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Naoto Kobayashi

Publications and source records attributed to Naoto Kobayashi.

At least 19 recordsLinked to original sources
arXiv2026

Bright Metal-Poor Star Survey - I. Tomo-e Gozen narrow-band photometric survey and medium-resolution spectroscopic follow-up

We present the Tomo-e Gozen Bright Metal-Poor Star Survey (TeMPS), a wide-area narrow-band photometric survey for bright metal-poor stars in the northern sky. The survey uses the Tomo-e Gozen camera on the 1.05 m Kiso Schmidt telescope with four narrow-band filters centered on the \ion{Ca}{2} H and K lines, the CH $G$ band, H$α$, and a reference wavelength region. We review the survey strategy, photometric processing, and calibration of metallicity and carbon abundance estimates derived from narrow-band colors. We further present medium-resolution spectroscopic follow-up with Nayuta/MALLS to validate the photometric selection and identify new metal-poor stars. The current data set covers $\gtrsim 22,000$deg$^{2}$ in all four bands with a total integration time of $\sim 100$ hr. The median limiting magnitudes at S/N = 20 correspond to $G\sim12.5$. By combining narrow-band photometry with archival broad-band photometry and {\it Gaia} distances, we estimate $T\_{\rm eff}$, $\log (g)$, metallicity, and carbon abundance. Calibration against literature abundances derived from high-resolution spectra shows typical scatters of $< 0.3$ dex in metallicity and $< 0.4$ dex in carbon abundance. We estimate metallicities for $\sim 1.7$ million stars and identify $\sim 16,000$ very metal-poor candidates with [M/H]$_{\rm NB,fin}<-2$. We show that Nayuta/MALLS medium-resolution spectra provide metallicities consistent with high-resolution measurements, with a scatter of $\sim 0.27$ dex. Among 32 photometrically selected candidates followed up with MALLS, 24 are confirmed to have [M/H]$_{\rm MALLS}< -2$, including one newly identified star with [M/H]$_{\rm MALLS}\simeq -3.4$. These results demonstrate that Tomo-e Gozen narrow-band photometry and MALLS medium-resolution spectroscopic follow-up efficiently select bright metal-poor stars for future high-resolution abundance studies. (abbreviated)

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arXiv2026

Protoplanetary Disk Evolution in a Low-Metallicity Environment: JWST's First Mid-Infrared Census of Low-Mass Stars

This study presents the first high-resolution, high-sensitivity mid-infrared (MIR) investigation of protoplanetary disks in a low-metallicity environment, using JWST/NIRCam and MIRI observations of Digel Cloud 2, a star-forming region in the outer Galaxy ($D \simeq 8$ kpc, ${\rm [M/H]} \simeq -0.7$ dex). It hosts two very young ($\sim$0.1 Myr) embedded clusters, Cloud 2-N and Cloud 2-S, offering a window into disk evolution under conditions analogous to the early universe, where low metallicity implies reduced dust content. Imaging across 1-20 $μ$m, including F770W and complementary bands (F356W, F444W, F405N), enables probing disk properties with unprecedented spatial resolution and stellar mass sensitivity down to $\sim$0.1 $M_\odot$. Among 89 and 95 sources detected in F770W in Cloud 2-N and 2-S, respectively, we identify candidate stellar-mass cluster members using infrared photometry, from which stellar mass and extinction are estimated. Among these, $\simeq$75 % retain optically thick disks in both clusters based on MIR SED slopes, consistent with similarly aged solar-metallicity regions. In contrast, a lack of 2 $μ$m excess suggests diminished inner disk emission, possibly due to enhanced silicate grains with low sublimation temperatures. Using the F405N narrow-band filter covering Br$α$, we detect accretion signatures in $\simeq$35 % of sources selected by extinction criteria, with rates $\gtrsim$10$^{-6}$ $M_\odot$ yr$^{-1}$, comparable to or exceeding those in nearby low-mass stars. Brown dwarf candidates, identified across multiple bands including F770W and shorter wavelengths, exhibit a high disk fraction of $\sim$75 %, indicating robust disk retention across mass ranges even under low-metallicity conditions.

