arXiv subjects

Yasushi Nakajima

Publications and source records attributed to Yasushi Nakajima.

At least 37 records · Page 2Linked to original sources

arXiv2013

Near Infrared Circular Polarization Images of NGC 6334-V

We present results from deep imaging linear and circular polarimetry of the massive star-forming region NGC 6334- V. These observations show high degrees of circular polarization (CP), as much as 22 % in the Ks band, in the infrared nebula associated with the outflow. The CP has an asymmetric positive/negative pattern and is very extended (~80" or 0.65 pc). Both the high CP and its extended size are larger than those seen in the Orion CP region. Three-dimensional Monte Carlo light-scattering models are used to show that the high CP may be produced by scattering from the infrared nebula followed by dichroic extinction by an optically thick foreground cloud containing aligned dust grains. Our results show not only the magnetic field orientation of around young stellar objects but also the structure of circumstellar matter such as outflow regions and their parent molecular cloud along the line of sight. The detection of the large and extended CP in this source and the Orion nebula may imply the CP origin of the biological homochirality on Earth.

astro-ph.GA↗

arXiv2012

IRSF SIRIUS JHKs Simultaneous Transit Photometry of GJ1214b

We report high precision transit photometry of GJ1214b in JHKs bands taken simultaneously with the SIRIUS camera on the IRSF 1.4m telescope at Sutherland, South Africa. Our MCMC analyses show that the observed planet-to-star radius ratios in JHKs bands are R_{\rm p}/R_{\rm s,J} = 0.11833 \pm 0.00077, R_{\rm p}/R_{\rm s,H} = 0.11522 \pm 0.00079, R_{\rm p}/R_{\rm s,Ks} = 0.11459 \pm 0.00099, respectively. The radius ratios are well consistent with the previous studies by Bean et al. (2011) within 1σ, while our result in Ks band is shallower than and inconsistent at 4σ level with the previous measurements in the same band by Croll et al. (2011). We have no good explanation for this discrepancy at this point. Our overall results support a flat transmission spectrum in the observed bands, which can be explained by a water-dominated atmosphere or an atmosphere with extensive high-altitude clouds or haze. To solve the discrepancy of the radius ratios and to discriminate a definitive atmosphere model for GJ1214b in the future, further transit observations around Ks band would be especially important.

astro-ph.EP↗

arXiv2011

Complex Scattered Radiation Fields and Multiple Magnetic Fields in the Protostellar Cluster in NGC 2264

Near-infrared (IR) imaging polarimetry in the J, H, and Ks bands has been carried out for the protostellar cluster region around NGC 2264 IRS 2 in the Monoceros OB1 molecular cloud. Various infrared reflection nebulae clusters (IRNCs) associated with NGC 2264 IRS 2 and IRAS 12 S1 core were detected as well as local infrared reflection nebulae (IRNe). The illuminating sources of the IRNe were identified with known or new near- and mid-IR sources. In addition, 314 point-like sources were detected in all three bands and their aperture polarimetry was studied. Using a color-color diagram, reddened field stars and diskless pre-main sequence stars were selected to trace the magnetic field (MF) structure of the molecular cloud. The mean polarization position angle of the point-like sources is 81 \pm 29 degree in the cluster core, and 58 \pm 24 degree in the perimeter of the cluster core, which is interpreted as the projected direction on the sky of the MF in the observed region of the cloud. The Chandrasekhar-Fermi method gives a rough estimate of the MF strength to be about 100 μG. A comparison with recent numerical simulations of the cluster formation implies that the cloud dynamics is controlled by the relatively strong MF. The local MF direction is well associated with that of CO outflow for IRAS 12 S1 and consistent with that inferred from submillimeter polarimetry. In contrast, the local MF direction runs roughly perpendicular to the Galactic MF direction.

astro-ph.SR↗

arXiv2010

Extended High Circular Polarization in the Orion Massive Star Forming Region: Implications for the Origin of Homochirality in the Solar System

