Protostellar accretion traced with chemistry: Comparing synthetic C18O maps of embedded protostars to real observations

Context: Understanding how protostars accrete their mass is a central question of star formation. One aspect of this is trying to understand whether the time evolution of accretion rates in deeply embedded objects is best characterised by a smooth decline from early to late stages or by intermittent bursts of high accretion. Aims: We create synthetic observations of deeply embedded protostars in a large numerical simulation of a molecular cloud, which are compared directly to real observations. The goal is to compare episodic accretion events in the simulation to observations and to test the methodology used for analysing the observations. Methods: Simple freeze-out and sublimation chemistry is added to the simulation, and synthetic C$^{18}$O line cubes are created for a large number of simulated protostars. The spatial extent of C$^{18}$O is measured for the simulated protostars and compared directly to a sample of 16 deeply embedded protostars observed with the Submillimeter Array. If CO is distributed over a larger area than predicted based on the protostellar luminosity, it may indicate that the luminosity has been higher in the past and that CO is still in the process of refreezing. Results: Approximately 1% of the protostars in the simulation show extended C$^{18}$O emission, as opposed to approximately 50% in the observations, indicating that the magnitude and frequency of episodic accretion events in the simulation is too low relative to observations. The protostellar accretion rates in the simulation are primarily modulated by infall from the larger scales of the molecular cloud, and do not include any disk physics. The discrepancy between simulation and observations is taken as support for the necessity of disks, even in deeply embedded objects, to produce episodic accretion events of sufficient frequency and amplitude.Comment: Accepted for publication in A&A, 11 pages, 8 figures; v2 contains minor updates to the languag

Warm water deuterium fractionation in IRAS 16293-2422 - The high-resolution ALMA and SMA view

Measuring the water deuterium fractionation in the inner warm regions of low-mass protostars has so far been hampered by poor angular resolution obtainable with single-dish ground- and space-based telescopes. Observations of water isotopologues using (sub)millimeter wavelength interferometers have the potential to shed light on this matter. Observations toward IRAS 16293-2422 of the 5(3,2)-4(4,1) transition of H2-18O at 692.07914 GHz from Atacama Large Millimeter/submillimeter Array (ALMA) as well as the 3(1,3)-2(2,0) of H2-18O at 203.40752 GHz and the 3(1,2)-2(2,1) transition of HDO at 225.89672 GHz from the Submillimeter Array (SMA) are presented. The 692 GHz H2-18O line is seen toward both components of the binary protostar. Toward one of the components, "source B", the line is seen in absorption toward the continuum, slightly red-shifted from the systemic velocity, whereas emission is seen off-source at the systemic velocity. Toward the other component, "source A", the two HDO and H2-18O lines are detected as well with the SMA. From the H2-18O transitions the excitation temperature is estimated at 124 +/- 12 K. The calculated HDO/H2O ratio is (9.2 +/- 2.6)*10^(-4) - significantly lower than previous estimates in the warm gas close to the source. It is also lower by a factor of ~5 than the ratio deduced in the outer envelope. Our observations reveal the physical and chemical structure of water vapor close to the protostars on solar-system scales. The red-shifted absorption detected toward source B is indicative of infall. The excitation temperature is consistent with the picture of water ice evaporation close to the protostar. The low HDO/H2O ratio deduced here suggests that the differences between the inner regions of the protostars and the Earth's oceans and comets are smaller than previously thought.Comment: Accepted for publication in Astronomy & Astrophysic