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arXiv2026

An optical transient candidate of $\lesssim$ 2-second duration captured by wide-field video observations

Recent time-domain surveys have revealed rapid transients that evolve on timescales of $\lesssim 10$ days, expanding the transient population into the short-duration regime. The transient search on even shorter timescales, particularly those lasting only seconds or less, remains a largely unexplored frontier. Very short-duration optical transients could serve as potential counterparts to millisecond-duration fast radio bursts (FRBs), providing clues to their origins. However, the optical search for transients on such short timescales has been limited primarily by instrumental constraints. Here we report the discovery of an optical transient candidate (TMG20200322) with a duration of $\lesssim 2$~s by wide-field video observations in the direction of the Earth's shadow. TMG20200322 was detected in just two consecutive images of 1-second exposure time, with its shape becoming elongated in the second frame. PSF shape variability analysis of field stars reveals that such an elongated PSF cannot be explained by atmospheric fluctuations. We investigate the potential origins of TMG20200322 in two scenarios: meteoroid impact flashes on near-Earth asteroids (NEAs) and head-on meteors in the Earth's atmosphere. None of the scenarios provides a satisfactory explanation for this transient. We derive a sky-projected rate of the TMG20200322 event of $R_{\mathrm{trans}} = (3.4 \times 10^{-2})^{+0.13}_{-0.028}$~deg$^{-2}$~day$^{-1}$ and an upper limit on second-timescale transients with durations of $1~\mathrm{s} \leq τ\lesssim 15~\mathrm{s}$ of $R_{\mathrm{trans}} \lesssim 0.10$~deg$^{-2}$~day$^{-1}$ for the non-detection case. We highlight that continuous monitoring observations in the direction of the Earth's shadow could be a key strategy to unveil a new population of optical transients on timescales of seconds or less.

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arXiv2025

Second-timescale Glints from Satellites and Space Debris Detected with Tomo-e Gozen

A search for second-timescale optical transients is one of the frontiers of time-domain astronomy. However, it has been pointed out that reflections of sunlight from Earth-orbiting objects can also produce second-timescale ``glints.'' We conducted wide-field observations at 2 frames per second using Tomo-e Gozen on the 1.05 m Kiso Schmidt telescope. We identified 1554 point-source glints that appeared in only one frame (0.5 sec). Their brightness ranges from 11 to 16 mag, with fainter glints being more numerous. These glints are likely caused by satellites and space debris in high-altitude orbits such as the geosynchronous Earth orbit or highly elliptical orbits. Many glints brighter than 14 mag are associated with known satellites or debris with large apogees ($>$ 30,000 km). In contrast, most fainter glints are not associated with cataloged objects and may be due to debris with sizes of 0.3--1 m. The event rate of second-timescale glints is estimated to be $4.7 \pm 0.2\ {\rm deg^{-2}\ hr^{-1}}$ (average) and $9.0 \pm 0.3\ {\rm deg^{-2}\ hr^{-1}}$ (near the equator) at 15.5 mag. Our results demonstrate that high-altitude debris can represent a significant foreground in searches for second-timescale optical transients. They also imply that deep surveys such as Rubin/LSST will detect many of these glints in single-exposure images.

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arXiv2025

MAGIS (Measuring Abundances of red super Giants with Infrared Spectroscopy) project I. Establishment of an abundance analysis procedure for red supergiants and its evaluation with nearby stars

