Chemically tracing the water snowline in protoplanetary disks with HCO<sup>+</sup>

Leemker, M.; Hoff, M. L. R. ’t van; Trapman, Leon; Gelder, M. L. van; Hogerheijde, M. R.; Ruíz-Rodríguez, Dary; Dishoeck, E. F. van

doi:10.1051/0004-6361/202039387

Cited by 34 publications

(37 citation statements)

References 106 publications

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“…With the advent of ALMA, spatially resolved observations have been used together with detailed modelling of individual disks with large dust cavities (e.g., Bruderer et al 2012;van der Marel et al 2015van der Marel et al , 2016Kama et al 2016;Schwarz et al 2021) and without large dust cavities (e.g., Schwarz et al 2016;Pinte et al 2018;Calahan et al 2021). The temperature of the (outer) disk midplane is derived from the location of the snowlines of major species such as H 2 O and CO (Mathews et al 2013;Qi et al 2013;van 't Hoff et al 2017van 't Hoff et al , 2018Qi et al 2019;Leemker et al 2021). Additionally rotational diagrams are used to derive the temperature of intermediate layers in disks (e.g., Schwarz et al 2016;Loomis et al 2018;Pegues et al 2020;Terwisscha van Scheltinga et al 2021).…”

Section: Introductionmentioning

confidence: 99%

Gas temperature structure across transition disk cavities

Leemker,

Booth,

van Dishoeck

et al. 2022

Preprint

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Context.Most disks observed at high angular resolution show signs of substructures like rings, gaps, arcs, and cavities in both the gas and the dust. To understand the physical mechanisms responsible for these structures, knowledge about the gas surface density is essential. This, in turn, requires information on the gas temperature. Aims. The aim of this work is to constrain the gas temperature as well as the gas surface densities inside and outside the mm-dust cavities of two transition disks: LkCa15 and HD 169142, with dust cavities of 68 AU and 25 AU, respectively. Methods. We use some of the few existing ALMA observations of the J = 6 − 5 transition of 13 CO together with archival J = 2 − 1 data of 12 CO, 13 CO and C 18 O. The ratio of the 13 CO J = 6 − 5 to the J = 2 − 1 transition is used to constrain the temperature and is compared with that found from peak brightness temperature of optically thick lines. The spectra are used to resolve the innermost disk regions to a spatial resolution better than the beam of the observations. Furthermore, we use the thermochemical code DALI to model the temperature and density structure of a typical transition disk as well as the emitting regions of the CO isotopologues. Results. The 13 CO J = 6−5 and J = 2−1 transitions peak inside the dust cavity in both disks, indicating that gas is present in the dust cavities. The kinematically derived radial profiles show that the gas is detected down to 10 and 5 − 10 AU, much further in than the dust cavities in the LkCa15 and HD 169142 disks, respectively. For LkCa15, the steep increase towards the star in the 13 CO J = 6 − 5 transition, in contrast to the J = 2 − 1 line, shows that the gas is too warm to be traced by the J = 2 − 1 line and that molecular excitation is important for analysing the line emission. Quantitatively, the 6 − 5/2 − 1 line ratio constrains the gas temperature in the emitting layers inside the dust cavity to be up to 65 K, warmer than in the outer disk at 20−30 K. For the HD 169142, the lines are optically thick, complicating a line ratio analysis. In this case, the peak brightness temperature constrains the gas in the dust cavity of HD 169142 to be 170 K, whereas that in the outer disk is only 100 K. The data indicate a vertical structure in which the 13 CO 6 − 5 line emits from a higher layer than the 2 − 1 line in both disks, consistent with exploratory thermo-chemical DALI models. Such models also show that a more luminous central star, a lower abundance of PAHs and the absence of a dusty inner disk increase the temperature of the emitting layers and hence the line ratio in the gas cavity. The gas column density in the LkCa15 dust cavity drops by a factor > 2 compared to the outer disk, with an additional drop of an order of magnitude inside the gas cavity at 10 AU. In the case of HD 169142, the gas column density drops by a factor of 200−500 inside the gas cavity. Conclusions. The gas temperatures inside the dust cavities steeply increase towards the star and reach temperatures up to 65 K (LkCa15) an...

