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Register a new userDetection of Photospheric Features in the Near-Infrared Spectrum of a Class 0 Protostar
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Thomas P. Greene
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We present a near-infrared $K$-band $R simeq 1500$ Keck spectrum of S68N, a Class 0 protostar in the Serpens molecular cloud. The spectrum shows a very red continuum, CO absorption bands, weak or non-existent atomic metal absorptions, and H$_2$ emission lines. The near-IR H$_2$ emission is consistent with excitation in shocks or by X-rays but not by UV radiation. We model the absorption component as a stellar photosphere plus circumstellar continuum emission with wavelength-dependent extinction. A Markov Chain Monte Carlo analysis shows that the most likely model parameters are consistent with a low-temperature, low-gravity photosphere with significant extinction and no more than modest continuum veiling. Its $T_{mathrm{eff}} simeq 3260$ K effective temperature is similar to that of older, more evolved pre-main-sequence stars, but its surface gravity log $g simeq 2.4$ cm s$^{-2}$ is approximately 1 dex lower. This implies that the radius of this protostar is a factor of $sim 3$ larger than that of $10^6$ yr old T Tauri stars. Its low veiling is consistent with a circumstellar disk having intrinsic near-IR emission that is less than or equal to that of more evolved Class I protostars. Along with the high extinction, this suggests that most of the circumstellar material is in a cold envelope, as expected for a Class 0 protostar. This is the first known detection and analysis of a Class 0 protostar absorption spectrum.
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Context: The protostellar envelopes, outflow and large-scale chemistry of Class~0 and Class~I objects have been well-studied, but while previous works have hinted at or found a few Keplerian disks at the Class~0 stage, it remains to be seen if their presence in this early stage is the norm. Likewise, while complex organics have been detected toward some Class~0 objects, their distribution is unknown as they could reside in the hottest parts of the envelope, in the emerging disk itself or in other components of the protostellar system, such as shocked regions related to outflows. Aims: In this work, we aim to address two related issues regarding protostars: when rotationally supported disks form around deeply embedded protostars and where complex organic molecules reside in such objects. Methods: We observed the deeply embedded protostar, L483, using Atacama Large Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a high angular resolution down to $sim$~0.1$^{primeprime}$ (20~au) scales. Results: We find that the kinematics of CS~$J=7$--$6$ and H$^{13}$CN~$J=4$--$3$ are best fitted by the velocity profile from infall under conservation of angular momentum and not by a Keplerian profile. The spatial extents of the observed complex organics are consistent with an estimated ice sublimation radius of the envelope at $sim$~50~au, suggesting that the complex organics exist in the hot corino of L483. Conclusions: We find that L483 does not harbor a Keplerian disk down to at least $15$~au in radius. Instead, the innermost regions of L483 are undergoing a rotating collapse. This result highlights that some Class~0 objects contain only very small disks, or none at all, with the complex organic chemistry taking place on scales inside the hot corino of the envelope, in a region larger than the emerging disk.
We present a characterization of the protostar embedded within the BHR7 dark cloud, based on both photometric measurements from the near-infrared to millimeter and interferometric continuum and molecular line observations at millimeter wavelengths. We find that this protostar is a Class 0 system, the youngest class of protostars, measuring its bolometric temperature to be 50.5~K, with a bolometric luminosity of 9.3~L$_{odot}$. The near-infrared and textit{Spitzer} imaging show a prominent dark lane from dust extinction separating clear bipolar outflow cavities. Observations of $^{13}$CO ($J=2rightarrow1$), C$^{18}$O ($J=2rightarrow1$), and other molecular lines with the Submillimeter Array (SMA) exhibit a clear rotation signature on scales $<$1300~AU. The rotation can be traced to an inner radius of $sim$170~AU and the rotation curve is consistent with an R$^{-1}$ profile, implying that angular momentum is being conserved. Observations of the 1.3~mm dust continuum with the SMA reveal a resolved continuum source, extended in the direction of the dark lane, orthogonal to the outflow. The deconvolved size of the continuum indicates a radius of $sim$100~AU for the continuum source at the assumed distance of 400~pc. The visibility amplitude profile of the continuum emission cannot be reproduced by an envelope alone and needs a compact component. Thus, we posit that the resolved continuum source could be tracing a Keplerian disk in this very young system. If we assume that the continuum radius traces a Keplerian disk (R$sim$120~AU) the observed rotation profile is consistent with a protostar mass of 1.0~$M_{odot}$.