[Abbreviated] Context. Given their high luminosities (L>~10^4Lsun), red supergiants (RSGs) are good tracers of the chemical abundances of the young stellar population in the Milky Way and nearby galaxies. However, previous abundance analyses tailored to RSGs suffer some systematic uncertainties originating in, most notably, the synthesized molecular spectral lines for RSGs. Aims. We establish a new abundance analysis procedure for RSGs that circumvents difficulties faced in previous works, and test the procedure with ten nearby RSGs observed with the near-infrared high-resolution spectrograph WINERED (0.97--1.32 micron, R=28,000). Results. We determined the [X/Fe] of ten elements (Na I, Mg I, Al I, Si I, K I, Ca I, Ti I, Cr I, Ni I, and Y II). We estimated the relative precision in the derived abundances to be 0.04--0.12 dex for elements with more than two lines analyzed (e.g., Fe I and Mg I) and up to 0.18 dex for the other elements (e.g., Y II). We compared the resultant abundances of RSGs with the well-established abundances of another type of young star, namely the Cepheids, in order to evaluate the potential systematic bias in our abundance measurements, assuming that the young stars (i.e., both RSGs and Cepheids) in the solar neighborhood have common chemical abundances. We find that the determined RSG abundances are highly consistent with those of Cepheids within <~0.1 dex for some elements (notably [Fe/H] and [Mg/Fe]), which means the bias in the abundance determination for these elements is likely to be small. In contrast, the consistency is worse for some other elements (e.g., [Si/Fe] and [Y/Fe]). Nevertheless, the dispersion of the chemical abundances among our target RSGs is comparable with the individual statistical errors on the abundances. Hence, the procedure is likely to be useful to evaluate the relative difference in chemical abundances among RSGs.

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arXiv2024

Revealing Potential Initial Mass Function variations with metallicity: JWST observations of young open clusters in a low-metallicity environment

We present the substellar mass function of star-forming clusters ($\simeq$0.1 Myr old) in a low-metallicity environment ($\simeq$$-$0.7 dex). We performed deep JWST/NIRCam and MIRI imaging of two star-forming clusters in Digel Cloud 2, a star-forming region in the Outer Galaxy ($R_G \gtrsim 15$ kpc). The very high sensitivity and spatial resolution of JWST enable us to resolve cluster members clearly down to a mass detection limit of 0.02 $M_\odot$, enabling the first detection of brown dwarfs in low-metallicity clusters. Fifty-two and ninety-one sources were extracted in mass-$A_V$-limited samples in the two clusters, from which Initial mass functions (IMFs) were derived by model-fitting the F200W band luminosity function, resulting in IMF peak masses (hereafter $M_C$) $\log M_C / M_\odot \simeq -1.5 \pm 0.5$ for both clusters. Although the uncertainties are rather large, the obtained $M_C$ values are lower than those in any previous study ($\log M_C / M_\odot \sim -0.5$). Comparison with the local open clusters with similar ages to the target clusters ($\sim$$10^6$-$10^7$ yr) suggests a metallicity dependence of $M_C$, with lower $M_C$ at lower metallicities, while the comparison with globular clusters, similarly low metallicities but considerably older ($\sim$$10^{10}$ yr), suggests that the target clusters have not yet experienced significant dynamical evolution and remain in their initial physical condition. The lower $M_C$ is also consistent with the theoretical expectation of the lower Jeans mass due to the higher gas density under such low metallicity. The $M_C$ values derived from observations in such an environment would place significant constraints on the understanding of star formation.

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arXiv2024

Overview Results of JWST Observations of Star-Forming Clusters in the Extreme Outer Galaxy

The extreme outer Galaxy (EOG), which we define as the region of the Milky Way with a galactocentric radius of more than 18 kpc, provides an excellent opportunity to study star formation in an environment significantly different from that in the solar neighborhood because of its lower metallicity and lower gas density. We carried out near- and mid-infrared (NIR and MIR) imaging observations toward two star-forming clusters located in the EOG using JWST NIRCam and MIRI with nine filters: F115W, F150W, F200W, F350W, F405N, F444W, F770W, F1280W, and F2100W. In this paper, we present an overview of the observations, data reduction, and initial results. The NIR sensitivity is approximately 10--80 times better than our previous observation with the Subaru 8.2 m telescope. Accordingly, the mass detection limit reaches to about 0.01--0.05 $M_\odot$, which is about 10 times better than the previous observations. At MIR wavelengths, the high sensitivity and resolution data enable us to resolve individual young stellar objects in such a distant region for the first time. The mass detection limit at MIR F770W filter reaches about 0.1--0.3 $M_\odot$. With these new observations, we have identified components of the clusters that previous surveys did not detect, including class 0 candidates, outflow/jet components, and distinctive nebular structures. These data will enable us to investigate the properties of star formation in the EOG at the same depth of detail as previous observations of star formation in the solar neighborhood.

