The Circumstellar Environment around the Embedded Protostar EC 53

Johnstone

et al. 2021

ApJ

Self Cite

We present the four-year survey results of monthly submillimeter monitoring of eight nearby (<500 pc) starforming regions by the JCMT Transient Survey. We apply the Lomb-Scargle Periodogram technique to search for and characterize variability on 295 submillimeter peaks brighter than 0.14 Jy beam −1 , including 22 disk sources (Class II), 83 protostars (Class 0/I), and 190 starless sources. We uncover 18 secular variables, all of them protostars. No single-epoch burst or drop events and no inherently stochastic sources are observed. We classify the secular variables by their timescales into three groups: Periodic, Curved, and Linear. For the Curved and Periodic cases, the detectable fractional amplitude, with respect to mean peak brightness, is ∼4% for sources brighter than ∼0.5 Jy beam −1 . Limiting our sample to only these bright sources, the observed variable fraction is 37% (16 out of 43). Considering source evolution, we find a similar fraction of bright variables for both Class 0 and Class I. Using an empirically motivated conversion from submillimeter variability to variation in mass accretion rate, six sources (7% of our full sample) are predicted to have years-long accretion events during which the excess mass accreted reaches more than 40% above the total quiescently accreted mass: two previously known eruptive Class I sources, V1647 Ori and EC 53 (V371 Ser), and four Class 0 sources, HOPS 356, HOPS 373, HOPS 383, and West 40. Considering the full protostellar ensemble, the importance of episodic accretion on few years timescale is

Section: B2 Serpens Main Ec 53 (V371 Serp): Periodic Groupmentioning

confidence: 99%

The JCMT Transient Survey: Four-year Summary of Monitoring the Submillimeter Variability of Protostars

Johnstone

et al. 2021

ApJ

Self Cite

“…Lee et al (2020) obtained spatially resolved images of CH 3 OH and H 13 CO + emission lines with ALMA towards the embedded protostar EC53, in which quasi-periodic emission was reported by the near-infrared monitoring observations (Hodapp et al 2012) and the submillimeter (JCMT 7 ) monitoring survey (Herczeg et al 2017;Yoo et al 2017), which strongly suggests variable accretion rates. Its luminosities L bol of 1.7-4.8L ⊙ (e.g., Evans et al 2009) and envelope mass M env of 0.86-1.25 (Lee et al 2020) are similar to those in IRAS 2A and IRAS 4A (the differences are within a factor of 4-5 times). This source is classified as Class I (Giardino et al 2007), and L X is around 1 − 3 × 10 30 erg s −1 , according to XMM-Newton X-ray observations (Preibisch 2003) and Chandra X-ray observations (Giardino et al 2007).…”

Section: Hco + Ch 3 Oh and Other Molecular Line Observationsmentioning

confidence: 91%

“…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%

“…According to Table 2, the binding energy of CH 3 OH is somewhat smaller than that of H 2 O (E des (CH 3 OH)=3820 K and E des (H 2 O)=4880 K), and the CH 3 OH snowline position (∼ 2 × 10 2 au) is located outside the water snowline (∼ 10 2 au). Thus, CH 3 OH has been considered to probe the 100 K region in hot cores (Nomura & Millar 2004;Garrod & Herbst 2006;Taquet et al 2014), and also provides an outer limit to the water snowline position in protostellar envelopes (e.g., Jørgensen et al 2013;van 't Hoff et al 2018b;Lee et al 2019Lee et al , 2020.…”

Section: Ch 3 Oh Fractional Abundancesmentioning

confidence: 99%

“…In the FU Orionis type stars, sudden increases in the luminosity of the central star will quickly expand the sublimation front (the so-called snowline) to larger radii, which provide good opportunities to study the abundances of COMs in the planet forming materials (Lee et al 2019). Lee et al (2020) obtained spatially resolved images of CH 3 OH and H 13 CO + emission lines with ALMA towards the embedded protostar EC53, in which quasi-periodic emission was reported by the near-infrared monitoring observations (Hodapp et al 2012) and the submillimeter (JCMT 7 ) monitoring survey (Herczeg et al 2017;Yoo et al 2017), which strongly suggests variable accretion rates. Its luminosities L bol of 1.7-4.8L ⊙ (e.g., Evans et al 2009) and envelope mass M env of 0.86-1.25 (Lee et al 2020) are similar to those in IRAS 2A and IRAS 4A (the differences are within a factor of 4-5 times).…”

Section: Hco + Ch 3 Oh and Other Molecular Line Observationsmentioning

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

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.

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

Notsu

Dishoeck

Walsh

et al. 2021

A&A

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 to total hydrogen density) in the inner warm envelopes (r < 102 au). Water destruction by X-rays is thought to influence the water abundances in these regions, but the detailed chemistry, including the nature of alternative oxygen carriers, is not yet understood. Aims. Our aim is 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 computed the chemical composition of two proto-typical low-mass protostellar envelopes using a 1D gas-grain chemical reaction network. We varied the X-ray luminosities of the central protostars, and thus the X-ray ionization 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 H2O snowline (Tgas ~ 102 K, r ~ 102 au). Outside, the water gas abundance increases with LX, from ~10−10 for low LX to ~10−8–10−7 at LX > 1030 erg s−1. Inside, water maintains a high abundance of ~10−4 for LX ≲ 1029–1030 erg s−1, with water and CO being the dominant oxygen carriers. For LX ≳ 1030–1031 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 Tgas ≳ 250 K (~10−6). For these cases, the fractional abundances of O2 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 CH3OH, which have been used as tracers of the water snowline, significantly increase and decrease, respectively, within the water snowline as the X-ray fluxes become larger. The fractional abundances of some other dominant molecules, such as CO2, OH, CH4, HCN, and NH3, 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, O2, HCO+, and CH3OH, 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.