We present observations toward HOPS 383, the first known outbursting Class 0 protostar located within the Orion molecular cloud using ALMA, VLA, and SMA. The SMA observations reveal envelope scale continuum and molecular line emission surrounding HOPS 383 at 0.85 mm, 1.1 mm, and 1.3 mm. The images show that HCO$^+$ and H$^{13}$CO$^+$ peaks on or near the continuum, while N$_2$H$^+$ is reduced at the same position. This reflects the underlying chemistry where CO evaporating close to the protostar destroys N$_2$H$^+$ while forming HCO$^+$. We also observe the molecular outflow traced by $^{12}$CO ($J = 2 rightarrow 1$) and ($J = 3 rightarrow 2$). A disk is resolved in the ALMA 0.87 mm dust continuum, orthogonal to the outflow direction, with an apparent radius of $sim$62 AU. Radiative transfer modeling of the continuum gives disk masses of 0.02 M$_{odot}$ when fit to the ALMA visibilities. The models including VLA 8 mm data indicate that the disk mass could be up to a factor of 10 larger due to lower dust opacity at longer wavelengths. The disk temperature and surface density profiles from the modeling, and an assumed protostar mass of 0.5 M$_{odot}$ suggest that the Toomre $Q$ parameter $< 1$ before the outburst, making gravitational instability a viable mechanism to explain outbursts at an early age if the disk is sufficiently massive.
We present near-infrared K-band spectra for a sample of 7 Class 0 protostars in the Perseus and Orion star-forming regions. We detect Br gamma, CO overtone, and H2 emission, features that probe the near circumstellar environment of the protostar and reveal evidence of magnetospheric accretion, a hot inner disk atmosphere, and outflows, respectively. Comparing the properties of these features with those of Class I sources from the literature, we find that their Br gamma and CO emission are generally consistent in strength and velocity width. The Br gamma line profiles are broad and centrally peaked, with FWHMs of 200 km/s and wings extending to 300 km/s. The line ratios of our H2 emission features, which are spatially extended for some sources, are consistent with shock excitation and indicate the presence of strong jets or a disk wind. Within our small sample, the frequency of CO band emission (67%) is high relative to that of Class I samples (15%), indicating that Class 0s have high inner disk accretion rates, similar to those of the most actively accreting Class I sources. Collectively, our results suggest that Class 0 sources have similar accretion mechanisms to the more evolved classes, with strong organized stellar magnetic fields established at the earliest observable stage of evolution.
During the protostellar phase of stellar evolution, accretion onto the star is expected to be variable, but this suspected variability has been difficult to detect because protostars are deeply embedded. In this paper, we describe a sub-mm luminosity burst of the Class I protostar EC 53 in Serpens Main, the first variable found during our dedicated JCMT/SCUBA-2 monitoring program of eight nearby star-forming regions. EC 53 remained quiescent for the first 6 months of our survey, from February to August 2016. The sub-mm emission began to brighten in September 2016, reached a peak brightness of $1.5$ times the faint state, and has been decaying slowly since February 2017. The change in sub-mm brightness is interpreted as dust heating in the envelope, generated by a luminosity increase of the protostar of a factor of $ge 4$. The 850~$mu$m lightcurve resembles the historical $K$-band lightcurve, which varies by a factor of $sim 6$ with a 543 period and is interpreted as accretion variability excited by interactions between the accretion disk and a close binary system. The predictable detections of accretion variability observed at both near-infrared and sub-mm wavelengths make the system a unique test-bed, enabling us to capture the moment of the accretion burst and to study the consequences of the outburst on the protostellar disk and envelope.
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