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arXiv2024

Optical Variability of Blazars in the Tomo-e Gozen Northern Sky Transient Survey

We studied the optical variability of 241 BL Lacs and 83 flat-spectrum radio quasars (FSRQ) from the 4LAC catalog using data from the Tomo-e Gozen Northern Sky Transient Survey, with $\sim$ 50 epochs per blazar on average. We excluded blazars whose optical variability may be underestimated due to the influence of their host galaxy, based on their optical luminosity ($L_O$). FSRQs with $γ$-ray photon index greater than 2.6 exhibit very low optical variability, and their distribution of standard deviation of repeated photometry is significantly different from that of the other FSRQs (KS test P value equal to $5 \times 10^{-6}$ ). Among a sample of blazars at any particular cosmological epoch, those with lower $γ$-ray luminosity ($L_γ$) tend to have lower optical variability, and those FSRQs with $γ$-ray photon index greater than 2.6 tend to have low $L_γ$. We also measured the structure function of optical variability and found that the amplitude of the structure function for FSRQs is higher than previously measured and higher than that of BL Lacs at multiple time lags. Additionally, the amplitude of the structure function of FSRQs with high $γ$-ray photon index is significantly lower than that of FSRQs with low $γ$-ray photon index. The structure function of FSRQs of high $γ$-ray photon index shows a characteristic timescale of more than 10 days, which may be the variability timescale of the accretion disk. In summary, we infer that the optical component of FSRQs with high $γ$-ray photon index may be dominated by the accretion disk.

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arXiv2024

[N I] 10400/10410 Å Lines as Possible Disk Wind Tracers in a Young Intermediate-Mass Star

In this study, we performed high-resolution near-infrared (NIR) spectroscopy (R = 28,000; $λ = 0.90$-1.35 $μ$m) with a high signal-to-noise ratio on HD 200775, a very young (${\sim}$0.1 Myr old) and massive intermediate-mass star (a binary star with a mass of about 10 $M_{\odot}$ each) with a protoplanetary disk. The obtained spectra show eight forbidden lines of three elements: two of [S II] (10289 and 10323 Å), two of [N I] (10400 and 10410 Å), and four of [Fe II] (12570, 12946, 12981, and 13209 Å). This is the first time that the [N I] lines are detected in a young stellar object with a doublet deblended. Gaussian fitting of the spectra indicates that all line profiles have low-velocity components and exhibit blueshifted features, suggesting that all lines originate from the disk winds (magnetohydrodynamic disk wind and/or photoevaporative wind). Based on the fit, the [N I] and [Fe II] lines are categorized into narrow components, while the [S II] lines are at the boundary between broad and narrow components. These forbidden lines are suggested to be very promising disk wind tracers among the existing ones because they are in the NIR-wavelength range, which can be observed from early stages with high sensitivities. Among these lines, [N I] lines would be a rather powerful probe for deriving the basic physical parameters of disk wind gases. However, the study of these lines herein is limited to one object; thus, further studies are needed to examine their properties.

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arXiv2024

Intermediate-luminosity Type IIP SN 2021gmj: a low-energy explosion with signatures of circumstellar material

We present photometric, spectroscopic and polarimetric observations of the intermediate-luminosity Type IIP supernova (SN) 2021gmj from 1 to 386 days after the explosion. The peak absolute V-band magnitude of SN 2021gmj is -15.5 mag, which is fainter than that of normal Type IIP SNe. The spectral evolution of SN 2021gmj resembles that of other sub-luminous supernovae: the optical spectra show narrow P-Cygni profiles, indicating a low expansion velocity. We estimate the progenitor mass to be about 12 Msun from the nebular spectrum and the 56Ni mass to be about 0.02 Msun from the bolometric light curve. We also derive the explosion energy to be about 3 x 10^{50} erg by comparing numerical light curve models with the observed light curves. Polarization in the plateau phase is not very large, suggesting nearly spherical outer envelope. The early photometric observations capture the rapid rise of the light curve, which is likely due to the interaction with a circumstellar material (CSM). The broad emission feature formed by highly-ionized lines on top of a blue continuum in the earliest spectrum gives further indication of the CSM at the vicinity of the progenitor. Our work suggests that a relatively low-mass progenitor of an intermediate-luminosity Type IIP SN can also experience an enhanced mass loss just before the explosion, as suggested for normal Type IIP SNe.