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Section: Introductionmentioning

confidence: 99%

Gas temperature structure across transition disk cavities

Leemker,

Booth,

van Dishoeck

et al. 2022

Preprint

Self Cite

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“…The inner circular region around source A where H 13 CO + is not present (Fig. 5) coincides with the 100 K radius (Jacobsen et al 2018), tracing the water snowline (e.g., van 't Hoff et al 2018;Leemker et al 2021). It is worth highlighting that the east-west outflow does not present H 13 CO + emission in either transition.…”

Section: Comparison With Higher Transitionsmentioning

confidence: 56%

A cold accretion flow onto one component of a multiple protostellar system

Murillo,

van Dishoeck,

Hacar

et al. 2021

Preprint

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Context. Gas accretion flows transport material from the cloud core onto the protostar. In multiple protostellar systems, it is not clear if the delivery mechanism is preferential or more evenly distributed among the components. Aims. The distribution of gas accretion flows within the cloud core of the deeply embedded, chemically rich, low-mass multiple protostellar system IRAS 16293−2422 is explored out to 6000 AU. Methods. Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 observations of low-J transitions of various molecules such as HNC, cyanopolyynes (HC 3 N, HC 5 N), and N 2 H + are used to probe the cloud core structure of IRAS 16293−2422 at ∼100 AU resolution. Additional Band 3 archival data provide low-J HCN and SiO lines. These data are compared with the corresponding higher-J lines from the PILS Band 7 data for excitation analysis. The HNC/HCN ratio is used as a temperature tracer. Results. The low-J transitions of HC 3 N, HC 5 N, HNC and N 2 H + trace extended and elongated structures from 6000 AU down to ∼100 AU, without accompanying dust continuum emission. Two structures are identified: one traces a flow that is likely accreting toward the most luminous component of the system IRAS 16293−2422 A. Temperatures inferred from the HCN/HNC ratio suggest that the gas in this flow is cold, between 10 and 30 K. The other structure is part of an UV-irradiated cavity wall entrained by one of the outflows driven by the source. The two outflows driven by IRAS 16293−2422 A present different molecular gas distributions. Conclusions. Accretion of cold gas is seen from 6000 AU scales onto IRAS 16293−2422 A, but not onto source B, indicating that cloud core material accretion is competitive due to feedback onto a dominant component in an embedded multiple protostellar system. The preferential delivery of material could explain the higher luminosity and multiplicity of source A compared to source B. The results of this work demonstrate that several different molecular species, and multiple transitions of each species, are needed to confirm and characterize accretion flows in protostellar cloud cores.

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“…HCO + has been considered as a chemical tracer of the water snowline, since its most abundant destroyer in warm dense gas is water via the following reaction (Jørgensen et al 2013;Visser et al 2015;van 't Hoff et al 2018a;Hsieh et al 2019;Lee et al 2020;Leemker et al 2021),…”

Section: Hco + Fractional Abundancesmentioning

confidence: 99%

“…On the basis of our modeling, for high X-ray luminosities, the water gas abundance sharply decreases inside the water snowline and thus HCO + is not efficiently destroyed. Formation of HCO + is dominated by the ion-molecule reaction between H + 3 and CO (Schwarz et al 2018;van 't Hoff et al 2018a;Leemker et al 2021), and H + 3 is mainly formed by the ionization of H 2 . Therefore, the HCO + abundances increase as the X-ray ionisation rate increases, and they have relatively radially flat profiles for high X-ray luminosities (see Figure 6).…”

