deep absorption features expected for a cool, late-type object. This may be a result of accretion-induced veiling, especially in light of its strong and sustained Ha emission (EW(H alpha)greater than or similar to 290 angstrom). Alternatively, FW Tau b may be a slightly warmer (M5-M8) accreting low-mass star or brown dwarf (0.03-0.15 M circle dot) with an edge-on disk. Regardless, its young evolutionary stage is in stark contrast to its Class III host FW Tau AB, indicating a more rapid disk clearing timescale for the host binary system than for its wide companion. Finally, we present near-infrared spectra of the young (similar to 2-10 Myr) low-mass (12-15 M-Jup) companions GSC 6214-210 B and SR 12 C and find they best resemble low- gravity M9.5 and M9 substellar templates.