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arXiv2023

Shock Excitation in Narrow Line Regions Powered by AGN Outflows

Outflows in the Active Galactic Nucleus (AGN) are considered to play a key role in the host galaxy evolution through transfer of a large amount of energy. A Narrow Line Region (NLR) in the AGN is composed of ionized gas extending from pc-scales to kpc-scales. It has been suggested that shocks are required for ionization of the NLR gas. If AGN outflows generate such shocks, they will sweep through the NLR and the outflow energy will be transferred into a galaxy-scale region. In order to study contribution of the AGN outflow to the NLR-scale shock, we measure the [\ion{Fe}{2}]$\lambda12570$/[\ion{P}{2}]$\lambda11886$ line ratio, which is a good tracer of shocks, using near-infrared spectroscopic observations with WINERED (Warm INfrared Echelle spectrograph to Realize Extreme Dispersion and sensitivity) mounted on the New Technology Telescope. Among 13 Seyfert galaxies we observed, the [\ion{Fe}{2}] and [\ion{P}{2}] lines were detected in 12 and 6 targets, respectively. The [\ion{Fe}{2}]/[\ion{P}{2}] ratios in 4 targets were found to be higher than 10, which implies the existence of shocks. We also found that the shock is likely to exist where an ionized outflow, i.e., a blue wing in [\ion{S}{3}]$\lambda9533$, is present. Our result implies that the ionized outflow present over a NLR-scale region sweeps through the interstellar medium and generates a shock.

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arXiv2023

A search for extragalactic fast optical transients in the Tomo-e Gozen high-cadence survey

The population of optical transients evolving within a time-scale of a few hours or a day (so-called fast optical transients, FOTs) has recently been debated extensively. In particular, our understanding of extragalactic FOTs and their rates is limited. We present a search for extragalactic FOTs with the Tomo-e Gozen high-cadence survey. Using the data taken from 2019 August to 2022 June, we obtain 113 FOT candidates. Through light curve analysis and cross-matching with other survey data, we find that most of these candidates are in fact supernovae, variable quasars, and Galactic dwarf novae, that were partially observed around their peak brightness. We find no promising candidate of extragalactic FOTs. From this non-detection, we obtain upper limits on the event rate of extragalactic FOTs as a function of their time-scale. For a very luminous event (absolute magnitude $M<-26$ mag), we obtain the upper limits of $4.4 \times 10^{-9}$ Mpc$^{-3}$ yr$^{-1}$ for a time-scale of 4 h, and $7.4 \times 10^{-10}$ Mpc$^{-3}$ yr$^{-1}$ for a time-scale of 1 d. Thanks to our wide (although shallow) surveying strategy, our data are less affected by the cosmological effects, and thus, give one of the more stringent limits to the event rate of intrinsically luminous transients with a time-scale of $< 1$ d.

