arXiv subjects

Naoto Kobayashi

Publications and source records attributed to Naoto Kobayashi.

At least 55 records · Page 3Linked to original sources

arXiv2017

Method to Estimate the Effective Temperatures of Late-Type Giants using Line-Depth Ratios in the Wavelength Range 0.97-1.32$μ$m

The effective temperature, one of the most fundamental atmospheric parameters of a star, can be estimated using various methods, and here we focus on the method using line-depth ratios (LDRs). This method combines low- and high-excitation lines and makes use of relations between LDRs of these line pairs and the effective temperature. It has an advantage, for example, of being minimally affected by interstellar reddening, which changes stellar colours. We report 81 relations between LDRs and the effective temperature established with high-resolution, $λ/Δλ\sim 28,000$, spectra of nine G- to M-type giants in Y and J bands. Our analysis gives the first comprehensive set of LDR relations for this wavelength range. The combination of all these relations can be used to determine the effective temperatures of stars that have 3700<Teff<5400K and -0.5<[Fe/H]<+0.3dex to the precision of $\pm $10K in the best cases.

astro-ph.SR↗

arXiv2017

New classical Cepheids in the inner part of the northern Galactic disk and their kinematics

The characteristics of the inner Galaxy remain obscured by significant dust extinction, and hence infrared surveys are useful to find young Cepheids whose distances and ages can be accurately determined. A near-infrared photometric and spectroscopic survey was carried out and three classical Cepheids were unveiled in the inner disk, around 20 and 30 degrees in Galactic longitude. The targets feature small Galactocentric distances, 3-5 kpc, and their velocities are important as they may be under the environmental influence of the Galactic bar. While one of the Cepheids has radial velocity consistent with the Galactic rotation, the other two are moving significantly slower. We also compare their kinematics with that of high-mass star-forming regions with parallactic distances measured.

astro-ph.GA↗

arXiv2017

Discovery of a distant molecular cloud in the extreme outer Galaxy with the Nobeyama 45-m telescope

We report the discovery of the molecular cloud whose kinematic distance is the largest in the Galaxy at the present moment, named G213.042$+$0.003, at $l =$ 213.042$^\circ$ and $b =$ 0.003$^\circ$ in the $^{12}$CO($J =$ 1-0) line using the Nobeyama 45-m telescope and a multi-beam receiver BEARS. This molecular cloud is located at the heliocentric distance of 21$_{-7}^{+12}$ kpc and Galactocentric distance of 29$_{-7}^{+12}$ kpc, which are estimated as the kinematic distances with the Galactic parameters obtained by Reid et al. (2014, ApJ, 783, 130). Its major and minor diameters and line width were measured to be 4.0$_{-1.3}^{+2.3}$ pc, 3.0$_{-1.0}^{+1.7}$ pc, and 1.5 km s$^{-1}$, respectively. The cloud mass was estimated to be 2.5$_{-1.4}^{+3.7}$ $\times$ 10$^2$ $M_\odot$ using the CO-to-H$_2$ conversion factor of 5.6 $\times$ 10$^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ obtained in far outer Galaxy.

astro-ph.GA↗

arXiv2016

A lack of classical Cepheids in the inner part of the Galactic disk

Recent large-scale infrared surveys have been revealing stellar populations in the inner Galaxy seen through strong interstellar extinction in the disk. In particular, classical Cepheids with their period-luminosity and period-age relations are useful tracers of Galactic structure and evolution. Interesting groups of Cepheids reported recently include four Cepheids in the Nuclear Stellar Disk (NSD), about 200 pc around the Galactic Centre, found by Matsunaga et al. and those spread across the inner part of the disk reported by Dekany and collaborators. We here report our discovery of nearly thirty classical Cepheids towards the bulge region, some of which are common with Dekany et al., and discuss the large impact of the reddening correction on distance estimates for these objects. Assuming that the four Cepheids in the NSD are located at the distance of the Galactic Centre and that the near-infrared extinction law, i.e. wavelength dependency of the interstellar extinction, is not systematically different between the NSD and other bulge lines-of-sight, most of the other Cepheids presented here are located significantly further than the Galactic Centre. This suggests a lack of Cepheids in the inner 2.5 kpc region of the Galactic disk except the NSD. Recent radio observations show a similar distribution of star-forming regions.

