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Register a new userK-band High-Resolution Spectroscopy of Embedded Massive Protostars
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A classical paradox in high-mass star formation is that powerful radiation pressure can halt accretion, preventing further growth of a central star. Disk accretion has been proposed to solve this problem, but the disks and the accretion process in high-mass star formation are poorly understood. We executed high-resolution ($R$=35,000-70,000) iSHELL spectroscopy in $K$-band for eleven high-mass protostars. Br-$gamma$ emission was observed toward eight sources, and the line profiles for most of these sources are similar to those of low-mass PMS stars. Using an empirical relationship between the Br-$gamma$ and accretion luminosities, we tentatively estimate disk accretion rates ranging from $lesssim$10$^{-8}$ and $sim$10$^{-4}$ $M_odot$ yr$^{-1}$. These low-mass-accretion rates suggest that high-mass protostars gain more mass via episodic accretion as proposed for low-mass protostars. Given the detection limits, CO overtone emission ($v$=2-0 and 3-1), likely associated with the inner disk region ($r ll 100$ au), was found towards two sources. This low-detection rate compared with Br-$gamma$ emission is consistent with previous observations. Ten out of the eleven sources show absorption at the $v$=0-2 ${rm R(7)-R(14)}$ CO R-branch. Most of them are either blueshifted or redshifted, indicating that the absorption is associated with an outflow or an inflow with a velocity of up to $sim50$ km s$^{-1}$. Our analysis indicates that the absorption layer is well thermalized (and therefore $n_{mathrm H_2} gtrsim 10^6$ cm$^{-3}$) at a single temperature of typically 100-200 K, and located within 200-600 au of the star.
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We present full spectral scans from 200-670$mu$m of 26 Class 0+I protostellar sources, obtained with $Herschel$-SPIRE, as part of the COPS-SPIRE Open Time program, complementary to the DIGIT and WISH Key programs. Based on our nearly continuous, line-free spectra from 200-670 $mu$m, the calculated bolometric luminosities ($L_{rm bol}$) increase by 50% on average, and the bolometric temperatures ($T_{rm bol}$) decrease by 10% on average, in comparison with the measurements without Herschel. Fifteen protostars have the same Class using $T_{rm bol}$ and $L_{rm bol}$/$L_{rm submm}$. We identify rotational transitions of CO lines from J=4-3 to J=13-12, along with emission lines of $^{13}$CO, HCO$^+$, H$_{2}$O, and [CI]. The ratios of $^{12}$CO to $^{13}$CO indicate that $^{12}$CO emission remains optically thick for $J_{rm up}$ < 13. We fit up to four components of temperature from the rotational diagram with flexible break points to separate the components. The distribution of rotational temperatures shows a primary population around 100 K with a secondary population at $sim$350 K. We quantify the correlations of each line pair found in our dataset, and find the strength of correlation of CO lines decreases as the difference between $J$-level between two CO lines increases. The multiple origins of CO emission previously revealed by velocity-resolved profiles are consistent with this smooth distribution if each physical component contributes to a wide range of CO lines with significant overlap in the CO ladder. We investigate the spatial extent of CO emission and find that the morphology is more centrally peaked and less bipolar at high-$J$ lines. We find the CO emission observed with SPIRE related to outflows, which consists two components, the entrained gas and shocked gas, as revealed by our rotational diagram analysis as well as the studies with velocity-resolved CO emission.
We present high resolution (R=25,000-35,000) K-band spectroscopy of two young stars, MWC 480 and V1331 Cyg. Earlier spectrally dispersed (R=230) interferometric observations of MWC 480 indicated the presence of an excess continuum emission interior to the dust sublimation radius, with a spectral shape that was interpreted as evidence for hot water emission from the inner disk of MWC 480. Our spectrum of V1331 Cyg reveals strong emission from CO and hot water vapor, likely arising in a circumstellar disk. In comparison, our spectrum of MWC 480 appears mostly featureless. We discuss possible ways in which strong water emission from MWC 480 might go undetected in our data. If strong water emission is in fact absent from the inner disk, as our data suggest, the continuum excess interior to the dust sublimation radius that is detected in the interferometric data must have another origin. We discuss possible physical origins for the continuum excess.