We present a wide-field (~6'x6') and deep near-infrared (Ks band: 2.14 micro m) circular polarization image in the Orion nebula, where massive stars and many low-mass stars are forming. Our results reveal that a high circular polarization region is spatially extended (~0.4 pc) around the massive star-forming region, the BN/KL nebula. However, other regions, including the linearly polarized Orion bar, show no significant circular polarization. Most of the low-mass young stars do not show detectable extended structure in either linear or circular polarization, in contrast to the BN/KL nebula. If our solar system formed in a massive star-forming region and was irradiated by net circularly polarized radiation, then enantiomeric excesses could have been induced, through asymmetric photochemistry, in the parent bodies of the meteorites and subsequently delivered to Earth. These could then have played a role in the development of biological homochirality on Earth.

astro-ph.GA↗

arXiv2008

A Chain of Dark Clouds in Projection Against the Galactic Center

In the J, H, and Ks bands survey of the the Galactic Center region over an area of 2deg x 5deg, we have found many dark clouds, among which a distinguished chain of dark clouds can be identified with a quiescent CO cloud. The distances of the clouds is estimated to be 3.2-4.2 kpc, corresponding to the Norma arm by our new method to determine distance to dark clouds using the cumulative number of stars against J-Ks colors. Adopting these estimated distances, the size is about 70 pc in length and the total mass of the cloud is 6x10^4 M_solar. Three compact HII regions harbor in the cloud, indicating that star forming activities are going on at the cores of the quiescent CO cloud on the spiral arm.

astro-ph↗

arXiv2008

Near-Infrared Imaging Polarimetry of S106 Cluster-Forming Region with SIRPOL

We present the results of wide-field JHKs polarimetry toward the HII region S106 using the IRSF (Infrared Survey Facility) telescope. Our polarimetry data revealed an extended (up to ~ 5') polarized nebula over S106. We confirmed the position of the illuminating source of most of the nebula as consistent with S106 IRS4 through an analysis of polarization vectors. The bright portion of the polarized intensity is consistent with the red wing component of the molecular gas. Diffuse polarized intensity emission is distributed along the north--south molecular gas lanes. We found the interaction region between the radiation from S106 IRS4 and the dense gas. In addition, we also discovered two small polarization nebulae, SIRN1 and SIRN2, associated with a young stellar objects (YSO). Aperture polarimetry of point-like sources in this region was carried out for the first time. The regional magnetic field structures were derived using point-like source aperture polarimetry, and the magnetic field structure position angle around the cluster region in S106 was found to be ~ 120$\arcdeg$. The magnetic fields in the cluster region, however, have three type position angles: ~ 20$\arcdeg$, ~ 80$\arcdeg$, and ~ 120$\arcdeg$. The present magnetic field structures are consistent with results obtained by submillimeter continuum observations. We found that the magnetic field direction in the dense gas region is not consistent with that of the low density gas region.

astro-ph↗

arXiv2008

Magnetic Field Configuration at the Galactic Center Investigated by Wide Field Near-Infrared Polarimetry

We present a polarimetric map of a 20'x20' area toward the Galactic center. The polarization of point sources has been measured in the J, H, and Ks bands using the near-infrared polarimetric camera SIRPOL on the 1.4 m telescope IRSF. One percent or better accuracy of polarization degree is achieved for sources with J<14.5, H<13.5, and Ks<12.0. Comparing the Stokes parameters between high extinction stars and relatively low extinction ones, we have obtained a polarization originating from magnetically aligned dust grains at the central region of our Galaxy of at most 1-2 kpc. The distribution of the position angles shows a peak at about 20 deg, nearly parallel to the Galactic plane, suggesting a toroidal magnetic configuration. The derived direction of the magnetic field is in good agreement with that obtained from far-infrared/submillimeter observations, which detect polarized thermal emission from dust in the molecular clouds at the Galactic center. Our results show that by subtracting foreground components, near-infrared polarimetry allows investigation of the magnetic field structure "at" the Galactic center.

astro-ph↗

arXiv2008

Mapping dust column density in dark clouds by using NIR scattered light : Case of the Lupus 3 dark cloud

