Koji Sugitani

INTERSTELLAR EXTINCTION LAW TOWARD THE GALACTIC CENTER III: J, H, KS BANDS IN THE 2MASS AND THE MKO SYSTEMS, AND 3.6, 4.5, 5.8, 8.0 μm IN THE SPITZER/IRAC SYSTEM

We have determined interstellar extinction law toward the Galactic center (GC) at the wavelength from 1.2 to 8.0 μm, using point sources detected in the IRSF/SIRIUS near-infrared (NIR) survey and those in the Two Micron All Sky Survey (2MASS) and Spitzer/IRAC/GLIMPSE II catalogs. The central region l 30 and b 10 has been surveyed in the J, H, and KS bands with the IRSF telescope and the SIRIUS camera whose filters are similar to the Mauna Kea Observatories (MKO) NIR photometric system. Combined with the GLIMPSE II point source catalog, we made KS versus KS – λ color-magnitude diagrams (CMDs) where λ=3.6, 4.5, 5.8, and 8.0 μm. The KS magnitudes of bulge red clump stars and the KS – λ colors of red giant branches are used as a tracer of the reddening vector in the CMDs. From these magnitudes and colors, we have obtained the ratios of total-to-selective extinction for the four IRAC bands. Combined with for the J and H bands derived by Nishiyama et al., we obtain AJ :AH ::A [3.6]:A [4.5]:A [5.8]:A [8.0] = 3.02:1.73:1:0.50:0.39:0.36:0.43 for the line of sight toward the GC. This confirms the flattening of the extinction curve at λ 3 μm from a simple extrapolation of the power-law extinction at shorter wavelengths, in accordance with recent studies. The extinction law in the 2MASS J, H, and KS bands has also been calculated, and good agreement with that in the MKO system is found. Thus, it is established that the extinction in the wavelength range of J, H, and KS is well fitted by a power law of steep decrease A λ ∝ λ–2.0 toward the GC. In nearby molecular clouds and diffuse interstellar medium, the lack of reliable measurements of the total-to-selective extinction ratios hampers unambiguous determination of the extinction law; however, observational results toward these lines of sight cannot be reconciled with a single extinction law.

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 20 and 05 b 10 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/E = 1.44 ± 0.01, A/E = 0.494 ± 0.006, and AH/EJ-H = 1.42 ± 0.02, which are significantly smaller than those obtained in previous studies. From these ratios, we estimate that AJ : AH : A = 1 : 0.573 ± 0.009 : 0.331 ± 0.004 and EJ-H/E = 1.72 ± 0.04, and we find that the power law Aλ ∝ λ-1.99±0.02 is a good approximation over these wavelengths. Moreover, we find a small variation in A/E 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.

A catalog of bright-rimmed clouds with IRAS point sources : candidates for star formation by radiation-driven implosion. I, The northern hemisphere

Forty-four bright-rimmed clouds associated with IRAS sources have been selected from the Palomar Sky Survey prints. They are good candidates for the sites of star formation induced by radiation-driven implosions. Nine of the bright-rimmed clouds are known to be associated with molecular outflows and two (including one with an outflow) with HH objects. Most of their sizes are ≃ 1 pc, similar to those of Bok globules. The luminosities of the associated IRAS sources are relatively large compared to those of the IRAS sources associated with dark globules or dense cores in quark cloud complexes. IRAS luminosity to cloud mass ratios are significantly greater than those in dark globules or in dense cores of dark cloud complexes

The Interstellar Extinction Law toward the Galactic Center. II. V, J, H, and Ks Bands

We have determined the ratios of total to selective extinction directly from observations in the optical V band and near-infrared J band toward the Galactic center. The OGLE (Optical Gravitational Lensing Experiment) Galactic bulge fields have been observed with the SIRIUS camera on the Infrared Survey Facility telescope, and we obtain AV/EV–J = 1.251 ± 0.014 and AJ/EV–J = 0.225 ± 0.007. From these ratios, we derive AJ/AV = 0.188 ± 0.005; combining this with the near-infrared extinction ratios obtained in Paper I for more reddened fields near the Galactic center, we obtain AV:AJ:AH:AKs = 1:0.188:0.108:0.062, which implies steeply declining extinction toward longer wavelengths. In particular, it is striking that the Ks-band extinction is ≈1/16 the visual extinction AV, much smaller than the 1/10 usually employed.

Near-Infrared Imaging Survey of Bok Globules: Density Structure

On the basis of near-infrared imaging observations, we derived the visual extinction (AV) distribution toward 10 Bok globules through measurements of both the color excess (EH-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 10 globules and four globules in the literature, we investigated the stability of globules on the basis of ξmax, which characterizes the Bonnor-Ebert sphere, as well as the stability of the equilibrium state against gravitational collapse. We found that more than half the starless globules are located near the critical state (ξmax = 6.5 ± 2). Thus, we suggest that a nearly critical Bonnor-Ebert sphere characterizes the typical density structure of starless globules. The remaining starless globules show clearly unstable states (ξmax > 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 nonthermal support. It was also found that all the star-forming globules show unstable solutions of ξmax > 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 ξmax for the observed starless globules is consistent with that from model calculations of the collapsing sphere. In addition to the near-infrared observations, we carried out radio molecular line observations (C18O and N2H + ) toward the same 10 globules. We confirmed that most of the globules are dominated by thermal support. The line width of each globule was used to estimate the cloud temperature including the contribution from turbulence, with which we estimated the distance to the globules from the Bonnor-Ebert model fitting.

