20 research outputs found
Carbon-grain sublimation: a new top-down component of protostellar chemistry
Earth's carbon deficit has been an outstanding problem in our understanding
of the formation of our Solar System. A possible solution would be the
sublimation of carbon grains at the so-called soot line (~300 K) early in the
planet-formation process. Here, we argue that the most likely signatures of
this process are an excess of hydrocarbons and nitriles inside the soot line,
and a higher excitation temperature for these molecules compared to
oxygen-bearing complex organics that desorb around the water snowline (~100 K).
Such characteristics have been reported in the literature, for example, in
Orion KL, although not uniformly, potentially due to differences in
observational settings and analysis methods of different studies or related to
the episodic nature of protostellar accretion. If this process is active, this
would mean that there is a heretofore unknown component to the carbon chemistry
during the protostellar phase that is acting from the top down - starting from
the destruction of larger species - instead of from the bottom up from atoms.
In the presence of such a top-down component, the origin of organic molecules
needs to be re-explored.Comment: Accepted for publication in ApJL. 14 pages, 2 figures, 1 table (4
pages
The young embedded disk L1527 IRS: constraints on the water snowline and cosmic ray ionization rate from HCO+ observations
The water snowline in circumstellar disks is a crucial component in planet
formation, but direct observational constraints on its location remain sparse
due to the difficulty of observing water in both young embedded and mature
protoplanetary disks. Chemical imaging provides an alternative route to locate
the snowline, and HCO isotopologues have been shown to be good tracers in
protostellar envelopes and Herbig disks. Here we present
0.5 resolution (35 au radius) Atacama Large
Millimeter/submillimeter Array (ALMA) observations of HCO and
HCO toward the young (Class 0/I) disk L1527 IRS. Using a
source-specific physical model with the midplane snowline at 3.4 au and a small
chemical network, we are able to reproduce the HCO and HCO
emission, but for HCO only when the cosmic ray ionization rate is lowered
to s. Even though the observations are not sensitive to the
expected HCO abundance drop across the snowline, the reduction in HCO
above the snow surface and the global temperature structure allow us to
constrain a snowline location between 1.8 and 4.1 au. Deep observations are
required to eliminate the envelope contribution to the emission and to derive
more stringent constraints on the snowline location. Locating the snowline in
young disks directly with observations of HO isotopologues may therefore
still be an alternative option. With a direct snowline measurement, HCO
will be able to provide constraints on the ionization rate.Comment: Accepted for publication in ApJ, 15 pages, 6 figures and appendi
Imaging the water snowline in a protostellar envelope with HCO
Snowlines are key ingredients for planet formation. Providing observational
constraints on the locations of the major snowlines is therefore crucial for
fully connecting planet compositions to their formation mechanism.
Unfortunately, the most important snowline, that of water, is very difficult to
observe directly in protoplanetary disks due to its close proximity to the
central star. Based on chemical considerations, HCO is predicted to be a
good chemical tracer of the water snowline, because it is particularly abundant
in dense clouds when water is frozen out. This work maps the optically thin
isotopologue HCO () toward the envelope of the low-mass
protostar NGC1333-IRAS2A (observed with NOEMA at ~0.9" resolution), where the
snowline is at larger distance from the star than in disks. The HCO
emission peaks ~2" northeast of the continuum peak, whereas the previously
observed HO shows compact emission on source. Quantitative modeling
shows that a decrease in HCO abundance by at least a factor of six
is needed in the inner ~360 AU to reproduce the observed emission profile.
