arXiv subjects

Chikako Yasui

Publications and source records attributed to Chikako Yasui.

48 records · Page 3Linked to original sources

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

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↗

arXiv2010

Deep CO Observations and the CO-to-H_2 Conversion Factor in DDO 154, a Low Metallicity Dwarf Irregular Galaxy

We present a deep spectroscopic search for CO emission in the dwarf irregular galaxy DDO154, which has an Oxygen abundance of only 1/20 the solar value. The observations were conducted in order to constrain the CO-to-$\mathrm{H_2}$ conversion factor at low metallicity. No CO was detected, however, despite being one of the sensitive observations done towards galaxies of this type. We succeed in putting a strong lower limit on the conversion factor, at least 10 times the Galactic value. Our result supports previous studies which argue for a high conversion factor at low metallicity.

astro-ph.CO↗

arXiv2010

Short Lifetime of Protoplanetary Disks in Low-metallicity Environments

We studied near-infrared disk fractions of six young clusters in the low-metallicity environments with [O/H$] \sim -0.7$ using deep $JHK$ images with Subaru 8.2\,m telescope. We found that disk fraction of the low-metallicity clusters declines rapidly in $<$1\,Myr, which is much faster than the $\sim$5--7\,Myr observed for the solar-metallicity clusters, suggesting that disk lifetime shortens with decreasing metallicity possibly with an $\sim$$10^Z$ dependence. Since the shorter disk lifetime reduces the time available for planet formation, this could be one of the major reasons for the strong planet--metallicity correlation. Although more quantitative observational and theoretical assessments are necessary, our results present the first direct observational evidence that can contribute to explaining the planet--metallicity correlation.

astro-ph.SR↗

arXiv2009

The Lifetime of Protoplanetary Disks in a Low-Metallicity Environment

The extreme outer Galaxy (EOG), the region with a Galactic radius of more than 18 kpc, is known to have very low metallicity, about one-tenth that of the solar neighborhood. We obtained deep near-infrared (NIR) images of two very young ($\sim$0.5 Myr) star-forming clusters that are one of the most distant embedded clusters in the EOG. We find that in both clusters the fraction of stars with NIR excess, which originates from the circumstellar dust disk at radii of $\leq$0.1 AU, is significantly lower than those in the solar neighborhood. Our results suggest that most stars forming in the low-metallicity environment experience disk dispersal at an earlier stage ($<$1 Myr) than those forming in the solar metallicity environment (as much as $\sim$5--6 Myr). Such rapid disk dispersal may make the formation of planets difficult, and the shorter disk lifetime with lower metallicity could contribute to the strong metallicity dependence of the well-known "planet-metallicity correlation", which states the probability of a star hosting a planet increases steeply with stellar metallicity. The reason for the rapid disk dispersal could be increase of the mass accretion rate and/or the effective far-ultraviolet photoevaporation due to the low extinction; however, another unknown mechanism for the EOG environment could be contributing significantly.

astro-ph.SR↗

arXiv2008

Star Formation in the Most Distant Molecular Cloud in the Extreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of the Galaxy's Formation

We report the discovery of active star formation in Digel's Cloud 2, which is one of the most distant giant molecular clouds known in the extreme outer Galaxy (EOG). At the probable Galactic radius of ~20 kpc, Cloud 2 has a quite different environment from that in the solar neighborhood, including lower metallicity, much lower gas density, and small or no perturbation from spiral arms. With new wide-field near-infrared (NIR) imaging that covers the entire Cloud 2, we discovered two young embedded star clusters located in the two dense cores of the cloud. Using our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we discuss the detailed star formation processes in this unique environment. We show clear evidences of a sequential star formation triggered by the nearby huge supernova remnant, GSH 138-01-94. The two embedded clusters show a distinct morphology difference: the one in the northern molecular cloud core is a loose association with isolated-mode star formation, while the other in the southern molecular cloud core is a dense cluster with cluster-mode star formation. We propose that high compression by the combination of the SNR shell and an adjacent shell caused the dense cluster formation in the southern core. Along with the low metallicity range of the EOG, we suggest that EOG could be an excellent laboratory for the study of star formation processes, such as those triggered by supernovae, that occured during an early epoch of the Galaxy's formation. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches with that of the EOG well.

astro-ph↗

arXiv2007

Star Formation in the Extreme Outer Galaxy: the IMF in a low metallicity environment