astro-ph.GA↗

arXiv2016

Low-metallicity Young Clusters in the Outer Galaxy. II. Sh 2-208

We obtained deep near-infrared images of Sh 2-208, one of the lowest-metallicity HII regions in the Galaxy, [O/H] = -0.8 dex. We detected a young cluster in the center of the HII region with a limiting magnitude of K = 18.0 mag (10sigma), which corresponds to a mass detection limit of ~0.2 M_sun. This enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. We identified 89 cluster members. From the fitting of the K-band luminosity function (KLF), the age and distance of the cluster are estimated to be ~0.5 Myr and ~4 kpc, respectively. The estimated young age is consistent with the detection of strong CO emission in the cluster region and the estimated large extinction of cluster members (Av ~ 4--25 mag). The observed KLF suggests that the underlying initial mass function (IMF) of the low-metallicity cluster is not significantly different from canonical IMFs in the solar neighborhood in terms of both high-mass slope and IMF peak (characteristic mass). Despite the very young age, the disk fraction of the cluster is estimated at only 27pm6 %, which is significantly lower than those in the solar metallicity. Those results are similar to Sh 2-207, which is another low-metallicity star-forming region close to Sh 2-208 with a separation of 12 pc,suggesting that their star-forming activities in low-metallicity environments are essentially identical to those in the solar neighborhood, except for the disk dispersal timescale. From large-scale mid-infrared images, we suggest that sequential star formation is taking place in Sh 2-207, Sh 2-208 and the surrounding region, triggered by an expanding bubble with a ~30 pc radius.

astro-ph.GA↗

arXiv2016

Near infrared diffuse interstellar bands toward the Cygnus OB2 association

We obtained the near-infrared (NIR) high-resolution ($R\equivλ/Δλ\sim20,000$) spectra of the seven brightest early-type stars in the Cygnus OB2 association for investigating the environmental dependence of diffuse interstellar bands (DIBs). The WINERED spectrograph mounted on the Araki 1.3m telescope in Japan was used to collect data. All 20 of the known DIBs within the wavelength coverage of WINERED ($0.91<λ<1.36μ$m) were clearly detected along all lines of sight because of their high flux density in the NIR wavelength range and the large extinction. The equivalent widths (EWs) of DIBs were not correlated with the column densities of C$_2$ molecules, which trace the patchy dense component, suggesting that the NIR DIB carriers are distributed mainly in the diffuse component. On the basis of the correlations among the NIR DIBs both for stars in Cyg OB2 and stars observed previously, $λλ$10780, 10792, 11797, 12623, and 13175 are found to constitute a "family", in which the DIBs are correlated well over the wide EW range. In contrast, the EW of $λ$10504 is found to remain almost constant over the stars in Cyg OB2. The extinction estimated from the average EW of $λ$10504 ($A_V\sim3.6$mag) roughly corresponds to the lower limit of the extinction distribution of OB stars in Cyg OB2. This suggests that $λ$10504 is absorbed only by the foreground clouds, implying that the carrier of $λ$10504 is completely destroyed in Cyg OB2, probably by the strong UV radiation field. The different behaviors of the DIBs may be caused by different properties of the DIB carriers.

astro-ph.GA↗

arXiv2016

Herbig Ae/Be candidate stars in the innermost Galactic disk: Quartet cluster

In order to investigate the Galactic-scale environmental effects on the evolution of protoplanetary disks, we explored the near-infrared (NIR) disk fraction of the Quartet cluster, which is a young cluster in the innermost Galactic disk at the Galactocentric radius Rg ~ 4 kpc. Because this cluster has a typical cluster mass of ~10^3 M_sun as opposed to very massive clusters, which have been observed in previous studies (>10^4 M_sun), we can avoid intra-cluster effects such as strong UV field from OB stars. Although the age of the Quartet is previously estimated to be 3-8 Myr old, we find that it is most likely ~3-4.5 Myr old. In moderately deep JHK images from the UKIDSS survey, we found eight HAeBe candidates in the cluster, and performed K-band medium-resolution ($R \equiv Δλ/ λ~ 800$) spectroscopy for three of them with the Subaru 8.2 m telescope. These are found to have both Brγabsorption lines as well as CO bandhead emission, suggesting that they are HAeBe stars with protoplanetary disks. We estimated the intermediate-mass disk fraction (IMDF) to be ~25 % for the cluster, suggesting slightly higher IMDF compared to those for young clusters in the solar neighborhood with similar cluster age, although such conclusion should await future spectroscopic study of all candidates of cluster members.