We present moderate-resolution ($Rsim4000$) $K$ band spectra of the super-Jupiter, $kappa$ Andromedae b. The data were taken with the OSIRIS integral field spectrograph at Keck Observatory. The spectra reveal resolved molecular lines from H$_{2}$O and CO. The spectra are compared to a custom $PHOENIX$ atmosphere model grid appropriate for young planetary-mass objects. We fit the data using a Markov Chain Monte Carlo forward modeling method. Using a combination of our moderate-resolution spectrum and low-resolution, broadband data from the literature, we derive an effective temperature of $T_mathrm{eff}$ = 1950 - 2150 K, a surface gravity of $log g=3.5 - 4.5$, and a metallicity of [M/H] = $-0.2 - 0.0$. These values are consistent with previous estimates from atmospheric modeling and the currently favored young age of the system ($<$50 Myr). We derive a C/O ratio of 0.70$_{-0.24}^{+0.09}$ for the source, broadly consistent with the solar C/O ratio. This, coupled with the slightly subsolar metallicity, implies a composition consistent with that of the host star, and is suggestive of formation by a rapid process. The subsolar metallicity of $kappa$ Andromedae b is also consistent with predictions of formation via gravitational instability. Further constraints on formation of the companion will require measurement of the C/O ratio of $kappa$ Andromedae A. We also measure the radial velocity of $kappa$ Andromedae b for the first time, with a value of $-1.4pm0.9,mathrm{km},mathrm{s}^{-1}$ relative to the host star. We find that the derived radial velocity is consistent with the estimated high eccentricity of $kappa$ Andromedae b.
We present multi-wavelength images observed with SOFIA-FORCAST from $sim$10 to 40 $mu$m of seven high luminosity massive protostars, as part of the SOFIA Massive (SOMA) Star Formation Survey. Source morphologies at these wavelengths appear to be influenced by outflow cavities and extinction from dense gas surrounding the protostars. Using these images, we build spectral energy distributions (SEDs) of the protostars, also including archival data from Spitzer, Herschel and other facilities. Radiative transfer (RT) models of Zhang & Tan (2018), based on Turbulent Core Accretion theory, are then fit to the SEDs to estimate key properties of the protostars. Considering the best five models fit to each source, the protostars have masses $m_{*} sim 12-64 : M_{odot}$ accreting at rates of $dot{m}_{*} sim 10^{-4}-10^{-3} : M_{odot} : rm yr^{-1}$ inside cores of initial masses $M_{c} sim 100-500 : M_{odot}$ embedded in clumps with mass surface densities $Sigma_{rm cl} sim 0.1-3 : rm g : cm^{-2}$ and span a luminosity range of $10^{4} -10^{6} : L_{odot}$. Compared with the first eight protostars in Paper I, the sources analyzed here are more luminous, and thus likely to be more massive protostars. They are often in a clustered environment or have a companion protostar relatively nearby. From the range of parameter space of the models, we do not see any evidence that $Sigma_{rm cl}$ needs to be high to form these massive stars. For most sources the RT models provide reasonable fits to the SEDs, though the cold clump material often influences the long wavelength fitting. However, for sources in very clustered environments, the model SEDs may not be such a good description of the data, indicating potential limitations of the models for these regions.
Sensitive 5-38 um Spitzer Space Telescope (SST) and ground based 3-5 um spectra of the embedded low mass protostars B5 IRS1 and HH46 IRS show deep ice absorption bands superposed on steeply rising mid-infrared continua. The ices likely originate in the circumstellar envelopes. The CO2 bending mode at 15 um is a particularly powerful tracer of the ice composition and processing history. Toward these protostars, this band shows little evidence for thermal processing at temperatures above 50 K. Signatures of lower temperature processing are present in the CO and OCN- bands, however. The observed CO2 profile indicates an intimate mixture with H2O, but not necessarily with CH3OH, in contrast to some high mass protostars. This is consistent with the low CH3OH abundance derived from the ground based L band spectra. The CO2/H2O column density ratios are high in both B5 IRS1 and HH46 IRS (~35%). Clearly, the SST spectra are essential to study ice evolution in low mass protostellar environments, and to eventually determine the relation between interstellar and solar system ices.
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