Massive galaxies in the early Universe have been shown to be forming stars at surprisingly high rates. Prominent examples are dust-obscured galaxies which are luminous when observed at sub-millimetre ...wavelengths and which may be forming stars at a rate of 1,000 solar masses (M ) per year. These intense bursts of star formation are believed to be driven by mergers between gas-rich galaxies. Probing the properties of individual star-forming regions within these galaxies, however, is beyond the spatial resolution and sensitivity of even the largest telescopes at present. Here we report observations of the sub-millimetre galaxy SMMJ2135-0102 at redshift z = 2.3259, which has been gravitationally magnified by a factor of 32 by a massive foreground galaxy cluster lens. This magnification, when combined with high-resolution sub-millimetre imaging, resolves the star-forming regions at a linear scale of only 100 parsecs. We find that the luminosity densities of these star-forming regions are comparable to the dense cores of giant molecular clouds in the local Universe, but they are about a hundred times larger and 107 times more luminous. Although vigorously star-forming, the underlying physics of the star-formation processes at z 2 appears to be similar to that seen in local galaxies, although the energetics are unlike anything found in the present-day Universe.
Millimeter observations of CO gas in planetesimal belts show a high detection rate around A stars, but few detections for later type stars. We present the first CO detection in a planetesimal belt ...around an M star, TWA 7. The optically thin CO (J = 3-2) emission is colocated with previously identified dust emission from the belt, and the emission velocity structure is consistent with Keplerian rotation around the central star. The detected CO is not well shielded against photodissociation, and must thus be continuously replenished by gas release from exocomets within the belt. We analyze in detail the process of exocometary gas release and destruction around young M dwarfs and how this process compares to earlier type stars. Taking these differences into account, we find that CO generation through exocometary gas release naturally explains the increasing CO detection rates with stellar luminosity, mostly because the CO production rate from the collisional cascade is directly proportional to stellar luminosity. More luminous stars will therefore on average host more massive (and hence more easily detectable) exocometary CO disks, leading to the higher detection rates observed. The current CO detection rates are consistent with a ubiquitous release of exocometary gas in planetesimal belts, independent of spectral type.
Context. Deuterium fractionation has been used to study the thermal history of prestellar environments. Their formation pathways trace different regions of the disk and may shed light into the ...physical structure of the disk, including locations of important features for planetary formation. Aims. We aim to constrain the radial extent of the main deuterated species; we are particularly interested in spatially characterizing the high and low temperature pathways for enhancing deuteration of these species. Methods. We observed the disk surrounding the Herbig Ae star HD 163296 using ALMA in Band 6 and obtained resolved spectral imaging data of DCO+ (J = 3 − 2), DCN (J = 3 − 2) and N2D+ (J = 3 − 2) with synthesized beam sizes of 0.̋53 × 0.̋42, 0.̋53 × 0.̋42, and 0.̋50 × 0.̋39, respectively. We adopted a physical model of the disk from the literature and use the 3D radiative transfer code LIME to estimate an excitation temperature profile for our detected lines. We modeled the radial emission profiles of DCO+, DCN, and N2D+, assuming their emission is optically thin, using a parametric model of their abundances and our excitation temperature estimates. Results. DCO+ can be described by a three-region model with constant-abundance rings centered at 70 AU, 150 AU, and 260 AU. The DCN radial profile peaks at about 60 AU and N2D+ is seen in a ring at 160 AU. Simple models of both molecules using constant abundances reproduce the data. Assuming reasonable average excitation temperatures for the whole disk, their disk-averaged column densities (and deuterium fractionation ratios) are 1.6–2.6×1012 cm-2 (0.04–0.07), 2.9–5.2×1012 cm-2 (~0.02), and 1.6–2.5×1011 cm-2 (0.34–0.45) for DCO+, DCN, and N2D+, respectively. Conclusions. Our simple best-fit models show a correlation between the radial location of the first two rings in DCO+ and the DCN and N2D+ abundance distributions that can be interpreted as the high and low temperature deuteration pathways regimes. The origin of the third DCO+ ring at 260 AU is unknown but may be due to a local decrease of ultraviolet opacity allowing the photodesorption of CO or due to thermal desorption of CO as a consequence of radial drift and settlement of dust grains. The derived Df values agree with previous estimates of 0.05 for DCO+/HCO+ and 0.02 for DCN/HCN in HD 163296, and 0.3−0.5 for N2D+/N2H+ in AS 209, a T Tauri disk. The high N2D+/N2H+ confirms N2D+ as a good candidate for tracing ionization in the cold outer disk.