We present a method of mapping dust column density in dark clouds by using near-infrared scattered light. Our observations of the Lupus 3 dark cloud indicate that there is a well defined relation between (1) the H-Ks color of an individual star behind the cloud, i.e., dust column density, and (2) the surface brightness of scattered light toward the star in each of the J, H, and Ks bands. In the relation, the surface brightnesses increase at low H-Ks colors, then saturate and decrease with increasing H-Ks. Using a simple one-dimensional radiation transfer model, we derive empirical equations which plausibly represent the observed relationship between the surface brightness and the dust column density. By using the empirical equations, we estimate dust column density of the cloud for any directions toward which even no background stars are seen. We obtain a dust column density map with a pixel scale of 2.3 x 2.3 arcsec^2 and a large dynamic range up to Av = 50 mag. Compared to the previous studies by Juvela et al., this study is the first to use color excess of the background stars for calibration of the empirical relationship and to apply the empirical relationship beyond the point where surface brightness starts to decrease with increasing column density.

astro-ph↗

arXiv2008

Near-Infrared Imaging Polarimetry of M42: Aperture Polarimetry of Point-like Sources

We have conducted aperture polarimetry of ~500 stars of the Orion Nebula Cluster (ONC) in M42 based on our wide-field (~8'\times 8') $JHKs$ band polarimetry. Most of the near-infrared (NIR) polarizations are dichroic, with position angles of polarization agreeing, both globally and locally, with previous far-infrared (FIR) and submillimeter observations, having taken into account the 90$^\circ $ difference in angles between dichroic absorption and emission. This is consistent with the idea that both NIR dichroic polarizations and FIR/submillimeter thermal polarizations trace the magnetic fields in the OMC-1 region. The magnetic fields inferred from these observations show a pinch at scales less than 0.5 pc with a centroid near IRc2. The hourglass-shaped magnetic field pattern is explained by the models in which the magnetic field lines are dragged along with the contracting gas and then wound up by rotation in a disk. The highly polarized region to the northwest of IRc2 and the low-polarized region near the bright bar are also common among NIR and FIR/submillimeter data, although a few regions of discrepancy exist. We have also discerned ~50 possible highly polarized sources whose polarizations are more likely to be intrinsic rather than dichroic. Their polarization efficiencies ($P(H)/A(H)$) are too large to be explained by the interstellar polarization. These include 10 young brown dwarfs that suggest a higher polarization efficiency, which may present geometrical evidence for (unresolved) circumstellar structures around young brown dwarfs.

astro-ph↗

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 Study of the Carina Nebula

We have carried out near-infrared (NIR) imaging observations of the Carina Nebula for an area of ~400 sq. arcmin. including the star clusters Trumpler 14 (Tr 14) and Trumpler 16 (Tr 16). With 10 sigma limiting magnitudes of J ~ 18.5, H ~ 17.5 and K_s ~ 16.5, we identified 544 Class II and 11 Class I young star candidates. We find some 40 previously unknown very red sources with H-K_s > 2, most of which remain undetected at the J band. The red NIR sources are found to be concentrated to the south-east of Tr 16, along the `V' shaped dust lane, where the next generation of stars seems to be forming. In addition, we find indications of ongoing star formation near the three MSX point sources, G287.51-0.49, G287.47-0.54, and G287.63-0.72. A handful of red NIR sources are seen to populate around each of these MSX sources. Apart from this, we identified two hard Chandra X-ray sources near G287.47-0.54, one of which does not have an NIR counterpart and may be associated with a Class I/Class 0 object. The majority of the Class II candidates, on the other hand, are seen to be distributed in the directions of the clusters, demarcating different evolutionary stages in this massive star-forming region. A comparison of the color-magnitude diagrams of the clusters with pre-main sequence model tracks shows that the stellar population of these clusters is very young (< 3 Myr). The K_s band luminosity function (KLF) of Tr 14 shows structure at the faint end, including a sharp peak due to the onset of deuterium burning, implying an age of 1-2 Myr for the cluster. The KLF of Tr 16, in contrast, is found to rise smoothly until it turns over. The slopes of the mass functions derived for the clusters are found to be in agreement with the canonical value of the field star initial mass function derived by Salpeter.

