Importance of source structure on complex organics emission III. Effect of disks around massive protostars

Complex organic molecules are only detected toward a fraction of high-mass protostars. The goal of this work is to investigate whether high-mass disks can explain the lack of methanol emission from some massive protostellar systems. We consider an envelope-only and an envelope-plus-disk model and use RADMC-3D to calculate the methanol emission. High and low millimeter (mm) opacity dust are considered for both models separately and the methanol abundance is parameterized. Viscous heating is included due to the high accretion rates of these objects in the disk. In contrast with low-mass protostars, the presence of a disk does not significantly affect the temperature structure and methanol emission. The shadowing effect of the disk is not as important for high-mass objects and the disk mid-plane is hot because of viscous heating, which is effective due to the high accretion rates. Consistent with observations of infrared absorption lines toward high-mass protostars, we find a vertical temperature inversion, i.e. higher temperatures in the disk mid-plane than the disk surface, at radii < 50au for the models with $L=10^4$ L$_{\odot}$ and large mm opacity dust as long as the envelope mass is >550 M$_{\odot}$. The large observed scatter in methanol emission from massive protostars can be mostly explained toward lower luminosity objects with the envelope-plus-disk models including low and high mm opacity dust. The methanol emission variation toward sources with high luminosities cannot be explained by models with or without a disk. However, the $L/M$ of these objects suggest that they could be associated with hypercompact/ultracompact HII regions. Therefore, the low methanol emission toward the high-luminosity sources can be explained by them hosting an HII region where methanol is absent.Comment: 25 pages, 24 figures, Accepted for publication in A&

Constraining turbulence in protoplanetary discs using the gap contrast: an application to the DSHARP sample

Constraining the strength of gas turbulence in protoplanetary discs is an open problem that has relevant implications for the physics of gas accretion and planet formation. In this work, we gauge the amount of turbulence in 6 of the discs observed in the DSHARP programme by indirectly measuring the vertical distribution of their dust component. We employ the differences in the gap contrasts observed along the major and the minor axes due to projection effects, and build a radiative transfer model to reproduce these features for different values of the dust scale heights. We find that (a) the scale heights that yield a better agreement with data are generally low ($\lesssim 4$ AU at a radial distance of $100$ AU), and in almost all cases we are only able to place upper limits on their exact values; these conclusions imply (assuming an average Stokes number of $\approx10^{-2}$) low turbulence levels of $\alpha_{\rm SS}\lesssim10^{-3}-10^{-4}$; (b) for the 9 other systems we considered out of the DSHARP sample, our method yields no significant constraints on the disc vertical structure; we conclude that this is because these discs have either a low inclination or gaps that are not deep enough. Based on our analysis we provide an empirical criterion to assess whether a given disc is suitable to measure the vertical scale height.Comment: Accepted for publication in MNRAS. 13 pages + appendix, 12 figure

The Nineteenth Century Engagement Between Geological and Adventist Thought and its Bearing on the Twentieth Century Flood Geology Movement

The Seventh-day Adventist Church has from the early years of its existence reacted to the perceived challenge of geological thought to their nascent theology. In particular, the Sabbath of the fourth Commandment in Genesis 2 and the catastrophic global Flood described in Genesis 7 and 8 were targeted. The nineteenth century Adventist response has been one of shifting focus, changing strategies, and increasing intensity. Ellen White, the church’s co-founder and prophetess, was one of the first to sound a warning on theological implications of geology. Her perception of geology contained many pre-nineteenth century concepts disconnected from contemporary geological thinking. Long-time editor Uriah Smith used external documents, notably Presbyterian writings to guide the Adventist congregation with ways of responding to geological thought as it impacted on their faith. The first authentic Adventist evaluation of geology and its perceived link with evolution by Alonzo Jones took place in the mid-1880s. With his spirited response, Jones criticised geological stratigraphic concepts in order to neutralise the threat of burgeoning theistic evolutionary thought. His searching in the geological literature involved the use of contextomy. George McCready Price next ventured to nullify the established stratigraphic principles of geology in order to justify a single, global flood-based hypothesis to explain all fossiliferous sedimentary formations. To achieve this, he presented from established scientists selected citations out of their intended context. A special case is presented on Price’s questionable use of the reports of American field geologists McConnell and Willis on thrust faults in the Rocky Mountains. Price modified diagrams and failed to convey unmistakable evidence of a dynamic cause of complex stratigraphy to present his case for the global existence of reverse sequences of rock strata. He argued that since the geologists’ evidence for a fossil sequence of life in the rock stratigraphy is so greatly flawed, there must have been a single catastrophic event that better explained this. Adventist engagement with geological thought during this period saw a noticeable increase in the disregard of intellectual integrity. This study argues that intellectual dishonesty is not a valid way to support a preconceived interpretation of the scriptural narrative. History provides several examples where skewed accounts of events due to questionable intellectual sincerity have eventually been corrected. This research provides access points for interested persons to further investigate the historical aspects of the nineteenth century geology and Adventist thought engagement