Tentative detection of ethylene glycol toward W51/e2 and G34.3+0.2

How complex organic - and potentially prebiotic - molecules are formed in regions of low- and high-mass star-formation remains a central question in astrochemistry. In particular, with just a few sources studied in detail, it is unclear what role environment plays in complex molecule formation. In this light, a comparison of relative abundances of related species between sources might be useful to explain observed differences. We seek to measure the relative abundance between three important complex organic molecules, ethylene glycol ((CH$_2$OH)$_2$), glycolaldehyde (CH$_2$OHCHO) and methyl formate (HCOOCH$_3$), toward high-mass protostars and thereby provide additional constraints on their formation pathways. We use IRAM 30-m single dish observations of the three species toward two high-mass star-forming regions - W51/e2 and G34.3+0.2 - and report a tentative detection of (CH2OH)2 toward both sources. Assuming that (CH$_2$OH)$_2$, CH$_2$OHCHO and HCOOCH$_3$ spatially coexist, relative abundance ratios, HCOOCH$_3$/(CH$_2$OH)$_2$, of 31 and 35 are derived for G34.3+0.2 and W51/e2, respectively. CH$_2$OHCHO is not detected, but the data provide lower limits to the HCOOCH$_3$/CH$_2$OHCHO abundance ratios of $\ge$193 for G34.3+0.2 and $\ge$550 for W51/e2. A comparison of these results to measurements from various sources in the literature indicates that the source luminosities may be correlated with the HCOOCH$_3$/(CH$_2$OH)$_2$ and HCOOCH$_3$/CH$_2$OHCHO ratios. This apparent correlation may be a consequence of the relative timescales each source spend at different temperatures-ranges in their evolution. Furthermore, we obtain lower limits to the ratio of (CH$_2$OH)$_2$/CH2OHCHO for G34.3+0.2 ($\ge$6) and W51/e2 ($\ge$16). This result confirms that a high (CH$_2$OH)$_2$/CH$_2$OHCHO abundance ratio is not a specific property of comets, as previously speculated.Comment: Accepted for publication by A&

Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments

It is not known whether the original carriers of Earth's nitrogen were molecular ices or refractory dust. To investigate this question, we have used data and results of Herschel observations towards two protostellar sources: the high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422. Towards Orion KL, our analysis of the molecular inventory of Crockett et al. (2014) indicates that HCN is the organic molecule that contains by far the most nitrogen, carrying $74_{-9}^{+5}\%$ of nitrogen-in-organics. Following this evidence, we explore HCN towards IRAS 16293-2422, which we consider a solar analog. Towards IRAS 16293-2422, we have reduced and analyzed Herschel spectra of HCN, and fit these observations against "jump" abundance models of IRAS 16293-2422's protostellar envelope. We find an inner-envelope HCN abundance $X_{\textrm{in}} = 5.9\pm0.7 \times 10^{-8}$ and an outer-envelope HCN abundance $X_{\textrm{out}} = 1.3 \pm 0.1 \times 10^{-9}$. We also find the sublimation temperature of HCN to be $T_{\textrm{jump}} = 71 \pm 3$~K; this measured $T_{\textrm{jump}}$ enables us to predict an HCN binding energy $E_{\textrm{B}}/k = 3840 \pm 140$~K. Based on a comparison of the HCN/H2O ratio in these protostars to N/H2O ratios in comets, we find that HCN (and, by extension, other organics) in these protostars is incapable of providing the total bulk N/H2O in comets. We suggest that refractory dust, not molecular ices, was the bulk provider of nitrogen to comets. However, interstellar dust is not known to have 15N enrichment, while high 15N enrichment is seen in both nitrogen-bearing ices and in cometary nitrogen. This may indicate that these 15N-enriched ices were an important contributor to the nitrogen in planetesimals and likely to the Earth.Comment: Accepted to ApJ; 21 pages, 4 figure

