arXiv subjects

Chie Nagashima

Publications and source records attributed to Chie Nagashima.

13 recordsLinked to original sources

arXiv2018

A near infrared variable star survey in the Magellanic Clouds: The Small Magellanic Cloud data

A very long term near-infrared variable star survey towards the Large and Small Magellanic Clouds was carried out using the 1.4m InfraRed Survey Facility at the South African Astronomical Observatory. This project was initiated in December 2000 in the LMC, and in July 2001 in the SMC. Since then an area of 3 square degrees along the bar in the LMC and an area of 1 square degree in the central part of the SMC have been repeatedly observed. This survey is ongoing, but results obtained with data taken until December 2017 are reported in this paper. Over more than 15 years we have observed the two survey areas more than one hundred times. This is the first survey that provides near-infrared time-series data with such a long time baseline and on such a large scale. This paper describes the observations in the SMC and publishes a point source photometric catalogue, a variable source catalogue, and time-series data.

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arXiv2008

Sequential star formation in a cometary globule (BRC37) of IC1396

We have carried out near-IR/optical observations to examine star formation toward a bright-rimmed cometary globule (BRC37) facing the exciting star(s) of an HII region (IC1396) containing an IRAS source, which is considered to be an intermediate-mass protostar. With slit-less spectroscopy we detected ten H_alpha emission stars around the globule, six of which are near the tip of the globule and are aligned along the direction to the exciting stars. There is evidence that this alignment was originally towards an O9.5 star, but has evolved to align towards a younger O6 star when that formed. Near-IR and optical photometry suggests that four of these six stars are low-mass young stellar objects (YSOs) with masses of ~0.4 M_sun. Their estimated ages of ~1 Myr indicate that they were formed at the tip in advance of the formation of the IRAS source. Therefore, it is likely that sequential star formation has been taking place along the direction from the exciting stars towards the IRAS source, due to the UV impact of the exciting star(s). Interestingly, one faint, H_alpha emission star, which is the closest to the exciting star(s), seems to be a young brown dwarf that was formed by the UV impact in advance of the formation of other YSOs at the tip.

astro-ph↗

arXiv2007

Near-Infrared Polarimetry of the Eagle Nebula (M16)

We carried out deep and wide (about 8' x 8') JHKs imaging polarimetry in the southern region of the Eagle Nebula (M16). The polarization intensity map reveals that two YSOs with near-IR reflection nebulae are located at the tips of two famous molecular pillars (Pillars 1 and 2) facing toward the exciting stars of M16. The centrosymmetric polarization pattern are consistent with those around class I objects having circumstellar envelopes, confirming that star formation is now taking place at the two tips of the pillars under the influence of UV radiation from the exciting stars. Polarization measurements of point sources show that magnetic fields are aligned along some of the pillars but in a direction that is quite different to the global structure in M16.

astro-ph↗

arXiv2007

Near-Infrared Imaging Polarimetry of the NGC 2071 Star Forming Region with SIRPOL

We have conducted deep JHKs imaging polarimetry of a ~8' x 8' area of the NGC 2071 star forming region. Our polarization data have revealed various infrared reflection nebulae (IRNe) associated with the central IR young star cluster NGC2071IR and identified their illuminating sources. There are at least 4 IRNe in NGC2071IR and several additional IRNe are identified around nearby young stars in the same field-of-view. Each illuminating source coincides with a known near-IR source except for IRS3, which is only a part of IRN2 and is illuminated by the radio source 1c. Aperture polarimetry of each cluster source is used to detect unresolved circumstellar disk/outflow systems. Aperture polarimetry of the other point-like sources within the field is made in this region for the first time. The magnetic field structures (from ~1 pc down to \~0.1 pc) are derived using both aperture polarimetry of the point-like sources and imaging polarimetry of the shocked H2 emission that is seen as the dominant knotty nebulae in the Ks band image; they are both of dichroic origin and the derived field directions are consistent with each other. The magnetic field direction projected on the sky is also consistent with that inferred from the 850 micron thermal continuum emission polarimetry of the central 0.2 pc region, but running roughly perpendicular (~75 degrees) to the direction of the large scale outflow. We argue that the field strength is too weak to align the outflow in the large scale field direction via magnetic braking.