The distribution of accretion rates as a diagnostic of protoplanetary disc evolution

We show that the distribution of observed accretion rates is a powerful diagnostic of protoplanetary disc physics. Accretion due to turbulent ("viscous") transport of angular momentum results in a fundamentally different distribution of accretion rates than accretion driven by magnetised disc winds. We find that a homogeneous sample of $\gtrsim$300 observed accretion rates would be sufficient to distinguish between these two mechanisms of disc accretion at high confidence, even for pessimistic assumptions. Current samples of T Tauri star accretion rates are not this large, and also suffer from significant inhomogeneity, so both viscous and wind-driven models are broadly consistent with the existing observations. If accretion is viscous, the observed accretion rates require low rates of disc photoevaporation ($\lesssim$$10^{-9}$M$_{\odot}$yr$^{-1}$). Uniform, homogeneous surveys of stellar accretion rates can therefore provide a clear answer to the long-standing question of how protoplanetary discs accrete.Comment: 10 pages, 8 figures. Accepted for publication in MNRA

Directly tracing the vertical stratification of molecules in protoplanetary disks

We aim to directly trace the vertical location of the emitting surface of multiple molecular tracers in protoplanetary disks. Our sample of disks includes Elias 2-27, WaOph 6 and the sources targeted by the MAPS ALMA Large Program. The set of molecules studied include CO isotopologues in various transitions, HCN, CN, H2CO, HCO+, C2H and c-C3H2. The vertical emitting region is determined directly from the channel maps, implementing accurate masking of the channel emission to recover the vertical location of the emission surface even at large radial distances from the star and for low-SNR lines. The vertical location of the emitting layer is obtained for 4-10 lines in each disk. IM Lup, HD163296 and MWC 480 12CO and 13CO show vertical modulations, which are coincident with dust gaps and kinematical perturbations. We also present estimates of the gas pressure scale height in the disks from the MAPS sample. Compared to physical-chemical models we find good agreement with the vertical location of CO isotopologues. In HD 163296 CN and HCN trace a similar intermediate layer, for the other disks, the UV flux tracers and the vertical profiles of HCN and C2H are lower than predicted in theoretical models. HCN and H2CO show a highly structured vertical profile, possibly indicative of different formation pathways. It is possible to trace the vertical locations of multiple molecular species that trace a wide variety of physical and chemical disk properties. The distribution of CO isotopologues are found at a wide range of vertical heights $z/r =$ 0.5-0.05. Other molecular lines are mostly found at $z/r \leq$0.15. The vertical layering of molecules is in agreement with theory in some systems, but not in all, therefore dedicated chemical-physical models are needed to further study and understand the emission surfaces.Comment: Accepted for publication in A&A. 29 pages, 28 figure

Lil....

Ġabra ta’ poeżiji u proża li tinkludi: Biki ta’ Omm ta’ Dun Karm – Ras il-Pellegrin ta’ Manwel Agius – L-Aħħar Tislima ta’ Ġużè Galea – Lil Ibni Herman-Baruch ta’ Karmenu Vassallo – Flus ir-Regħba Ħajja Qasira ta’ P. P. Saydon – Frak ta’ R. M. B. – Id-Dolliegħa u l-Qargħa ta’ R. M. B. – Il-Barrakka ta’ Katrin ta’ P. Pawl Tabone – Ħniena! ta’ Fran. Camilleri – Lil.... ta’ M. Agius.N/