Externally heated protostellar cores in the Ophiuchus star-forming region

We present APEX 218 GHz observations of molecular emission in a complete sample of embedded protostars in the Ophiuchus star-forming region. To study the physical properties of the cores, we calculate H$_2$CO and c-C$_3$H$_2$ rotational temperatures, both of which are good tracers of the kinetic temperature of the molecular gas. We find that the H$_2$CO temperatures range between 16 K and 124 K, with the highest H$_2$CO temperatures toward the hot corino source IRAS 16293-2422 (69-124 K) and the sources in the $\rho$ Oph A cloud (23-49 K) located close to the luminous Herbig Be star S 1, which externally irradiates the $\rho$ Oph A cores. On the other hand, the c-C$_3$H$_2$ rotational temperature is consistently low (7-17 K) in all sources. Our results indicate that the c-C$_3$H$_2$ emission is primarily tracing more shielded parts of the envelope whereas the H$_2$CO emission (at the angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the outer irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino emission dominates. In some sources, a secondary velocity component is also seen, possibly tracing the molecular outflow.Comment: 19 pages, 9 figures, accepted for publication in Ap

The Class 0 Protostar BHR71: Herschel Observations and Dust Continuum Models

We use Herschel spectrophotometry of BHR71, an embedded Class 0 protostar, to provide new constraints on its physical properties. We detect 645 (non-unique) spectral lines amongst all spatial pixels. At least 61 different spectral lines originate from the central region. A CO rotational diagram analysis shows four excitation temperature components, 43 K, 197 K, 397 K, and 1057 K. Low-J CO lines trace the outflow while the high-J CO lines are centered on the infrared source. The low-excitation emission lines of H2O trace the large-scale outflow, while the high-excitation emission lines trace a small-scale distribution around the equatorial plane. We model the envelope structure using the dust radiative transfer code, Hyperion, incorporating rotational collapse, an outer static envelope, outflow cavity, and disk. The evolution of a rotating collapsing envelope can be constrained by the far-infrared/millimeter SED along with the azimuthally-averaged radial intensity profile, and the structure of the outflow cavity plays a critical role at shorter wavelengths. Emission at 20-40 um requires a cavity with a constant-density inner region and a power-law density outer region. The best fit model has an envelope mass of 19 solar mass inside a radius of 0.315 pc and a central luminosity of 18.8 solar luminosity. The time since collapse began is 24630-44000 yr, most likely around 36000 yr. The corresponding mass infall rate in the envelope (1.2x10$^{-5}$ solar mass per year) is comparable to the stellar mass accretion rate, while the mass loss rate estimated from the CO outflow is 20% of the stellar mass accretion rate. We find no evidence for episodic accretion.Comment: Accepted for publication in ApJ. 33 pages; 34 figures; 4 table

The organic chemistry in the innermost, infalling envelope of the Class 0 protostar L483

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$^{\prime\prime}$ (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.Comment: 19 pages, 11 figure

The Co-evolution of Disk and Star in Embedded Stages: The Case of the Very Low-mass Protostar

We have observed the CCH (N=3-2, J=7/2-5/2, F=4-3 and 3-2) and SO (6_7-5_6) emission at a 0"2 angular resolution toward the low-mass Class 0 protostellar source IRAS 15398-3359 with ALMA. The CCH emission traces the infalling-rotating envelope near the protostar with the outflow cavity extended along the northeast-southwest axis. On the other hand, the SO emission has a compact distribution around the protostar. The CCH emission is relatively weak at the continuum peak position, while the SO emission has a sharp peak there. Although the maximum velocity shift of the CCH emission is about 1 km s^-1 from the systemic velocity, a velocity shift higher than 2 km s^{-1} is seen for the SO emission. This high velocity component is most likely associated with the Keplerian rotation around the protostar. The protostellar mass is estimated to be 0.007^{+0.004}_{-0.003} from the velocity profile of the SO emission. With this protostellar mass, the velocity structure of the CCH emission can be explained by the model of the infalling-rotating envelope, where the radius of the centrifugal barrier is estimated to be 40 au from the comparison with the model. The disk mass evaluated from the dust continuum emission by assuming the dust temperature of 20 K-100 K is 0.1-0.9 times the stellar mass, resulting in the Toomre Q parameter of 0.4-5. Hence, the disk structure may be partly unstable. All these results suggest that a rotationally-supported disk can be formed in the earliest stages of the protostellar evolution