Spectroscopic Survey of Hα Emission Line Stars Associated with Bright Rimmed Clouds

The results of a spectroscopic survey of H alpha emission line stars associated with fourteen bright rimmed clouds are presented. Slit-less optical spectroscopy was carried out with the Inter University Centre for Astronomy and Astrophysics (IUCAA) 2m telescope and IUCAA Faint Object Spectrograph and Camera (IFOSC). H alpha emission line was detected from 173 objects. Among them 85 objects have a strong H alpha emission line with its equivalent width larger than 10 A. Those are classical T Tauri stars. 52 objects have a weak H alpha emission line with its equivalent width less than 10 A and do not show intrinsic near-infrared excess. Those are weak-line T Tauri stars. On the other hand, 36 objects have a weak H alpha emission line (<10 A), although they show intrinsic near-infrared excess. Such objects are not common in low-mass star forming regions. Those are misfits of the general concept on formation process of a low-mass star, in which it evolves from a classical T Tauri star to a weak-line T Tauri star. Those might be weak-line T Tauri stars with a flared disk in which gas is heated by ultraviolet radiation from a nearby early-type star. Alternatively, we propose pre-transitional disk objects as their evolutional stage.

slits. IFOSC has a 2048 × 2048 CCD with the field of view of 10.5' × 10.5'. We had spectra centered at 6563 A with a width of 80Å and spectral resolution of ∼ 9Å. We also obtained V -band images. The targets were 14 BRCs listed in [7] and [8] that were observable on the observing date (Table 1). Three frames of 300-s exposure each were obtained for spectroscopy and one frame of 60 s was obtained for imaging. The OB star making the HII region and the bright rim is located in the observing fields of view for BRCs 15, 24, and 25. However, these OB stars are so bright that their spectra were saturated. The general seeing conditions varied between 0.9" and 2.0". The object frames were calibrated with the Image Reduction and Analysis Facility (IRAF). Data were processed in standard manner, namely bias subtraction and flat fielding with the twilight frames. We detected point sources in the V -band image with SExtractor program. The limiting magnitude is approximately 19 mag. Based on the coordinate of the source, a spectrum image of each object was extracted from the spectral frame. The spectrum extends along a line. We average the counts of each line and the average count was then subtracted from each line of the image. In the process, the continuum flux of the object was subtracted and the emission line appeared as a point source. We detected the emission line via the SExtractor program. We also confirmed the emission line by eye inspection. For the image with the emission line, we extracted a 1-D spectrum from the image prior to the continuum subtraction. An equivalent width of the emission line was measured with the IRAF splot task. We did not fit a Gaussian profile. The minimum equivalent width of the detected Hα emission line was 0.3Å.

Results
Emission lines of Hα were identified from 173 objects ( Table 2). Their equivalent widths range from 0.3Å to 132Å. We investigated near-infrared properties of the emission line stars by using 2MASS photometries. The near-infrared color-color diagram of the emission line stars are presented in Figure 1. We defined the line parallel to the reddening vector through an M6 dwarf color as the near-infrared excess border. Among the emission line stars, 77 objects are plotted redward of the border. We identified that such objects have the intrinsic near-infrared excess. The color-color diagram of TTSs in the BRCs is significantly different from that of TTSs in Taurus. TTSs with the equivalent width less than 10Å in Taurus are plotted blueward of the near-infrared excess border (Meyer et al. 1997). On the other hand, such objects in the BRCs are plotted on either side of the near-infrared excess border, like CTTSs in the BRCs and also like CTTSs in Taurus. In the Taurus molecular cloud, WTTSs are older than CTTSs. The ages of the TTSs in the BRCs were estimated on the (I, I − J) color-magnitude diagram with the isochrone of [14]. I-magnitudes of the objects were taken from the USNO-B1.0 catalog. Because the extinction vector is relatively parallel to the isochrone on this diagram, it is possible to roughly estimate the age of the object. It is indicated that majority of the objects have the age between 1 Myr and 10 Myr. We did not find any differences in the ages between the objects with a strong Hα emission line and the objects with a weak Hα emission line.
Masses of the TTSs were estimated with the isochrones of [14], [15], and [16] on the (J, J − H) colormagnitude diagram. It is revealed that most of the objects have mass between 0.5 M ⊙ and 2 M ⊙ .