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arXiv2022

Mass function of a young cluster in a low-metallicity environment. Sh 2-209

We present deep near-infrared (NIR) imaging of Sh 2-209 (S209), a low-metallicity (${\rm [O/H]} = - 0.5$ dex) HII region in the Galaxy. From the NIR images, combined with astrometric data from Gaia EDR3, we estimate the distance to S209 to be 2.5 kpc. This is close enough to enable us to resolve cluster members clearly ($\simeq$1000 AU separation) down to a mass-detection limit of $\simeq$0.1 $M_\odot$, and we have identified two star-forming clusters in S209, with individual cluster scales $\sim$1 pc. We employ a set of model luminosity functions to derive the underlying initial mass functions (IMFs) and ages for both clusters. The IMFs we obtained for both clusters exhibit slightly flat high-mass slopes ($Γ\simeq -1.0$) compared to the Salpeter IMF ($Γ= -1.35$), and their break mass of $\simeq$0.1 $M_\odot$ is lower than those generally seen in the solar neighborhood ($\sim$0.3 $M_\odot$). In particular, because the S209 main cluster is a star-forming cluster with a larger number of members ($\sim$1500) than the number ($\sim$100) in regions previously studied in such environments, it is possible for the first time to derive the IMF in a low-metallicity environment with high accuracy over the wide mass range 0.1--20 $M_\odot$.

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arXiv2022

Deep simultaneous limits on optical emission from FRB 20190520B by 24.4 fps observations with Tomo-e Gozen

We conduct 24.4~fps optical observations of repeating Fast Radio Burst (FRB) 20190520B using Tomo-e Gozen, a high-speed CMOS camera mounted on the Kiso 105-cm Schmidt telescope, simultaneously with radio observations carried out using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We succeeded in the simultaneous optical observations of 11 radio bursts that FAST detected. However, no corresponding optical emission was found. The optical fluence limits as deep as 0.068 Jy ms are obtained for the individual bursts (0.029 Jy ms on the stacked data) corrected for the dust extinction in the Milky Way. The fluence limit is deeper than those obtained in the previous simultaneous observations for an optical emission with a duration $\gtrsim 0.1$ ms. Although the current limits on radio--optical spectral energy distribution (SED) of FRBs are not constraining, we show that SED models based on observed SEDs of radio variable objects such as optically detected pulsars, and a part of parameter spaces of theoretical models in which FRB optical emission is produced by inverse-Compton scattering in a pulsar magnetosphere or a strike of a magnetar blastwave into a hot wind bubble, can be ruled out once a similar fluence limit as in our observation is obtained for a bright FRB with a radio fluence $\gtrsim 5$ Jy ms.

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arXiv2022

Video Observations of Tiny Near-Earth Objects with Tomo-e Gozen

We report the results of video observations of tiny (diameter less than 100 m) near-Earth objects (NEOs) with Tomo-e Gozen on the Kiso 105 cm Schmidt telescope. A rotational period of a tiny asteroid reflects its dynamical history and physical properties since smaller objects are sensitive to the YORP effect. We carried out video observations of 60 tiny NEOs at 2 fps from 2018 to 2021 and successfully derived the rotational periods and axial ratios of 32 NEOs including 13 fast rotators with rotational periods less than 60 s. The fastest rotator found during our survey is 2020 HS7 with a rotational period of 2.99 s. We statistically confirmed that there is a certain number of tiny fast rotators in the NEO population, which have been missed with any previous surveys. We have discovered that the distribution of the tiny NEOs in a diameter and rotational period (D-P) diagram is truncated around a period of 10 s. The truncation with a flat-top shape is not explained well either by a realistic tensile strength of NEOs or suppression of YORP by meteoroid impacts. We propose that the dependence of the tangential YORP effect on the rotational period potentially explains the observed pattern in the D-P diagram.

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arXiv2022

Survey of near-infrared diffuse interstellar bands in $Y$ and $J$ bands. I. Newly identified bands

We searched for diffuse interstellar bands (DIBs) in the 0.91$<λ<$1.33 $μ$m region by analyzing the near-infrared (NIR) high-resolution ($R=20,000$ and 28,000) spectra of 31 reddened early-type stars ($0.04<E(B-V)<4.58$) and an unreddened reference star. The spectra were collected using the WINERED spectrograph, which was mounted on the 1.3 m Araki telescope at Koyama Astronomical Observatory, Japan, in 2012--2016, and on the 3.58 m New Technology Telescope at La Silla Observatory, Chile, in 2017--2018. We detected 54 DIBs -- 25 of which are newly detected by this study -- eight DIB candidates. Using this updated list, the DIB distributions over a wide wavelength range from optical to NIR are investigated. The FWHM values of the NIR DIBs are found to be narrower than those of the optical DIBs on average, which suggests that the DIBs at longer wavelengths tend to be caused by larger molecules. Assuming that the larger carriers are responsible for the DIBs at longer wavelengths and have the larger oscillator strengths, we found that the total column densities of the DIB carriers tend to decrease with increasing DIB wavelength. The candidate molecules and ions for the NIR DIBs are also discussed.