We investigate the apparent discrepancy between gas and dust outer radii derived from millimeter observations of protoplanetary disks. Using 230 and 345 GHz continuum and CO image data from the ...Submillimeter Array for four nearby disk systems (HD 163296, TW Hydrae, GM Aurigae, and MWC 480), we examine models of circumstellar disk structure and the effects of their treatment of the outer disk edge. We show that for these disks, models described by power laws in surface density and temperature that are truncated at an outer radius are incapable of reproducing both the gas and dust emission simultaneously: the outer radius derived from the dust continuum emission is always significantly smaller than the extent of the molecular gas disk traced by CO emission. However, a simple model motivated by similarity solutions of the time evolution of accretion disks that includes a tapered exponential edge in the surface density distribution (and the same number of free parameters) does much better at reproducing both the gas and dust emission. While this analysis does not rule out the disparate radii implied by the truncated power law models, a realistic alternative disk model, grounded in the physics of accretion, provides a consistent picture for the extent of both the gas and dust.
We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm (230 GHz) observations of the HD 32297 and HD 61005 debris disks, two of the most iconic debris disks because of their dramatic ...swept-back wings seen in scattered light images. These observations achieve sensitivities of 14 and 13 Jy beam−1 for HD 32297 and HD 61005, respectively, and provide the highest resolution images of these two systems at millimeter wavelengths to date. By adopting a Markov Chain Monte Carlo modeling approach, we determine that both disks are best described by a two-component model consisting of a broad (ΔR/R > 0.4) planetesimal belt with a rising surface density gradient and a steeply falling outer halo aligned with the scattered light disk. The inner and outer edges of the planetesimal belt are located at 78.5 8.1 au and 122 3 au for HD 32297, and 41.9 0.9 au and 67.0 0.5 au for HD 61005. The halos extend to 440 32 au and 188 8 au, respectively. We also detect 12CO J = 2-1 gas emission from HD 32297 co-located with the dust continuum. These new ALMA images provide observational evidence that larger, millimeter-sized grains may also populate the extended halos of these two disks previously thought to only be composed of small, micron-sized grains. We discuss the implications of these results for potential shaping and sculpting mechanisms of asymmetric debris disks.
Recent Atacama Large Millimeter/submillimeter Array observations present mounting evidence for the presence of exocometary gas released within Kuiper Belt analogs around nearby main-sequence stars. ...This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J = 2-1 emission colocated with dust emission from the cometary belt in the 440 Myr old Fomalhaut system. Through spectrospatial filtering, we achieve a 5.4 detection and determine that the ring's sky-projected rotation axis matches that of the star. The CO mass derived ( ) is the lowest of any circumstellar disk detected to date and must be of exocometary origin. Using a steady-state model, we estimate the CO+CO2 mass fraction of exocomets around Fomalhaut to be between 4.6% and 76%, consistent with solar system comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct interstellar medium inheritance. In addition, we find tentative evidence that % of the detected flux originates from a region near the eccentric belt's pericenter. If confirmed, the latter may be explained through a recent impact event or CO pericenter glow due to exocometary release within a steady-state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.
Resolved observations of millimeter-sized dust, tracing larger planetesimals, have pinpointed the location of 26 Edgeworth-Kuiper Belt analogs. We report that a belt's distance R to its host star ...correlates with the star's luminosity L , following with a low intrinsic scatter of ∼17%. Remarkably, our Edgeworth-Kuiper Belt in the solar system and the two CO snow lines imaged in protoplanetary disks lie close to this R-L relation, suggestive of an intrinsic relationship between protoplanetary disk structures and belt locations. To test the effect of bias on the relation, we use a Monte Carlo approach and simulate uncorrelated model populations of belts. We find that observational bias could produce the slope and intercept of the R-L relation but is unable to reproduce its low scatter. We then repeat the simulation taking into account the collisional evolution of belts, following the steady-state model that fits the belt population as observed through infrared excesses. This significantly improves the fit by lowering the scatter of the simulated R-L relation; however, this scatter remains only marginally consistent with the one observed. The inability of observational bias and collisional evolution alone to reproduce the tight relationship between belt radius and stellar luminosity could indicate that planetesimal belts form at preferential locations within protoplanetary disks. The similar trend for CO snow line locations would then indicate that the formation of planetesimals or planets in the outer regions of planetary systems is linked to the volatility of their building blocks, as postulated by planet formation models.