Section: Hco + Fractional Abundancesmentioning

confidence: 99%

“…However, it is not yet understood that the nature of the major oxygen carriers under these conditions. Moreover, it is important to investigate whether HCO + and CH 3 OH are also affected by strong X-ray fluxes, since they have been used as tracers of the water snowline (Visser et al 2015;van 't Hoff et al 2018a,b;Leemker et al 2021). The chemical model that Stäuber et al (2005Stäuber et al ( , 2006 adopted were limited.…”

Section: Introductionmentioning

confidence: 99%

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X-ray induced chemistry of water and related molecules in low-mass protostellar envelopes

Notsu,

van Dishoeck,

Walsh

et al. 2021

Preprint

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Context. Water is a key molecule in star and planet forming regions. Recent water line observations toward several low-mass protostars suggest low water gas fractional abundances (< 10 −6 with respect total hydrogen density) in the inner warm envelopes (r < 10 2 au). Water destruction by X-rays has been proposed to influence the water abundances in these regions, but the detailed chemistry, including the nature of alternative oxygen carriers, is not yet understood. Aims. We aim to understand the impact of X-rays on the composition of low-mass protostellar envelopes, focusing specifically on water and related oxygen bearing species. Methods. We compute the chemical composition of two proto-typical low-mass protostellar envelopes using a 1D gas-grain chemical reaction network. We vary X-ray luminosities of the central protostars and thus the X-ray ionisation rates in the protostellar envelopes.Results. The protostellar X-ray luminosity has a strong effect on the water gas abundances, both within and outside the H 2 O snowline (T gas ∼ 10 2 K, r ∼ 10 2 au). Outside, the water gas abundance increases with L X , from ∼ 10 −10 for low L X to ∼ 10 −8 − 10 −7 at L X > 10 30 erg s −1 . Inside, water maintains a high abundance of ∼ 10 −4 for L X 10 29 − 10 30 erg s −1 , with water and CO being the dominant oxygen carriers. For L X 10 30 − 10 31 erg s −1 , the water gas abundances significantly decrease just inside the water snowline (down to ∼ 10 −8 − 10 −7 ) and in the innermost regions with T gas 250 K (∼ 10 −6 ). For these cases, the fractional abundances of O 2 and O gas reach ∼ 10 −4 within the water snowline, and they become the dominant oxygen carriers. In addition, the fractional abundances of HCO + and CH 3 OH, which have been used as tracers of the water snowline, significantly increase/decrease within the water snowline, respectively, as the X-ray fluxes become larger. The fractional abundances of some other dominant molecules, such as CO 2 , OH, CH 4 , HCN, and NH 3 , are also affected by strong X-ray fields, especially within their own snowlines. These X-ray effects are larger in lower density envelope models. Conclusions. X-ray induced chemistry strongly affects the abundances of water and related molecules including O, O 2 , HCO + , and CH 3 OH, and can explain the observed low water gas abundances in the inner protostellar envelopes. In the presence of strong X-ray fields, gas-phase water molecules within the water snowline are mainly destroyed with ion-molecule reactions and X-ray induced photodissociation. Future observations of water and related molecules (using e.g., ALMA and ngVLA) will access the regions around protostars where such X-ray induced chemistry is effective.

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Chemically tracing the water snowline in protoplanetary disks with HCO⁺

Cited by 34 publications

References 106 publications

Gas temperature structure across transition disk cavities

Gas temperature structure across transition disk cavities

A cold accretion flow onto one component of a multiple protostellar system

X-ray induced chemistry of water and related molecules in low-mass protostellar envelopes

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Chemically tracing the water snowline in protoplanetary disks with HCO+

Cited by 34 publications

References 106 publications

Gas temperature structure across transition disk cavities

Gas temperature structure across transition disk cavities

A cold accretion flow onto one component of a multiple protostellar system

X-ray induced chemistry of water and related molecules in low-mass protostellar envelopes

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Product

Resources

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Chemically tracing the water snowline in protoplanetary disks with HCO⁺