We are conducting a deep near-infrared (NIR) imaging survey of young embedded clusters in the extreme outer Galaxy (hereafter EOG), at the Galactic radius (R_g) of more than 18 kpc. The EOG is an excellent laboratory to study the nature of the IMF in a low-metallicity environment with a great advantage of the proximity compared to nearby dwarf galaxies, such as LMC & SMC. As a first step, we obtained deep NIR images of Digel Cloud 2 clusters at R_g ~19 kpc using the Subaru 8.2-m telescope. The observed K-band luminosity function shows that IMF in the low metallicity environment down to ~ 0.1 M_{sun} is not significantly different from the typical IMFs in the field and in the nearby star clusters as was suggested in our earlier work.

astro-ph↗

arXiv2007

Extreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of Galaxy Formation

The extreme outer Galaxy (EOG) has a very different environment from that in the solar neighborhood, with low metallicity (less than -0.5 dex), much lower gas density, and small or no perturbation from spiral arms. The EOG is an excellent laboratory for the study of the star formation processes that happened during the formation period of the Galaxy. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches well with that of the EOG. We show an example of a molecular cloud in the EOG (Digel's Cloud 2), which is located at R_g ~ 20 kpc beyond the Outer arm. Based on our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we examined the detailed star formation processes in this unique environment, especially the supernova triggered star formation, which should have been the major star formation mode during the halo and thick disk formation.

astro-ph↗

arXiv2007

Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters

As a first step for studying star formation in the extreme outer Galaxy (EOG), we obtained deep near-infrared images of two embedded clusters at the northern and southern CO peaks of Cloud 2, which is one of the most distant star forming regions in the outer Galaxy (galactic radius R_g ~ 19 kpc). With high spatial resolution (FWHM ~ 0".35) and deep imaging (K ~ 21 mag) with the IRCS imager at the Subaru telescope, we detected cluster members with a mass detection limit of < 0.1 M_{sun}, which is well into the substellar regime. These high quality data enables a comparison of EOG to those in the solar neighborhood on the same basis for the first time. Before interpreting the photometric result, we have first constructed the NIR color-color diagram (dwarf star track, classical T Tauri star (CTTS) locus, reddening law) in the Mauna Kea Observatory filter system and also for the low metallicity environment since the metallicity in EOG is much lower than those in the solar neighborhood. The estimated stellar density suggests that an ``isolated type'' star formation is ongoing in Cloud 2-N, while a ``cluster type'' star formation is ongoing in Cloud 2-S. Despite the difference of the star formation mode, other characteristics of the two clusters are found to be almost identical: (1) K-band luminosity function (KLF) of the two clusters are quite similar, as is the estimated IMF and ages (~ 0.5--1 Myr) from the KLF fitting, (2) the estimated star formation efficiencies (SFEs) for both clusters are typical compared to those of embedded clusters in the solar neighborhood (~ 10 %). The similarity of two independent clusters with a large separation (~ 25 pc) strongly suggest that their star formation activities were triggered by the same mechanism, probably the supernova remnant (GSH 138-01-94).

astro-ph↗

arXiv2006

Deep Near-Infrared Imaging af an Embedded Cluster in the Extreme Outer Galaxy: Census of Supernovae Triggered Star Formation

While conducting a near-infrared (NIR) survey of ``Digel Clouds'', which are thought to be located in the extreme outer Galaxy (EOG), Kobayashi & Tokunaga found star formation activity in ``Cloud 2'', a giant molecular cloud at the Galactic radius of ~ 20 kpc. Additional infrared imaging showed two embedded young clusters at the densest regions of the molecular cloud. Because the molecular cloud is located in the vicinity of a supernova remnant (SNR) HI shell, GSH 138-01-94, it was suggested that the star formation activity in Cloud 2 was triggered by this expanding HI shell. We obtained deep J (1.25 um), H (1.65 um) and K (2.2 um) images of one of the embedded clusters in Cloud 2 with high spatial resolution (FWHM ~0".3) and high sensitivity (K ~ 20 mag, 10 sigma). We identified 52 cluster members. The estimated stellar density (~ 10 pc^{-2}) suggests that the cluster is a T-association. This is the deepest NIR imaging of an embedded cluster in the EOG. The observed K-band luminosity function (KLF) suggests that the underlying initial mass function (IMF) of the cluster down to the detection limit of ~ 0.1 M_sun is not significantly different from the typical IMFs in the field and in the near-by star clusters. The overall characteristics of this cluster appears to be similar to those of other embedded clusters in the far outer Galaxy. The estimated age of the cluster from the KLF, which is less than 1 Myr, is consistent with the view that the star formation was triggered by the HI shell whose age was estimated at 4.3 Myr (Stil & Irwin). The 3-dimensional geometry of SNR shell, molecular cloud and the embedded cluster, which is inferred from our data, as well as the cluster age strongly suggest that the star formation in Cloud 2 was triggered by the SNR shell.

astro-ph↗