Context. The gas and dust in circumstellar disks provide the raw materials to form planets. The study of organic molecules and their building blocks in such disks offers insight into the origin of ...the prebiotic environment of terrestrial planets. Aims. We aim to determine the distribution of formaldehyde, H2CO, in the disk around HD 163296 to assess the contribution of gas- and solid-phase formation routes of this simple organic. Methods. Three formaldehyde lines were observed (H2CO 303–202, H2CO 322–221, and H2CO 321–220) in the protoplanetary disk around the Herbig Ae star HD 163296 with ALMA at ~0.5″ (60 AU) spatial resolution. Different parameterizations of the H2CO abundance were compared to the observed visibilities, using either a characteristic temperature, a characteristic radius or a radial power law index to describe the H2CO chemistry. Similar models were applied to ALMA Science Verification data of C18O. In each scenario, χ2 minimization on the visibilities was used to determine the best-fit model in each scenario. Results. H2CO 303–202 was readily detected via imaging, while the weaker H2CO 322–221 and H2CO 321–220 lines required matched filter analysis to detect. H2CO is present throughout most of the gaseous disk, extending out to ~550 AU. An apparent 50 AU inner radius of the H2CO emission is likely caused by an optically thick dust continuum. The H2CO radial intensity profile shows a peak at ~100 AU and a secondary bump at ~300 AU, suggesting increased production in the outer disk. In all modeling scenarios, fits to the H2CO data show an increased abundance in the outer disk. The overall best-fit H2CO model shows a factor of two enhancement beyond a radius of 270 ± 20 AU, with an inner abundance (relative to H2) of 2 − 5 × 10-12. The H2CO emitting region has a lower limit on the kinetic temperature of T> 20 K. The C18O modeling suggests an order of magnitude depletion of C18O in the outer disk and an abundance of 4 − 12 × 10-8 in the inner disk. Conclusions. There is a desorption front seen in the H2CO emission that roughly coincides with the outer edge of the 1.3 millimeter continuum. The increase in H2CO outer disk emission could be a result of hydrogenation of CO ices on dust grains that are then sublimated via thermal desorption or UV photodesorption. Alternatively, there could be more efficient gas-phase production of H2CO beyond ~300 AU if CO is photodisocciated in this region.
We report the detection of the CO 4–3, 6–5, 9–8, 10–9, and 11–10 lines in the Broad Absorption Line quasar APM 08279+5255 at $z=3.9$ using the IRAM 30 m telescope. We also present IRAM PdBI high ...spatial resolution observations of the CO 4–3 and 9–8 lines, and of the 1.4 mm dust radiation as well as an improved spectrum of the HCN(5–4) line. Unlike CO in other QSO host galaxies, the CO line SED of APM 08279+5255 rises up to the CO(10–9) transition. The line fluxes in the CO ladder and the dust continuum fluxes are best fit by a two component model, a “cold” component at ~$ 65$ K with a high density of n(H2) = 1$\times$105 cm-3, and a “warm”, ~$ 220$ K component with a density of 1$\times$104 cm-3. We show that IR pumping via the 14 $\mu{\rm m}$ bending mode of HCN is the most likely channel for the HCN excitation. From our models we find, that the CO(1–0) emission is dominated by the dense gas component which implies that the CO conversion factor is higher than usually assumed for high-z galaxies with $\alpha \approx$ 5$ M_{\odot}$ (K km s-1 pc2)-1. Using brightness temperature arguments, the results from our high-resolution mapping, and lens models from the literature, we argue that the molecular lines and the dust continuum emission arise from a very compact (r ≈ 100-300 pc), highly gravitationally magnified ($ m = 60{-}110$) region surrounding the central AGN. Part of the difference relative to other high-z QSOs may therefore be due to the configuration of the gravitational lens, which gives us a high-magnification zoom right into the central 200-pc radius of APM 08279+5255 where IR pumping plays a significant role for the excitation of the molecular lines.
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