Evidence for ubiquitous carbon grain destruction in hot protostellar envelopes

Earth is deficient in carbon and nitrogen by up to ${\sim}4$ orders of magnitude compared with the Sun. Destruction of (carbon- and nitrogen-rich) refractory organics in the high-temperature planet forming regions could explain this deficiency. Assuming a refractory cometary composition for these grains, their destruction enhances nitrogen-containing oxygen-poor molecules in the hot gas ($\gtrsim 300$K) after the initial formation and sublimation of these molecules from oxygen-rich ices in the warm gas (${\sim}150$K). Using observations of $37$ high-mass protostars with ALMA, we find that oxygen-containing molecules (CH$_3$OH and HNCO) systematically show no enhancement in their hot component. In contrast, nitrogen-containing, oxygen-poor molecules (CH$_3$CN and C$_2$H$_3$CN) systematically show an enhancement of a factor ${\sim} 5$ in their hot component, pointing to additional production of these molecules in the hot gas. Assuming only thermal excitation conditions, we interpret these results as a signature of destruction of refractory organics, consistent with the cometary composition. This destruction implies a higher C/O and N/O in the hot gas than the warm gas, while, the exact values of these ratios depend on the fraction of grains that are effectively destroyed. This fraction can be found by future chemical models that constrain C/O and N/O from the abundances of minor carbon, nitrogen and oxygen carriers presented here.Comment: Accepted for publication in ApJ Letter

Modelling the secular evolution of protoplanetary disc dust sizes: a comparison between the viscous and magnetic wind case

Stars and planetary system

Suldat

Ġabra ta’ poeżiji u proża li tinkludi: Sursum Corda! ta’ R. M. B. – Tal-Qali ta’ Ġużè Ellul – Tfajla ta’ Dun Karm – Meta mort Għawdex bid-dawra ta’ Ġ. Cassar-Pullicino – Il-Maqdes tal-Mulej ta’ A. Buttigieg – Il-Barrakka ta’ Katrin ta’ P. Pawl Tabone – Ġlieġel ta’ Mary Meylak – Is-Suldat ta’ Ġużè Chetcuti.N/

Modeling accretion shocks at the disk–envelope interface: sulfur chemistry

Context. As material from an infalling protostellar envelope hits the forming disk, an accretion shock may develop which could (partially) alter the envelope material entering the disk. Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) indicate that emission originating from warm SO and SO2 might be good tracers of such accretion shocks.Aims. The goal of this work is to test under what shock conditions the abundances of gas-phase SO and SO2 increase in an accretion shock at the disk-envelope interface.Methods. Detailed shock models including gas dynamics were computed using the Paris-Durham shock code for nonmagnetized J-type accretion shocks in typical inner envelope conditions. The effect of the preshock density, shock velocity, and strength of the ultraviolet (UV) radiation field on the abundance of warm SO and SO2 is explored. Compared with outflows, these shocks involve higher densities (similar to 10(7) cm(-3)), lower shock velocities (similar to few km s(-1)), and large dust grains (similar to 0.2 mu m) and thus probe a different parameter space.Results. Warm gas-phase chemistry is efficient in forming SO under most J-type shock conditions considered. In lower-velocity (similar to 3 km s(-1)) shocks, the abundance of SO is increased through subsequent reactions starting from thermally desorbed CH4 toward H2CO and finally SO. In higher velocity (greater than or similar to 4 km s(-1)) shocks, both SO and SO2 are formed through reactions of OH and atomic S. The strength of the UV radiation field is crucial for SO and in particular SO2 formation through the photodissociation of H2O. Thermal desorption of SO and SO2 ice is only relevant in high-velocity (greater than or similar to 5 km s(-1)) shocks at high densities (greater than or similar to 10(7) cm(-3)). Both the composition in the gas phase, in particular the abundances of atomic S and O, and in ices such as H2S, CH4, SO, and SO2 play a key role in the abundances of SO and SO2 that are reached in the shock.Conclusions. Warm emission from SO and SO2 is a possible tracer of accretion shocks at the disk-envelope interface as long as a local UV field is present. Observations with ALMA at high-angular resolution could provide further constraints given that other key species for the gas-phase formation of SO and SO2, such as H2S and H2CO, are also covered. Moreover, the James Webb Space Telescope will give access to other possible slow, dense shock tracers such as H-2, H2O, and [SI} 25 mu m.NWOTOP-1 614.001.751Interstellar matter and star formatio