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arXiv2022

Fast optical flares from M dwarfs detected by a one-second-cadence survey with Tomo-e Gozen

We report a one-second-cadence wide-field survey for M-dwarf flares using the Tomo-e Gozen camera mounted on the Kiso Schmidt telescope. We detect 22 flares from M3-M5 dwarfs with rise times and amplitudes ranging from $5\, \mathrm{sec} \lesssim t_\mathrm{rise} \lesssim 100\,\mathrm{sec}$ and $0.5 \lesssim ΔF/F_{\star} \lesssim 20$, respectively. The flare light curves mostly show steeper rises and shallower decays than those obtained from the Kepler one-minute cadence data and tend to have flat peak structures. Assuming a blackbody spectrum with temperatures of $9,000-15,000\,\mathrm{K}$, the peak luminosities and bolometric energies are estimated to be $10^{29}\,\mathrm{erg\,sec^{-1}} \lesssim L_\mathrm{peak} \lesssim 10^{31}\,\mathrm{erg\,sec^{-1}}$ and $10^{31}\,\mathrm{erg} \lesssim E_{\rm bol} \lesssim 10^{34}\,\mathrm{erg}$, which constitutes the bright end of fast optical flares for M dwarfs. We confirm that more than 90\% of the host stars of the detected flares are magnetically active based on their H$α$ emission line intensities obtained by LAMOST. The estimated occurrence rate of the detected flares is $\sim 0.7$ per day per an active star, indicating they are common in magnetically active M dwarfs. We argue that the flare light curves can be explained by the chromospheric compression model; the rise time is broadly consistent with the Alfvén transit time of a magnetic loop with a length scale of $l_\mathrm{loop} \sim 10^4\,\mathrm{km}$ and a field strength of $1,000\,\mathrm{G}$, while the decay time is likely determined by the radiative cooling of the compressed chromosphere down to near the photosphere with a temperature of $\gtrsim 10,000\,\mathrm{K}$. These flares from M dwarfs could be a major contamination source for a future search of fast optical transients of unknown types.

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arXiv2022

Star Formation Activity Beyond the Outer Arm II: Distribution and Properties of Star Formation

The outer Galaxy beyond the Outer Arm represents a promising opportunity to study star formation in an environment vastly different from the solar neighborhood. In our previous study, we identified 788 candidate star-forming regions in the outer Galaxy (at galactocentric radii $R_{\rm G}$ $\ge$ 13.5 kpc) based on Wide-field Infrared Survey Explorer (WISE) mid-infrared (MIR) all-sky survey. In this paper, we investigate the statistical properties of the candidates and their parental molecular clouds derived from the Five College Radio Astronomy Observatory (FCRAO) CO survey. We show that the molecular clouds with candidates have a shallower slope of cloud mass function, a larger fraction of clouds bound by self-gravity, and a larger density than the molecular clouds without candidates. To investigate the star formation efficiency (SFE) at different $R_{\rm G}$, we used two parameters: 1) the fraction of molecular clouds with candidates and 2) the monochromatic MIR luminosities of candidates per parental molecular cloud mass. We did not find any clear correlation between SFE parameters and $R_{\rm G}$ at $R_{\rm G}$ of 13.5 kpc to 20.0 kpc, suggesting that the SFE is independent of environmental parameters such as metallicity and gas surface density, which vary considerably with $R_{\rm G}$. Previous studies reported that the SFE per year (SFE/yr) derived from the star-formation rate surface density per total gas surface density, HI plus H$_2$, decreases with increased $R_{\rm G}$. Our results might suggest that the decreasing trend is due to a decrease in HI gas conversion to H$_2$ gas.

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