Discussion
We identified Hα emission line stars in the BRCs. Some objects show intrinsic near-infrared excess and the others do not. We investigated the relationship between the intrinsic near-infrared excess and the Hα equivalent widths (Figure 2). The intersection of the reddening vector originating from the observed JHK colors of the TTSs and the dereddened CTTS line represents the amount of the intrinsic near-infrared excess of the TTS. Zero point of the intrinsic near-infrared excess is defined as the intersection of the near-infrared excess border and the dereddened CTTS line. We also defined the point of unity of the intrinsic near-infrared excess as the reddest intrinsic color of CTTSs (Meyer et al. 1997). The object with the intrinsic near-infrared excess > 0 have an intrinsic near-infrared excess. CTTSs and WTTSs are classified by the Hα equivalent width. We classified the objects into four types. Type 1 object has the Hα emission line with its equivalent width less than 10Å and does not show the intrinsic near-infrared excess. Type 2 object has the Hα emission line with its equivalent width less than 10Å and shows the intrinsic near-infrared excess. Type 3 object has the Hα emission line with its equivalent width larger than 10Å and does not show the intrinsic near-infrared excess. Type 4 object has the Hα emission line with its equivalent width larger than 10Å and shows the intrinsic near-infrared excess. For the Hα emission line stars in the observed BRCs, 30    nor extinction dependencies on the object types.
Hα emission line stars in other star forming regions were also investigated. We used the Hα equivalent widths listed in [25] for the objects in the Taurus molecular cloud, [26] for the Chamaeleon molecular cloud, [27] for the ρ Ophiuchi cloud, [28] and [29] for BRCs, and [30] for the Orion cluster. Near-infrared magnitudes of the Hα emission line stars were taken from 2MASS catalog. Table 3 shows the classification of the Hα emission line stars in the regions. It is revealed that Type 2 objects are abundant in the massive star forming regions such as BRCs compared to those in the low-mass star forming regions such as the Taurus molecular cloud. On the other hand, difference in the spatial distributions of Type 2 objects and the other type objects is not identified for the BRCs observed in this study and IC 1396 cluster.
The general concept that a low-mass star evolves from a CTTS to a WTTS is well established by many observational studies of nearby low-mass star forming regions. In our definition, Type 3 and Type 4 objects correspond to CTTSs and Type 1 objects to WTTSs. Dissipation process of a circumstellar disk around a low-mass star in strong UV field emanating from a nearby OB star has been widely discussed. [  the Arches cluster near the Galactic center. They identified a significant population of near-infrared excess sources. The disk fraction of B-type star was derived as 6 % in the Arches cluster. On the other hand, the fraction was as low as 3% in the vicinity of O-type stars in the cluster core. They concluded that disk dissipation process was more rapid in compact starburst clusters than in moderate star-forming environments. Disk dissipation process due to UV radiation is also examined by numerical simulations. [32] considered circumstellar disk evolution in strong far-UV radiation fields from external stars. It is revealed that the UV radiation from nearby OB stars heats the gas near the disk edge and effectively drives mass loss from circumstellar disks. They also found that the UV radiation photoevaporates disks and disk radii are truncated to less than ∼ 100 AU.
Type 2 objects have a weak Hα emission line but show the intrinsic near-infrared excess. Those are classified into WTTSs from optical spectroscopy, albeit into CTTSs from near-infrared photometry. The general concept of the formation process of a low-mass stars does not involve such objects. We propose two hypotheses for Type 2 objects. The first hypothesis involves the idea that a Type 2 object is a WTTS with a flaring circumstellar disk. The Hα emission line stars are associated with the BRCs. Photons from the nearby OB star ionize hydrogen atoms outside the BRCs and excite hydrogen atoms at the boundary of the BRCs. Type 2 objects may be irradiated by UV photons from the nearby OB star. [33] calculated the structure of a circumstellar disk irradiated by UV radiation emanating from a nearby massive