A Distinct Structure inside the Galactic Bar

We present the result of a near-infrared (JHKs) survey along the Galactic plane, -105 ≤ l ≤ 105 and b = +1°, with the IRSF 1.4 m telescope and the SIRIUS camera. Ks versus H - Ks color-magnitude diagrams reveal a well-defined population of red clump stars whose apparent magnitude peak changes continuously along the Galactic plane, from Ks = 13.4 at l = -10° to Ks = 12.2 at l = 10° after dereddening. This variation can be explained by the barlike structure found in previous studies, but we find an additional inner structure at l 4°, where the longitude-apparent magnitude relation is distinct from the outer bar and where the apparent magnitude peak changes by only ≈0.1 mag over the central 8°. The exact nature of this inner structure is as yet uncertain.

NEAR-INFRARED-IMAGING POLARIMETRY TOWARD SERPENS SOUTH: REVEALING THE IMPORTANCE OF THE MAGNETIC FIELD

The Serpens South embedded cluster, which is located in the constricted part of a long, filamentary, infrared dark cloud, is believed to be in a very early stage of cluster formation. We present results of near-infrared (JHKs) polarization observations of the filamentary cloud. Our polarization measurements of near-infrared point sources indicate a well-ordered global magnetic field that is perpendicular to the main filament, implying that the magnetic field is likely to have controlled the formation of the main filament. On the other hand, the sub-filaments, which converge on the central part of the cluster, tend to run along the magnetic field. The global magnetic field appears to be curved in the southern part of the main filament. Such morphology is consistent with the idea that the global magnetic field is distorted by gravitational contraction along the main filament toward the northern part, which contains larger mass. Applying the Chandrasekhar-Fermi method, the magnetic field strength is roughly estimated to be a few ×100 μG, suggesting that the filamentary cloud is close to magnetically critical.

Star formation in bright-rimmed globules: Evidence for radiation-driven implosion

Molecular outflows have been discovered in three bright-rimmed globules, Ori-I-2, L1206, and rim E in IC 1396, respectively. These outflows are associated with cold IRAS sources without optical counterparts. They are good candidates for recently formed stars. The IRAS sources are separated by only 0.1-0.2 pc from the bright rims. Shock waves preceding ionization fronts (bright rims) can propagate to the positions of the IRAS sources in times as short as several times 10,000 yr. This suggests that star formation was triggered by the ionization/shock fronts surrounding these globules. The data agree with the radiation-driven implosion model for star formation proposed in 1980 by Klein et al. The ratios of luminosity of the associated IRAS sources to globule mass, 3-13 L/M (solar units), are much higher than the values found for isolated dark globules, 0.03-0.3 L/M, indicating that the continuing radiation-driven implosion process may cause formation of more luminous and perhaps more massive stars than within isolated dark globules. 33 refs.

A Deep Near-Infrared Survey of the Chamaeleon I Dark Cloud Core

We have carried out a deep near-infrared imaging survey to search for low-mass young stellar objects (YSOs) in the densest star-forming core of the Chamaeleon I dark cloud. Our observations cover an area of 30 arcmin2, including an early B9 star (HD 97300) and an outflow source (HM 23). The 10 σ limiting magnitudes are 18.1, 17.0, and 16.2 mag at J, H, and K, respectively, which is sensitive enough to provide a census of the embedded stellar population down to substellar objects in the cloud. Source classification is performed based on the near-infrared (NIR) color-color diagram. Many of the YSO candidates with NIR excesses are more than 7 mag fainter than typical T Tauri stars in the same cloud. Some of them are even fainter than the known brown dwarfs in the Pleiades. The luminosities of newly identified YSO candidates and the recent evolutionary models for very low mass objects suggest that they appear to be substellar, if their typical age is assumed to be similar to that of classic T Tauri stars or, namely, 1 Myr with an upper limit of 10 Myr. Therefore it is highly likely that young brown dwarfs form in this molecular cloud core. The J-band luminosity function of the YSO candidates does not appear to turn over down to the completeness limit. In the Chamaeleon I dark cloud core, stars form in a clustered mode characterized by both a high star formation efficiency and high stellar density such as in the ρ Oph core.

Young Star Clusters in Bright-rimmed Clouds: Small-Scale Sequential Star Formation?

A JHK imaging survey has been made for 44 bright-rimmed clouds associated with IRAS point sources detected by Sugitani et al. We have found small clusters of near-infrared sources having young stellar object (YSO) colors in some of these objects; most of the cluster members are considered to be older than the IRAS point sources and to be pre-main-sequence stars such as T Tauri stars. In at least six bright-rimmed clouds, the clusters are elongated toward the bright-rim tip or the exciting star(s) of the bright rim with the IRAS sources situated near the other end. There is a tendency for bluer (i.e., older) stars to be located closer to the exciting star(s) and for redder (i.e., younger) stars to be closer to the IRAS sources. This asymmetric distribution of the cluster members strongly suggests small-scale sequential star formation or propagation of star formation from the side of the exciting star(s) to the IRAS position in a few times 105 yr, as a result of the advance of the shock caused by the UV radiation from the exciting star(s).