Yuri Aikawa

Evolution of Molecular Abundance in Protoplanetary Disks

We investigate the evolution of molecular abundance in quiescent protoplanetary disks that are presumed to be around weak-lined T Tauri stars. In the region of surface density less than 102 g cm-2 (distance from the star 10 AU in the minimum-mass solar nebula), cosmic rays are barely attenuated even in the midplane of the disk and produce chemically active ions such as He+ and H -->+3. Through reactions with these ions, CO and N2 are finally transformed into CO2, NH3, and HCN. In the region where the temperature is low enough for these products to freeze onto grains, a considerable amount of carbon and nitrogen is locked up in the ice mantle and is depleted from the gas phase in a timescale of 3 × 106 yr. Oxidized (CO2) ice and reduced (NH3 and hydrocarbon) ice naturally coexist in this part of the disk. The molecular abundance both in the gas phase and in the ice mantle varies significantly with the distance from the central star.

How do stars gain their mass? : A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star Forming Regions

Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion—prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In A James Clerk Maxwell Telescope/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions, we are monitoring monthly with SCUBA-2 the submillimeter emission in eight fields within nearby (<500 pc) star-forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 deg^2 includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125–0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of submillimeter observations to reach a precision of ~2%–3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any submillimeter variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for submillimeter variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars.

Depletion of Heavy Nitrogen in the Cold Gas of Star-forming Regions

We investigate nitrogen isotope fractionation in forming and evolving molecular clouds using gas-ice astrochemical simulations. We find that the bulk gas can become depleted in heavy nitrogen (15N) due to the formation of 15N-enriched ices. Around the chemical transition from atomic nitrogen to N2, N15N is selectively photodissociated, which results in the enrichment of 15N in atomic nitrogen. As 15N-enriched atomic nitrogen is converted to ammonia ice via grain surface reactions, the bulk gas is depleted in 15N. The level of 15N depletion in the bulk gas can be up to a factor of two compared to the elemental nitrogen isotope ratio, depending on the photodesorption yield of ammonia ice. Once the nitrogen isotopes are differentially partitioned between gas and solids in a molecular cloud, it should remain in the later stages of star formation (e.g., prestellar core) as long as the sublimation of ammonia ice is inefficient. Our model suggests that all the N-bearing molecules in the cold gas of star-forming regions can be depleted in 15N, which is at least qualitatively consistent with the observations toward prestellar core L1544. In our models, icy species show both 15N and deuterium fractionation. The fractionation pattern within ice mantles is different between 15N and deuterium, reflecting their fractionation mechanisms; while the concentration of deuterium almost monotonically increases from the lower layers of the ice mantles to the upper layers, the concentration of 15N reaches the maximum at a certain depth and declines towards the surface.

The JCMT Transient Survey : identifying submillimeter continuum variability over several year timescales using archival JCMT Gould Belt Survey observations

Investigating variability at the earliest stages of low-mass star formation is fundamental in understanding how a protostar assembles mass. While many simulations of protostellar disks predict non-steady accretion onto protostars, deeper investigation requires robust observational constraints on the frequency and amplitude of variability events characterised across the observable SED. In this study, we develop methods to robustly analyse repeated observations of an area of the sky for submillimetre variability in order to determine constraints on the magnitude and frequency of deeply embedded protostars. We compare \mbox{850

A Multiline Study of a High-mass Young Stellar Object in the Small Magellanic Cloud with ALMA: The Detection of Methanol Gas at 0.2 Solar Metallicity

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Multiple Paths of Deuterium Fractionation in Protoplanetary Disks

m} JCMT Transient Survey data with archival JCMT Gould Belt Survey data to investigate variability over 2-4 year timescales. Out of 175 bright, independent emission sources identified in the overlapping fields, we find 7 variable candidates, 5 of which we classify as \textit{Strong} and the remaining 2 as \textit{Extended} to indicate the latter are associated with larger-scale structure. For the \textit{Strong} variable candidates, we find an average fractional peak brightness change per year of |4.0|\% yr

The Distribution and Excitation of CH3CN in a Solar Nebula Analog

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Chemical Models of Circumstellar Disks

with a standard deviation of

Molecular-cloud-scale Chemical Composition. II. Mapping Spectral Line Survey toward W3(OH) in the 3 mm Band

2.7\%\mathrm{\:yr}^{-1}

ALMA Observations of the Protostar L1527 IRS: Probing Details of the Disk and the Envelope Structures

. In total, 7\% of the protostars associated with \mbox{850