star, based on the circumstellar disk model of [34] and [35]. They indicated that the disk has large scale height, because gas in the disk is heated by the UV radiation then expands. We constructed SEDs of the Hα emission line stars in IC 1396 cluster with the photometric data of Guide Star Catalog, 2MASS catalog, and WISE catalog, then fitted them with the SED model of [36]. Surface height of the circumstellar disk is expressed as, where H 0 is the disk half-thickness, R 0 is the radius of the central star, and β is the flaring parameter [37]. β is deduced to be between 1.00 and 1.20 for all objects. The average β are 1.067 ± 0.008, 1.095 ± 0.015, and 1.106±0.009 for Type 1, 3, and 4 objects, respectively, while that of Type 2 objects is as high as 1.125±0.011. Large β of Type 2 objects supports the idea that those are WTTSs with a highly flared disk, although the difference in β between Type 2 and Type 4 is less than 2 σ significance level. Spatial distribution of Type 2 objects should be inhomogeneous, if this hypothesis is valid. One may guess rich population of Type 2 objects outside the BRC or at the surface of the BRC. However, we do not find such spatial distribution of Type 2 objects. Another hypothesis is that Type 2 objects are pre-transitional disk objects. A transitional disk object has a circumstellar disk with an inner hole created by photo evaporation of a central star or planet formation. A pre-transitional disk object has a small and optically thick disk in the inner hole of the circumstellar disk. If near-infrared excess arises from the inner disk and a small amount of material accretes from the inner disk to the photosphere, then the object is classified into Type 2 object. We investigated Hα strength and nearinfrared excess of five pre-transitional disk objects listed in [38]. Four objects are located in the Type 2 region, or in Type 4 region near the border between Type 2 region and Type 4 region in the near-infrared excess and Hα equivalent width diagram. Similarities between Type 2 objects and pre-transitional disk objects is also found in the near-infrared and WISE colors. We plotted the TTSs in the BRCs, the transitional disk objects in the L 1641 cloud [39], and the known pre-transitional disk objects on the near-infrared and WISE color diagram (Figure 3). Majority of Type 2 objects are not plotted in the transitional disk object region, but their K− [4.6] colors are redder than that of the transitional disk objects. The known pre-transitional disk objects and Type 2 objects in the BRCs are plotted on the similar region in the diagram.
[40] carried out near-infrared polarimetry of BRC 74. They found that the magnetic field in the layer just behind the rim ran along the rim. The estimated magnetic field strength was ∼ 90µG, stronger than that far inside, ∼ 30µG, thereby suggesting that the magnetic field inside the rim is enhanced by the UVradiation-induced shock. A proto-planetary disk has an inner stable region and an outer unstable region, if it magnetizes. [41] indicated that the boundary between stable and unstable regions is located at ∼ 20 AU from the central star and the stable region shrinks in strong magnetic field environment. An abundant population of Type 2 objects in the BRCs may indicate slow evolution from CTTSs to WTTSs. Geometric K. Hosoya, et al. Figure 3: The near-infrared K-band and WISE 4.6µm-, 12µm-, and 22µm-bands color diagram of Type 2 objects, transitional disk objects, and pre-transitional disk objects. The region enclosed by a dashed line is the region of the transitional disk objects. Type 2 objects and pre-transitional disk objects have redder color in K - [4.6] color. structure and evolution timescale of a proto-planetary disk in the close vicinity of the central star under a strong magnetic field is to be investigated.

Conclusions
We have conducted slit-less optical spectroscopy for 14 bright rimmed clouds and found 173 Hα emission line stars. Among them, 36 objects have a weak Hα emission line, but show intrinsic near-infrared excess. Those are identified as WTTSs with optical spectroscopy, but as CTTSs with near-infrared photometry. The general concept of the formation process of a low-mass star does not involve such objects. Those might be weak-line T Tauri stars with a flared circumstellar disk in which gas is heated by ultraviolet radiation from a nearby early-type star. Alternatively, those might be pre-transitional disk objects.