分类： 天文学 >> 天文学 提交时间： 2023-02-19
Carlos E. Muñoz-Romero
Karin I. Öberg
Charles J. Law
Richard Teague
Yuri Aikawa
Jennifer B. Bergner
David J. Wilner
Jane Huang
Viviana V. Guzmán
L. Ilsedore Cleeves
摘要： Deuterium fractionation provides a window to the thermal history of volatiles in the solar system and protoplanetary disks. While evidence of active molecular deuteration has been observed towards a handful of disks, it remains unclear whether this chemistry affects the composition of forming planetesimals due to limited observational constraints on the radial and vertical distribution of deuterated molecules. To shed light on this question, we introduce new ALMA observations of DCO$^+$ and DCN $J=2-1$ at an angular resolution of $0.5"$ (30 au) and combine them with archival data of higher energy transitions towards the protoplanetary disk around TW Hya. We carry out a radial excitation analysis assuming both LTE and non-LTE to localize the physical conditions traced by DCO$^+$ and DCN emission in the disk, thus assessing deuterium fractionation efficiencies and pathways at different disk locations. We find similar disk-averaged column densities of $1.9\times10^{12}$ and $9.8\times10^{11}$ cm$^{-2}$ for DCO$^{+}$ and DCN, with typical kinetic temperatures for both molecules of 20-30K, indicating a common origin near the comet- and planet-forming midplane. The observed DCO$^+$/DCN abundance ratio, combined with recent modeling results, provide tentative evidence of a gas phase C/O enhancement within $<40$ au. Observations of DCO$^+$ and DCN in other disks, as well as HCN and HCO$^+$, will be necessary to place the trends exhibited by TW Hya in context, and fully constrain the main deuteration mechanisms in disks.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jamila Pegues
Karin I. Öberg
Chunhua Qi
Sean M. Andrews
Jane Huang
Charles J. Law
Romane Le Gal
Luca Matrà
David J. Wilner
摘要： Protoplanetary disks around Herbig AeBe stars are exciting targets for studying the chemical environments where giant planets form. Save for a few disks, however, much of Herbig AeBe disk chemistry is an open frontier. We present a Submillimeter Array (SMA) $\sim$213-268 GHz pilot survey of mm continuum, CO isotopologues, and other small molecules in disks around five Herbig AeBe stars (HD 34282, HD 36112, HD 38120, HD 142666, and HD 144432). We detect or tentatively detect $^{12}$CO 2--1 and $^{13}$CO 2--1 from four disks; C$^{18}$O 2--1 and HCO$^+$ 3--2 from three disks; HCN 3--2, CS 5--4, and DCO$^+$ 3--2 from two disks; and C$_2$H 3--2 and DCN 3--2 from one disk each. H$_2$CO 3--2 is undetected at the sensitivity of our observations. The mm continuum images of HD 34282 suggest a faint, unresolved source $\sim$5\farcs0 away, which could arise from a distant orbital companion or an extended spiral arm. We fold our sample into a compilation of T Tauri and Herbig AeBe/F disks from the literature. Altogether, most line fluxes generally increase with mm continuum flux. Line flux ratios between CO 2--1 isotopologues are nearest to unity for the Herbig AeBe/F disks. This may indicate emitting layers with relatively similar, warmer temperatures and more abundant CO relative to disk dust mass. Lower HCO$^+$ 3--2 flux ratios may reflect less ionization in Herbig AeBe/F disks. Smaller detection rates and flux ratios for DCO$^+$ 3--2, DCN 3--2, and H$_2$CO 3--2 suggest smaller regimes of cold chemistry around the luminous Herbig AeBe/F stars.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Charles J. Law
Richard Teague
Karin I. Öberg
Evan A. Rich
Sean M. Andrews
Jaehan Bae
Myriam Benisty
Stefano Facchini
Kevin Flaherty
Andrea Isella
Sheng Jin
Jun Hashimoto
Jane Huang
Ryan A. Loomis
Feng Long
Carlos E. Muñoz-Romero
Teresa Paneque-Carreño
Laura M. Pérez
Chunhua Qi
Kamber R. Schwarz
摘要： High spatial resolution observations of CO isotopologue line emission in protoplanetary disks at mid-inclinations (${\approx}$30-75{\deg}) allow us to characterize the gas structure in detail, including radial and vertical substructures, emission surface heights and their dependencies on source characteristics, and disk temperature profiles. By combining observations of a suite of CO isotopologues, we can map the 2D (r, z) disk structure from the disk upper atmosphere, as traced by CO, to near the midplane, as probed by less abundant isotopologues. Here, we present high angular resolution (${\lesssim}$0."1 to ${\approx}$0."2; ${\approx}$15-30 au) observations of CO, $^{13}$CO, and C$^{18}$O in either or both J=2-1 and J=3-2 lines in the transition disks around DM Tau, Sz 91, LkCa 15, and HD 34282. We derived line emission surfaces in CO for all disks and in $^{13}$CO for the DM Tau and LkCa 15 disks. With these observations, we do not resolve the vertical structure of C$^{18}$O in any disk, which is instead consistent with C$^{18}$O emission originating from the midplane. Both the J=2-1 and J=3-2 lines show similar heights. Using the derived emission surfaces, we computed radial and vertical gas temperature distributions for each disk, including empirical temperature models for the DM Tau and LkCa 15 disks. After combining our sample with literature sources, we find that $^{13}$CO line emitting heights are also tentatively linked with source characteristics, e.g., stellar host mass, gas temperature, disk size, and show steeper trends than seen in CO emission surfaces.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jenny K. Calahan
Edwin A. Bergin
Arthur D. Bosman
Evan Rich
Sean M. Andrews
Jennifer B. Bergner
L. Ilsedore Cleeves
Viviana V. Guzman
Jane Huang
John D. Ilee
Charles J. Law
Romane Le Gal
Karin I. Oberg
Richard Teague
Catherine Walsh
David J. Wilner
Ke Zhang
摘要： 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 ( 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Roberto Galván-Madrid
Qizhou Zhang
Andrés Izquierdo
Charles J. Law
Thomas Peters
Eric Keto
Hauyu Baobab Liu
Paul T. P. Ho
Adam Ginsburg
Carlos Carrasco-González
摘要： We present ALMA observations with a 800 au resolution and radiative-transfer modelling of the inner part ($r\approx6000$ au) of the ionized accretion flow around a compact star cluster in formation at the center of the luminous ultra-compact (UC) HII region G10.6-0.4. We modeled the flow with an ionized Keplerian disk with and without radial motions in its outer part, or with an external Ulrich envelope. The MCMC fits to the data give total stellar masses $M_\star$ from 120 to $200~M_\odot$, with much smaller ionized-gas masses $M_\mathrm{ion-gas} = 0.2$ to $0.25~M_\odot$. The stellar mass is distributed within the gravitational radius $R_g\approx 1000$ to 1500 au, where the ionized gas is bound. The viewing inclination angle from the face-on orientation is $i = 49$ to $56~\deg$. Radial motions at radii $r > R_g$ converge to $v_{r,0} \approx 8.7$ km/s, or about the speed of sound of ionized gas, indicating that this gas is marginally unbound at most. From additional constraints on the ionizing-photon rate and far-IR luminosity of the region, we conclude that the stellar cluster consists of a few massive stars with $M_\mathrm{star} = 32$ to $60~M_\odot$, or one star in this range of masses accompanied by a population of lower-mass stars. Any active accretion of ionized gas onto the massive (proto)stars is residual. The inferred cluster density is very large, comparable to that reported at similar scales in the Galactic Center. Stellar interactions are likely to occur within the next Myr.
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