astro-ph.SR↗

arXiv2015

Low-metallicity Young Clusters in the Outer Galaxy I. Sh 2-207

To study star formation in low metallicity environments ([M/H] ~ -1 ,dex), we obtained deep near-infrared (NIR) images of Sh 2-207 (S207), which is an HII region in the outer Galaxy with spectroscopically determined metallicity of [O/H] ~= -0.8 dex. We identified a young cluster in the western region of S207 with a limiting magnitude of Ks =19.0 mag (10 sigma) that corresponds to a mass detection limit of <~0.1 M_sun and enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. From the fitting of the K-band luminosity function (KLF), the age and distance of S207 cluster are estimated at 2-3Myr and ~4 kpc, respectively. The estimated age is consistent with the suggestion of small extinctions of stars in the cluster (Av ~ 3 mag) and the non-detection of molecular clouds. The reasonably good fit between observed KLF and model KLF suggests that the underlying initial mass function (IMF) of the cluster down to the detection limit is not significantly different from the typical IMFs in the solar metallicity. From the fraction of stars with NIR excesses, a low disk fraction (<10 %) in the cluster with relatively young age is suggested, as we had previously proposed.

astro-ph.SR↗

arXiv2015

Detection of Remnant Dust Cloud Associated with the 2007 Outburst of 17P/Holmes

This paper reports a new optical observation of 17P/Holmes one orbital period after the historical outburst event in 2007. We detected not only a common dust tail near the nucleus, but also a long narrow structure that extended along the position angle 274.6+/- 0.1 degree beyond the field of view of the Kiso Wide Field Camera, i.e., >0.2 degree eastward and >2.0 degree westward from the nuclear position. The width of the structure decreased westward with increasing distance from the nucleus. We obtained the total cross section of the long extended structure in the field of view, C= (2.3 +/- 0.5)x10^10 m^2. From the position angle, morphology and the mass, we concluded that the long narrow structure consists of materials ejected during the 2007 outburst. On the basis of the dynamical behavior of dust grains in the solar radiation field, we estimated that the long narrow structure would be composed of 1 mm-1 cm grains having an ejection velocity of >50 m/s. The velocity was more than one order of magnitude faster than that of millimeter - centimeter grains from typical comets around a heliocentric distance rh of 2.5 AU. We considered that sudden sublimation of a large amount of water ice (about 10^30 mol/s) would be responsible for the high ejection velocity. We finally estimated a total mass of M=(4-8)x10^11 kg and a total kinetic energy of E=(1-6)x10^15 J for the 2007 outburst ejecta, which are consistent with those of previous studies that conducted soon after the outburst.

astro-ph.EP↗

arXiv2015

Near-infrared diffuse interstellar bands in 0.91-1.32 micron

We present a comprehensive survey of diffuse interstellar bands (DIBs) in $0.91-1.32μ$m with a newly-developed near-infrared (NIR) spectrograph, WINERED, mounted on the Araki 1.3 m Telescope in Japan. We obtained high-resolution ($R=28,300$) spectra of 25 early-type stars with color excesses of $0.07<E(B-V)<3.4$. In addition to the five DIBs previously detected in this wavelength range, we identified 15 new DIBs, 7 of which were reported as DIB "candidates" by Cox. We analyze the correlations among NIR DIBs, strong optical DIBs, and the reddening of the stars. Consequently, we found that all NIR DIBs show weaker correlations with the reddening rather than the optical DIBs, suggesting that the equivalent widths of NIR DIBs depend on some physical conditions of the interstellar clouds, such as UV flux. Three NIR DIBs, $λλ$10780, 10792, and 11797, are found to be classifiable as a "family," in which the DIBs are well correlated with each other, suggesting that the carriers of these DIBs are connected with some chemical reactions and/or have similar physical properties such as ionization potential. We also found that three strongest NIR DIBs $λ\lambda10780, 11797, 13175$ are well correlated with the optical DIB $λ5780.5$, whose carrier is proposed to be a cation molecule with high ionization potential, indicating that the carriers of the NIR DIBs could be cation molecules.

astro-ph.GA↗

arXiv2015

A Warm Near-Infrared High-Resolution Spectrograph with Very High Throughput (WINERED)