astro-ph↗

arXiv2006

Near-Infrared Polarization Images of The Orion Molecular Cloud 1 South Region

We present the polarization images in the $J$, $H$, & $Ks$ bands of the Orion Molecular Cloud 1 South region. The polarization images clearly show at least six infrared reflection nebulae (IRNe) which are barely seen or invisible in the intensity images. Our polarization vector images also identify the illuminating sources of the nebulae: IRN 1 & 2, IRN 3, 4, & 5, and IRN 6 are illuminated by three IR sources, Source 144-351, Source 145-356, and Source 136-355, respectively. Moreover, our polarization images suggest the candidate driving sources of the optical Herbig-Haro objects for the first time; HH529, a pair of HH202 and HH528 or HH 203/204, HH 530 and HH269 are originated from Source 144-351, Source 145-356, and Source 136-355, respectively.

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arXiv2006

Probable association of T Tauri stars with the L1014 dense core

Using the Wide Field Grism Spectrograph 2 (WFGS2), we have carried out slit-less spectroscopy, g'r'i' photometry, and slit spectroscopy on the L1014 dense core. We detected three Halpha emission line stars. We interpret one as weak-line T Tauri star (WTTS) and the others as classical T Tauri stars (CTTS). Since their g'-i' colors and/or classified spectral types are consistent with those of T Tauri stars and two of them show less extinction than the cloud, these three stars are likely to be T Tauri stars associated with L1014. Adopting an age range for T Tauri stars, 1-10 Myr, the color-magnitude diagram suggests a distance of ~400-900 pc, rather than the previously assumed distance, 200 pc. This could strongly affect on the mass estimate of L1014-IRS, which is thought to be either a very young protostar or proto-brown dwarf.

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arXiv2006

Interstellar Extinction Law in the J, H, and Ks Bands toward the Galactic Center

We have determined the ratios of total to selective extinction in the near-infrared bands (J, H, Ks) toward the Galactic center from the observations of the region |l| < 2.0deg and 0.5deg < |b| < 1.0deg with the IRSF telescope and the SIRIUS camera. Using the positions of red clump stars in color-magnitude diagrams as a tracer of the extinction and reddening, we determine the average of the ratios of total to selective extinction to be A(Ks)/E(H-Ks) = 1.44+-0.01, A(Ks)/E(J-Ks) = 0.494+-0.006, and A(H)/E(J-H) = 1.42+-0.02, which are significantly smaller than those obtained in previous studies. From these ratios, we estimate that A(J) : A(H) : A(Ks) = 1 : 0.573+-0.009 : 0.331+-0.004 and E(J-H)/E(H-Ks) = 1.72+-0.04, and we find that the power law A(lambda) \propto lambda^{-1.99+-0.02} is a good approximation over these wavelengths. Moreover, we find a small variation in A(Ks)/E(H-Ks) across our survey. This suggests that the infrared extinction law changes from one line of sight to another, and the so-called ``universality'' does not necessarily hold in the infrared wavelengths.

astro-ph↗

arXiv2005

Near Infrared Imaging Survey of Bok Globules: Density Structure

On the basis of near-infrared imaging observations, we derived visual extinction (Av) distribution toward ten Bok globules through measurements of both the color excess (E_{H-K}) and the stellar density at J, H, and Ks (star count). Radial column density profiles for each globule were analyzed with the Bonnor-Ebert sphere model. Using the data of our ten globules and four globules in the literature, we investigated the stability of globules on the basis of xi_max, which characterizes the Bonnor-Ebert sphere as well as the stability of the equilibrium state against the gravitational collapse. We found that more than half of starless globules are located near the critical state (xi_max = 6.5 +/- 2). Thus, we suggest that a nearly critical Bonnor-Ebert sphere characterizes the typical density structure of starless globules. Remaining starless globules show clearly unstable states (xi_max > 10). Since unstable equilibrium states are not long maintained, we expect that these globules are on the way to gravitational collapse or that they are stabilized by non-thermal support. It was also found that all the star-forming globules show unstable solutions of xi_max >10, which is consistent with the fact that they have started gravitational collapse. We investigated the evolution of a collapsing gas sphere whose initial condition is a nearly critical Bonnor-Ebert sphere. We found that the column density profiles of the collapsing sphere mimic those of the static Bonnor-Ebert spheres in unstable equilibrium. The collapsing gas sphere resembles marginally unstable Bonnor-Ebert spheres for a long time. We found that the frequency distribution of xi_max for the observed starless globules is consistent with that from model calculations of the collapsing sphere.