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

A Near-Infrared Study of the Highly-Obscured Active Star-Forming Region W51B

We present wide-field JHKs-band photometric observations of the three compact HII regions G48.9-0.3, G49.0-0.3, and G49.2-0.3 in the active star-forming region W51B. The star clusters inside the three compact HII regions show the excess number of stars in the J-Ks histograms compared with reference fields. While the mean color excess ratio E(J-H)/E(H-Ks) of the three compact HII regions are similar to ~ 2.07, the visual extinctions toward them are somewhat different: ~ 17 mag for G48.9-0.3 and G49.0-0.3; ~ 23 mag for G49.2-0.3. Based on their sizes and brightnesses, we suggest that the age of each compact HII region is =< 2 Myr. The inferred total stellar mass, ~ 1.4 x 10^4 M_sun, of W51B makes it one of the most active star forming regions in the Galaxy with the star formation efficiency of ~ 10 %.

astro-ph↗

arXiv2007

Herbig Ae/Be Stars in the Magellanic Bridge

We have found Herbig Ae/Be star candidates in the western region of the Magellanic Bridge. Using the near infrared camera SIRIUS and the 1.4 m telescope IRSF, we surveyed about 3.0 deg x 1.3 deg (24 deg < RA < 36 deg, -75 deg < Dec. < -73.7 deg) in the J, H, and Ks bands. On the basis of colors and magnitudes, about 200 Herbig Ae/Be star candidates are selected. Considering the contaminations by miscellaneous sources such as foreground stars and early-type dwarfs in the Magellanic Bridge, we estimate that about 80 (about 40%) of the candidates are likely to be Herbig Ae/Be stars. We also found one concentration of the candidates at the young star cluster NGC 796, strongly suggesting the existence of pre-main-sequence (PMS) stars in the Magellanic Bridge. This is the first detection of PMS star candidates in the Magellanic Bridge, and if they are genuine PMS stars, this could be direct evidence of recent star formation. However, the estimate of the number of Herbig Ae/Be stars depends on the fraction of classical Be stars, and thus a more precise determination of the Be star fraction or observations to differentiate between the Herbig Ae/Be stars and classical Be stars are required.

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 Extinction in The Coalsack Globule 2

We have conducted J, H, and Ks imaging observations for the Coalsack Globule 2 with the SIRIUS infrared camera on the IRSF 1.4 m telescope at SAAO, and determined the color excess ratio, E(J-H)/E(H-Ks). The ratio is determined in the same photometric system as our previous study for the rho Oph and Cha clouds without any color transformation; this enables us to directly compare the near-infrared extinction laws among these regions. The current ratio E(J-H)/E(H-Ks) = 1.91 +- 0.01 for the extinction range 0.5 < E(J-H) <1.8 is significantly larger than the ratios for the rho Oph and Cha clouds (E(J-H)/E(H-Ks) = 1.60-1.69). This ratio corresponds to a large negative index alpha = 2.34 +- 0.01 when the wavelength dependence of extinction is approximated by a power law which might indicate little growth of dust grains, or larger abundance of dielectric non-absorbing components such as silicates, or both in this cloud. We also confirm that the color excess ratio for the Coalsack Globule 2 has a trend of increasing with decreasing optical depth, which is the same trend as the rho Oph and Cha clouds have.

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.

astro-ph↗

arXiv2005

Near-Infrared Photometric Monitoring of a Pre-Main-Sequence Object KH 15D

An extensive photometric monitoring of KH 15D, an enigmatic variable in the young star cluster NGC 2264, has been conducted. Simultaneous and accurate near-infrared (JHKs-bands) photometry is presented between 2003 December and 2005 March covering most of the variable phase. The infrared variability is characterized by large-amplitude and long-lasting eclipse, as observed at optical. The period of variability is 48.3 +/- 0.2 days, the maximum photometric amplitude of variability is ~4.2 mag, and the eclipse duration is \~0.5 in phase units. These are consistent with the most recent period, amplitude, and duration at optical. The blueing of the J-H color (~0.16 mag) during the eclipse, which has been suggested before, is unambiguously confirmed; a similar blueing at H-Ks is less clear but is probably present at a similar level. The overall shape of the JHKs light curves is very similar to the optical one, including a fair time-symmetry and a less stable flux during the eclipse with a slight hump near the zero phase. Most of these variability features of KH 15D observed at near-infrared wavelengths can be explained with the recent model employing an eclipse by the inclined, precessing disk and an outer scattering region around a pre-main-sequence binary.

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