Chemical modeling predicts indeed a steep increase in HCO just outside the
water snowline; the 50% decrease in gaseous HO at the snowline is not
enough to allow HCO to be abundant. This places the water snowline at 225
AU, further away from the star than expected based on the 1D envelope
temperature structure for NGC1333-IRAS2A. In contrast, DCO observations
show that the CO snowline is at the expected location, making an outburst
scenario unlikely. The spatial anticorrelation of the HCO and
HO emission provide a proof of concept that HCO can be used
as a tracer of the water snowline.Comment: 10 pages, 8 figures, 3 tables. Accepted for publication in A&
Temperature structures of embedded disks: young disks in Taurus are warm
The chemical composition of gas and ice in disks around young stars set the
bulk composition of planets. In contrast to protoplanetary disks (Class II),
young disks that are still embedded in their natal envelope (Class 0 and I) are
predicted to be too warm for CO to freeze out, as has been confirmed
observationally for L1527 IRS. To establish whether young disks are generally
warmer than their more evolved counterparts, we observed five young (Class 0/I
and Class I) disks in Taurus with the Atacama Large Millimeter/submillimeter
Array (ALMA), targeting CO , HCO , HDO
and CHOH transitions at resolution. The different freeze-out temperatures
of these species allow us to derive a global temperature structure. CO
and HCO are detected in all disks, with no signs of CO freeze-out in the
inner 100 au, and a CO abundance close to 10. HCO
emission originates in the surface layers of the two edge-on disks, as
witnessed by the especially beautiful V-shaped emission pattern in
IRAS~04302+2247. HDO and CHOH are not detected, with column density upper
limits more than 100 times lower than for hot cores. Young disks are thus found
to be warmer than more evolved protoplanetary disks around solar analogues,
with no CO freeze-out (or only in the outermost part of 100 au disks)
or CO processing. However, they are not as warm as hot cores or disks around
outbursting sources, and therefore do not have a large gas-phase reservoir of
complex molecules.Comment: Accepted for publication in ApJ. 19 pages, 11 figures, 3 tables (+
appendix
The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars. IV. Unveiling the Embedded Intermediate-Mass Protostar and Disk within OMC2-FIR3/HOPS-370
We present ALMA (0.87 and 1.3 mm) and VLA (9 mm) observations toward the candidate intermediate-mass protostar OMC2-FIR3 (HOPS-370; L_(bol) ~ 314 L_⊙) at ~0."1 (40 au) resolution for the continuum emission and ~0."25 (100 au) resolution of nine molecular lines. The dust continuum observed with ALMA at 0.87 and 1.3 mm resolves a near edge-on disk toward HOPS-370 with an apparent radius of ~100 au. The VLA observations detect both the disk in dust continuum and free–free emission extended along the jet direction. The ALMA observations of molecular lines (H₂CO, SO, CH₃OH, ¹³CO, C¹⁸O, NS, and H¹³CN) reveal rotation of the apparent disk surrounding HOPS-370 orthogonal to the jet/outflow direction. We fit radiative transfer models to both the dust continuum structure of the disk and molecular line kinematics of the inner envelope and disk for the H₂CO, CH₃OH, NS, and SO lines. The central protostar mass is determined to be ~2.5 M_⊙ with a disk radius of ~94 au, when fit using combinations of the H₂CO, CH₃OH, NS, and SO lines, consistent with an intermediate-mass protostar. Modeling of the dust continuum and spectral energy distribution yields a disk mass of 0.035 M_⊙ (inferred dust+gas) and a dust disk radius of 62 au; thus, the dust disk may have a smaller radius than the gas disk, similar to Class II disks. In order to explain the observed luminosity with the measured protostar mass, HOPS-370 must be accreting at a rate of (1.7−3.2) × 10⁻⁵ M_⊙ yr⁻¹
Early Planet Formation in Embedded Disks (eDisk) XII: Accretion streamers, protoplanetary disk, and outflow in the Class I source Oph IRS63
We present ALMA observations of the Class I source Oph IRS63 in the context
of the Early Planet Formation in Embedded Disks (eDisk) large program. Our ALMA
observations of Oph IRS63 show a myriad of protostellar features, such as a
shell-like bipolar outflow (in CO), an extended rotating envelope
structure (in CO), a streamer connecting the envelope to the disk (in
CO), and several small-scale spiral structures seen towards the edge of
the dust continuum (in SO). By analyzing the velocity pattern of CO and
CO, we measure a protostellar mass of ~ and confirm the presence of a disk rotating at almost Keplerian
velocity that extends up to au. These calculations also show that the
gaseous disk is about four times larger than the dust disk, which could
indicate dust evolution and radial drift. Furthermore, we model the CO
streamer and SO spiral structures as features originating from an infalling
rotating structure that continuously feeds the young protostellar disk. We
compute an envelope-to-disk mass infall rate of ~ and compare it to the disk-to-star mass accretion rate of ~, from which we infer that the protostellar
disk is in a mass build-up phase. At the current mass infall rate, we speculate
that soon the disk will become too massive to be gravitationally stable.Comment: 26 pages and 17 figure