WINERED is a newly built high-efficiency (throughput$ > 25-30\%$) and high-resolution spectrograph customized for short NIR bands at 0.9-1.35 ${\rm μ}$m. WINERED is equipped with ambient temperature optics and a cryogenic camera using a 1.7 ${\rm μ}$m cut-off HgCdTe HAWAII-2RG array detector. WINERED has two grating modes: one with a conventional reflective echelle grating (R$\sim$28,300), which covers 0.9-1.35 $μ$m simultaneously, the other with ZnSe or ZnS immersion grating (R$\sim$100,000). We have completed the development of WINERED except for the immersion grating, and started engineering and science observations at the Nasmyth platform of the 1.3 m Araki Telescope at Koyama Astronomical Observatory of Kyoto-Sangyo University in Japan. We confirmed that the spectral resolution ($R\sim$ 28,300) and the throughput ($>$ 40\% w/o telescope/atmosphere/array QE) meet our specifications. We measured ambient thermal backgrounds (e.g., 0.06 ${\rm [e^{-}/sec/pixel]}$ at 287 K), which are roughly consistent with that we expected. WINERED is a portable instrument that can be installed at any telescope with Nasmyth focus as a PI-type instrument. If WINERED is installed on a 10 meter telescope, the limiting magnitude is expected to be J=18-19, which can provide high-resolution spectra with high quality even for faint distant objects.

astro-ph.IM↗

arXiv2014

Discovery of Star Formation in the Extreme Outer Galaxy Possibly Induced by a High-velocity Cloud Impact

We report the discovery of star formation activity in perhaps the most distant molecular cloud in the extreme outer galaxy. We performed deep near infrared imaging with the Subaru 8.2 m telescope, and found two young embedded clusters at two CO peaks of Digel Cloud 1 at the kinematic distance of D = 16 kpc (Galactocentric radius RG = 22 kpc). We identified 18 and 45 cluster members in the two peaks, and the estimated stellar density are ~ 5 and ~ 3 pc^-2, respectively. The observed K-band luminosity function suggests that the age of the clusters is less than 1 Myr and also the distance to the clusters is consistent with the kinematic distance. On the sky, Cloud 1 is located very close to the H I peak of high-velocity cloud (HVC) Complex H, and there are some H I intermediate velocity structures between the Complex H and the Galactic disk, which could indicate an interaction between them. We suggest possibility that Complex H impacting on the Galactic disk has triggered star formation in Cloud 1 as well as the formation of Cloud 1 molecular cloud.

astro-ph.GA↗

arXiv2014

Discovery of Dramatic Optical Variability in SDSS J1100+4421: A Peculiar Radio-Loud Narrow-Line Seyfert 1 Galaxy?

We present our discovery of dramatic variability in SDSS J1100+4421 by the high-cadence transient survey Kiso Supernova Survey (KISS). The source brightened in the optical by at least a factor of three within about half a day. Spectroscopic observations suggest that this object is likely a narrow-line Seyfert 1 galaxy (NLS1) at z=0.840, however with unusually strong narrow emission lines. The estimated black hole mass of ~ 10^7 Msun implies bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is also extremely radio-loud, with a radio loudness parameter of R ~ 4 x 10^2 - 3 x 10^3, which implies the presence of relativistic jets. Rapid and large-amplitude optical variability of the target, reminiscent of that found in a few radio- and gamma-ray loud NLS1s, is therefore produced most likely in a blazar-like core. The 1.4 GHz radio image of the source shows an extended structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine NLS1, as suggested here, this radio structure would then be the largest ever discovered in this type of active galaxies.

astro-ph.GA↗

arXiv2014

Kiso Supernova Survey (KISS): Survey Strategy

The Kiso Supernova Survey (KISS) is a high-cadence optical wide-field supernova (SN) survey. The primary goal of the survey is to catch the very early light of a SN, during the shock breakout phase. Detection of SN shock breakouts combined with multi-band photometry obtained with other facilities would provide detailed physical information on the progenitor stars of SNe. The survey is performed using a 2.2x2.2 deg field-of-view instrument on the 1.05-m Kiso Schmidt telescope, the Kiso Wide Field Camera (KWFC). We take a three-minute exposure in g-band once every hour in our survey, reaching magnitude g~20-21. About 100 nights of telescope time per year have been spent on the survey since April 2012. The number of the shock breakout detections is estimated to be of order of 1 during our 3-year project. This paper summarizes the KISS project including the KWFC observing setup, the survey strategy, the data reduction system, and CBET-reported SNe discovered so far by KISS.

astro-ph.SR↗

arXiv2014

Rapid Evolution of the Innermost Dust Disk of Protoplanetary Disks Surrounding Intermediate-mass Stars

