202 research outputs found
Molecular Cloud Formation Behind Shock Waves
We examine the formation of molecular gas behind shocks in atomic gas using a
chemical/dynamical model, particular emphasis is given to constraints the
chemistry places on the dynamical evolution. The most important result of this
study is to stress the importance of shielding the molecular gas from the
destructive effects of UV radiation. For shock ram pressures comparable to or
exceeding typical local ISM pressures, self-shielding controls the formation
time of H2 but CO formation requires shielding of the interstellar radiation
field by dust grains. We find that the molecular hydrogen fractional abundance
can become significant well before CO forms. The timescale for (CO) molecular
cloud formation is not set by H2 formation, but rather by the timescale for
accumulating a sufficient column density or extinction, A_V > 0.7. The local
ratio of atomic to molecular gas (4:1), coupled with short estimates for cloud
lifetimes (3-5 Myr), suggests that the timescales for accumulating molecular
clouds from atomic material typically must be no longer than about 12-20 Myr.
Based on the shielding requirement, this implies that the typical product of
pre-shock density and velocity must be n*v > 20 cm^-3 km s^-1. Based on these
results we find that flow-driven formation of molecular clouds in the local
interstellar medium can occur sufficiently rapidly to account for observations.
We also provide detailed predictions of atomic and molecular emission and
absorption that track molecular cloud formation, with a view toward helping to
verify cloud formation by shock waves. Finally, we provide an analytic solution
for time-dependent H2 formation which may be of use in numerical hydrodynamic
calculations.Comment: 43 pages, 13 figures, accepted by ApJ main journa
UV-driven Chemistry as a Signpost for Late-stage Planet Formation
The chemical reservoir within protoplanetary disks has a direct impact on
planetary compositions and the potential for life. A long-lived carbon-and
nitrogen-rich chemistry at cold temperatures (<=50K) is observed within cold
and evolved planet-forming disks. This is evidenced by bright emission from
small organic radicals in 1-10 Myr aged systems that would otherwise have
frozen out onto grains within 1 Myr. We explain how the chemistry of a
planet-forming disk evolves from a cosmic-ray/X-ray-dominated regime to an
ultraviolet-dominated chemical equilibrium. This, in turn, will bring about a
temporal transition in the chemical reservoir from which planets will accrete.
This photochemical dominated gas phase chemistry develops as dust evolves via
growth, settling and drift, and the small grain population is depleted from the
disk atmosphere. A higher gas-to-dust mass ratio allows for deeper penetration
of ultraviolet photons is coupled with a carbon-rich gas (C/O > 1) to form
carbon-bearing radicals and ions. This further results in gas phase formation
of organic molecules, which then would be accreted by any actively forming
planets present in the evolved disk.Comment: Accepted to Nature Astronomy, Published Dec 8th 202
A Triple Protostar System Formed via Fragmentation of a Gravitationally Unstable Disk
Binary and multiple star systems are a frequent outcome of the star formation
process, and as a result, almost half of all sun-like stars have at least one
companion star. Theoretical studies indicate that there are two main pathways
that can operate concurrently to form binary/multiple star systems: large scale
fragmentation of turbulent gas cores and filaments or smaller scale
fragmentation of a massive protostellar disk due to gravitational instability.
Observational evidence for turbulent fragmentation on scales of 1000~AU has
recently emerged. Previous evidence for disk fragmentation was limited to
inferences based on the separations of more-evolved pre-main sequence and
protostellar multiple systems. The triple protostar system L1448 IRS3B is an
ideal candidate to search for evidence of disk fragmentation. L1448 IRS3B is in
an early phase of the star formation process, likely less than 150,000 years in
age, and all protostars in the system are separated by 200~AU. Here we
report observations of dust and molecular gas emission that reveal a disk with
spiral structure surrounding the three protostars. Two protostars near the
center of the disk are separated by 61 AU, and a tertiary protostar is
coincident with a spiral arm in the outer disk at a 183 AU separation. The
inferred mass of the central pair of protostellar objects is 1 M,
while the disk surrounding the three protostars has a total mass of 0.30
M_{\sun}. The tertiary protostar itself has a minimum mass of 0.085
M. We demonstrate that the disk around L1448 IRS3B appears susceptible
to disk fragmentation at radii between 150~AU and 320~AU, overlapping with the
location of the tertiary protostar. This is consistent with models for a
protostellar disk that has recently undergone gravitational instability,
spawning one or two companion stars.Comment: Published in Nature on Oct. 27th. 24 pages, 8 figure
The Birth of a Galaxy. II. The Role of Radiation Pressure
Massive stars provide feedback that shapes the interstellar medium of
galaxies at all redshifts and their resulting stellar populations. Here we
present three adaptive mesh refinement radiation hydrodynamics simulations that
illustrate the impact of momentum transfer from ionising radiation to the
absorbing gas on star formation in high-redshift dwarf galaxies. Momentum
transfer is calculated by solving the radiative transfer equation with a ray
tracing algorithm that is adaptive in spatial and angular coordinates. We find
that momentum input partially affects star formation by increasing the
turbulent support to a three-dimensional rms velocity equal to the circular
velocity of early haloes. Compared to a calculation that neglects radiation
pressure, the star formation rate is decreased by a factor of five to 1.8 x
10^{-2} Msun/yr in a dwarf galaxy with a dark matter and stellar mass of 2.0 x
10^8 and 4.5 x 10^5 solar masses, respectively, when radiation pressure is
included. Its mean metallicity of 10^{-2.1} Z_sun is consistent with the
observed dwarf galaxy luminosity-metallicity relation. However, what one may