astro-ph↗

arXiv2005

A Distinct Structure Inside the Galactic Bar

We present the result of a near-infrared (J H Ks) survey along the Galactic plane, -10.5deg < l < +10.5deg and b=+1.0deg, with the IRSF 1.4m telescope and the SIRIUS camera. Ks vs. H-Ks color-magnitude diagrams reveal a well-defined population of red clump (RC) stars whose apparent magnitude peak changes continuously along the Galactic plane, from Ks=13.4 at l=-10deg to Ks=12.2 at l=+10deg after dereddening. This variation can be explained by the bar-like structure found in previous studies, but we find an additional inner structure at |l| < 4deg, where the longitude - apparent magnitude relation is distinct from the outer bar, and the apparent magnitude peak changes by only 0.1 mag over the central 8deg. The exact nature of this inner structure is as yet uncertain.

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arXiv2004

Variable stars in the Magellanic Clouds: II. The data and infrared properties

The data of 8,852 and 2,927 variable stars detected by OGLE survey in the Large and Small Magellanic Clouds are presented. They are cross-identified with the SIRIUS JHK survey data, and their infrared properties are discussed. Variable red giants are well separated on the period-J - K plane, suggesting that it could be a good tool to distinguish their pulsation mode and type.

astro-ph↗

arXiv2003

The Infrared Counterpart of the X-Ray Nova XTE J1720-318

We report on the discovery of an infrared counterpart to the X-ray transient XTE J1720-318 on 2003 January 18, nine days after an X-ray outburst, and the infrared light curve during the first 130 days after the outburst. The infrared light curve shows a decline of about 1.2 mag from the peak magnitude of Ks about 15.3 over the observation period, and a secondary maximum, about 40 days after the outburst. Another small increase in the flux was also recorded about 20 days after the outburst. These increases were also detected in the X-ray light curve. The J H Ks colors are consistent with an X-ray irradiated accretion disk suffering an extinction of Av about 8, which is also inferred from its X-ray spectrum and the extinction map constructed from far-infrared dust emission of this line of sight. These J, H, and Ks observations demonstrate that useful data can be obtained even for such an object, which suffers heavy optical extinction, possibly located beyond the Galactic center.

astro-ph↗

arXiv2003

Variable Stars in the Magellanic Clouds: Results from OGLE and SIRIUS

We have performed a cross-identification between OGLE-II data and single-epoch SIRIUS JHK survey data in the LMC and SMC. After eliminating obvious spurious variables, we determined the pulsation periods for 9,681 and 2,927 variables in the LMC and SMC, respectively. Based on these homogeneous data, we studied the pulsation properties and metallicity effects on period-K magnitude (PK) relations by comparing the variable stars in the LMC and SMC. The sample analyzed here is much larger, and we found the following new features: (1) variable red giants in the SMC form parallel sequences on the PK plane, just like those found by Wood (2000) in the LMC; (2) both of the sequences A and B of Wood (2000) have discontinuities, and they occur at the K-band luminosity of the TRGB; (3) the sequence B of Wood (2000) separates into three independent sequences B+- and C'; (4) comparison between the theoretical pulsation models (Wood et al. 1996) and observational data suggests that the variable red giants on sequences C and newly discovered C' are pulsating in the fundamental and first overtone mode, respectively; (5) the theory can not explain the pulsation mode of sequences A+- and B+-, and they are unlikely to be the sequences for the first and second overtone pulsators, as was previously suggested; (6) the zero points of PK relations of Cepheids in the metal deficient SMC are fainter than those of LMC ones by ~0.1 mag but those of SMC Miras are brighter than those of LMC ones by ~0.13 mag, which are probably due to metallicity effects.

astro-ph↗

arXiv2002

Pulsation at the tip of the first giant branch?

The first results of our ongoing near-infrared (NIR) survey of the variable red giants in the Large Magellanic Cloud, using the Infrared Survey Facility (IRSF) and the SIRIUS infrared camera, are presented. Many very red stars were detected and found that most of them are variables. In the observed colour-magnitude diagram ($J-K, K$) and the stellar $K$ magnitude distribution, the tip of the first giant branch (TRGB), where helium burning in the core starts, is clearly seen. Apart from the genuine AGB variables, we found many variable stars at luminosities around the TRGB. From this result, we infer that a substantial fraction of them are RGB variables.

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