We derived the intermediate-mass (~=1.5--7 M_sun) disk fraction (IMDF) in the near-infrared JHK photometric bands as well as in the mid-infrared (MIR) bands for young clusters in the age range of 0 to ~10 Myr. From the JHK IMDF, the lifetime of the innermost dust disk (~0.3 AU; hereafter the K disk) is estimated to be ~3 Myr, suggesting a stellar mass (M*) dependence of K-disk lifetime proportional to M*^-0.7. However, from the MIR IMDF, the lifetime of the inner disk (~5 AU; hereafter the MIR disk) is estimated to be ~6.5 Myr, suggesting a very weak stellar mass dependence (proportional to M*^-0.2). The much shorter K-disk lifetime compared to the MIR-disk lifetime for intermediate-mass (IM) stars suggests that IM stars with transition disks, which have only MIR excess emission but no K-band excess emission, are more common than classical Herbig Ae/Be stars, which exhibit both. We suggest that this prominent early disappearance of the K disk for IM stars is due to dust settling/growth in the protoplanetary disk, and it could be one of the major reasons for the paucity of close-in planets around IM stars.

astro-ph.SR↗

arXiv2013

The Environment around the Young Massive Star Cluster RSGC 1 and HESS J1837-069

We report on Mopra observations toward the young massive star cluster RSGC 1, adjoined by, and possibly associated with the gamma-ray source HESS J1837-069. We measure the CO (J=1-0) distribution around the cluster and gamma-ray source, and find that the cluster is slightly higher than the velocity ranges associated with the Crux-Scutum arm. We reveal the cluster is associated with much less molecular gas compared with other young massive clusters in the Galaxy, Westerlund 1 (Wd 1) and 2 (Wd 2), which also radiate gamma-rays. We find no other structures that would otherwise indicate the action of supernova remnants, and due to the lack of material which may form gamma-rays by hadronic interaction, we conclude that the gamma-rays detected from HESS J1837-069 are not created through proton-proton interactions, and may more plausibly originate from the pulsar that was recently found near RSGC 1.

astro-ph.GA↗

arXiv2012

Cepheids and other short-period variables near the Galactic Centre

We report the result of our near-infrared survey of short-period variable stars (P<60d) in a field-of-view of 20'x30' towards the Galactic Centre. Forty-five variables are discovered and we classify the variables based on their light curve shapes and other evidence. In addition to 3 classical Cepheids reported previously, we find 16 type II Cepheids, 24 eclipsing binaries, one pulsating star with P=0.265d (RR Lyr or delta Sct) and one Cepheid-like variable whose nature is uncertain. Eclipsing binaries are separated into the foreground objects and those significantly obscured by interstellar extinction. One of the reddened binaries contains an O-type supergiant and its light curve indicates an eccentric orbit. We discuss the nature and distribution of type II Cepheids as well as the distance to the Galactic Centre based on these Cepheids and other distance indicators. The estimates of R0(GC) we obtained based on photometric data agree with previous results obtained with kinematics of objects around the GC. Furthermore, our result gives a support to the reddening law obtained by Nishiyama and collaborators, A(Ks)/E(H-Ks)=1.44, because a different reddening law would result in a rather different distance estimate.

astro-ph.SR↗

arXiv2012

Constraining stellar properties of intervening damped Lyα and MgII absorbing galaxies toward GRB 050730

We performed multi-band deep imaging of the field around GRB 050730 to identify the host galaxies of intervening absorbers, which consist of a damped Lyα absorption (DLA) system at zabs=3.564, a sub-DLA system at zabs=3.022, and strong MgII absorption systems at zabs=1.773 and 2.253. Our observations were performed after the gamma-ray burst afterglow had disappeared. Thus, our imaging survey has a higher sensitivity to the host galaxies of the intervening absorbers than the normal imaging surveys in the direction of QSOs, for which the QSO glare tends to hide the foreground galaxies. In this deep imaging survey, we could not detect any unambiguous candidates for the host galaxies of the intervening absorbers. Using the 3sigma upper limit of the flux in the optical to mid-infrared observing bands, which corresponds to the UV to optical bands in the rest-frame of the intervening absorbers, we constrained the star-formation rates and stellar masses of the hosts. We estimated the star-formation rates for the intervening absorbers as < 2.5 Msun/yr for z>3 DLAs and < 1.0 Msun/yr for z~2 MgII systems. Their stellar masses are estimated to be several times 10^9 Msun or smaller for all intervening galaxies. These properties are comparable to dwarf galaxies, rather than the massive star-forming galaxies commonly seen in the z>2 galaxy surveys based on emission-line selection or color selection.

astro-ph.CO↗