naively expect from the calculation without radiation pressure, the central
region of the galaxy overcools and produces a compact, metal-rich stellar
population with an average metallicity of 0.3 Z_sun, indicative of an incorrect
physical recipe. In addition to photo-heating in HII regions, radiation
pressure further drives dense gas from star forming regions, so supernovae
feedback occurs in a warmer and more diffuse medium, launching metal-rich
outflows. Capturing this aspect and a temporal separation between the start of
radiative and supernova feedback are numerically important in the modeling of
galaxies to avoid the "overcooling problem". We estimate that dust in early
low-mass galaxies is unlikely to aid in momentum transfer from radiation to the
gas.Comment: 18 pages, 11 figures, replaced with accepted version, MNRAS. Minor
changes with the conclusions unaffecte
Water-Rich Disks around Late M-stars Unveiled: Exploring the Remarkable Case of Sz114
We present an analysis of the JDISC JWST/MIRI-MRS spectrum of Sz~114, an
accreting M5 star surrounded by a large dust disk with a shallow gap at au. The spectrum is molecular-rich: we report the detection of water, CO,
CO, HCN, CH, and H. The only identified atomic/ionic transition
is from [NeII] at 12.81 micron. A distinct feature of this spectrum is the
forest of water lines with the 17.22 micron emission surpassing that of most
mid-to-late M-star disks by an order of magnitude in flux and aligning instead
with disks of earlier-type stars. Moreover, flux ratios of CH/HO
and HCN/HO in Sz~114 also resemble those of earlier-type disks, with a
slightly elevated CO/HO ratio. While accretional heating can boost all
infrared lines, the unusual properties of Sz~114 could be explained by the
young age of the source, its formation under unusual initial conditions (a
large massive disk), and the presence of dust substructures. The latter delays
the inward drift of icy pebbles and help preserve a lower C/O ratio over an
extended period. In contrast, mid-to-late M-star disks--which are typically
faint, small in size, and likely lack significant substructures--may have more
quickly depleted the outer icy reservoir and already evolved out of a
water-rich inner disk phase. Our findings underscore the unexpected diversity
within mid-infrared spectra of mid-to-late M-star disks, highlighting the need
to expand the observational sample for a comprehensive understanding of their
variations and thoroughly test pebble drift and planet formation models.Comment: 16 pages, 8 figures, accepted by ApJ
Molecules with ALMA at Planet-forming Scales (MAPS). VIII. CO gap in AS 209-gas depletion or chemical processing?
Funding: I.C. was supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51405.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. J.D.I. acknowledges support from the Science and Technology Facilities Council of the United Kingdom (STFC) under ST/T000287/1. C.W. acknowledges financial support from the University of Leeds, SFTC, and UKRI (grant Nos. ST/R000549/1, ST/T000287/1, and MR/T040726/1).Emission substructures in gas and dust are common in protoplanetary disks. Such substructures can be linked to planet formation or planets themselves. We explore the observed gas substructures in AS 209 using thermochemical modeling with RAC2D and high-spatial-resolution data from the Molecules with ALMA at Planet-forming Scales (MAPS) program. The observations of C18O J = 2-1 emission exhibit a strong depression at 88 au overlapping with the positions of multiple gaps in millimeter dust continuum emission. We find that the observed CO column density is consistent with either gas surface-density perturbations or chemical processing, while C2H column density traces changes in the C/O ratio rather than the H2 gas surface density. However, the presence of a massive planet (>0.2 MJup) would be required to account for this level of gas depression, which conflicts with constraints set by the dust emission and the pressure profile measured by gas kinematics. Based on our models, we infer that a local decrease of CO abundance is required to explain the observed structure in CO, dominating over a possible gap-carving planet present and its effect on the H2 surface density. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.Publisher PDFPeer reviewe
Molecules with ALMA at Planet-forming Scales (MAPS). VI. Distribution of the small organics HCN, C2H, and H2CO
Funding: I.C. was supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51405.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. C.W. acknowledges financial support from the University of Leeds, STFC, and UKRI (grant Nos. ST/R000549/1, ST/T000287/1, and MR/T040726/1). J.D.I. acknowledges support from the Science and Technology Facilities Council of the United Kingdom (STFC) under ST/T000287/1.Small organic molecules, such as C2H, HCN, and H2CO, are tracers of the C, N, and O budget in protoplanetary disks. We present high-angular-resolution (10-50 au) observations of C2H, HCN, and H2CO lines in five protoplanetary disks from the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program. We derive column density and excitation temperature profiles for HCN and C2H, and find that the HCN emission arises in a temperate (20-30 K) layer in the disk, while C2H is present in relatively warmer (20-60 K) layers. In the case of HD 163296, we find a decrease in column density for HCN and C2H inside one of the dust gaps near ~83 au, where a planet has been proposed to be located. We derive H2CO column density profiles assuming temperatures between 20 and 50 K, and find slightly higher column densities in the colder disks around T Tauri stars than around Herbig Ae stars. The H2CO column densities rise near the location of the CO snowline and/or millimeter dust edge, suggesting an efficient release of H2CO ices in the outer disk. Finally, we find that the inner 50 au of these disks are rich in organic species, with abundances relative to water that are similar to cometary values. Comets could therefore deliver water and key organics to future planets in these disks, similar to what might have happened here on Earth. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.Publisher PDFPeer reviewe
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