ABSTRACT

The results of a spectroscopic survey of Hα 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) 2 m telescope and IUCAA Faint Object Spectrograph and Camera (IFOSC). Hα emission line was detected from 173 objects. Among them 85 objects have a strong Hα emission line with its equivalent width larger than 10 Å. Those are classical T Tauri stars. 52 objects have a weak Hα emission line with its equivalent width less than 10 Å and do not show intrinsic near-infrared excess. Those are weak-line T Tauri stars. On the other hand, 36 objects have a weak Hα emission line (<10 Å), 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.

Keywords:

Star Formation, Pre-Main Sequence Stars, T Tauri Stars

1. Introduction

A T Tauri star (TTS) is a low-mass star in the pre-main sequence stage. It was first identified as a variable star [1] . Successive studies on low-mass star forming regions, such as the Taurus molecular cloud, revealed two types of TTSs, namely classical TTSs (CTTSs) and weak-line TTSs (WTTSs). CTTSs show the Hα emission line with its equivalent width larger than 10 Å. WTTSs also show the Hα emission line, but its equivalent width is less than 10 Å. [2] investigated near-infrared colors of TTSs in the Taurus molecular cloud. They estimated the amount of the interstellar extinction from $E\left(R-I\right)$ color and then calculated the dereddened colors of the TTSs. They noted that the WTTSs have the dereddened colors similar to those of normal main sequence stars. On the other hand, the dereddened colors of CTTSs occupy a narrow range in a $\left(J-H\right)-\left(H-K\right)$ color-color diagram. The range is well fitted by a linear line with an intersection with the reddening vector from an M6 dwarf at $A_{\text{V}}=1.9$ mag (hereafter termed as the dereddened CTTS line). Dereddened colors of approximately half of the CTTSs are redder than that intersection point. Such CTTSs have an intrinsic near-infrared excess. Those are brighter in the H- and/or K band than the radiation expected only from the photosphere. The intrinsic near-infrared excess of a TTS is caused by an optically thick circumstellar disk, and the amount of the excess depends on the optical thickness of the disk, geometric structure of the disk, and viewing angle of the disk [3] [4] . Approximately half of CTTSs show the intrinsic near-infrared excess while the other CTTSs do not. On the other hand, WTTSs do not show the intrinsic near-infrared excess.

A low-mass star evolves from a protostar to a CTTS and subsequently a WTTS. [5] investigated the ages of TTSs in the Taurus molecular cloud. They used Hipparcos parallaxes as well as photometric and spectroscopic information for 72 TTSs. They plotted them on the HR diagram and then estimated their ages by comparing them to a pre-main sequence evolutionary track. It is revealed that the ages of the CTTSs are younger than those of the WTTSs. A solar mass star evolves from a CTTS to a WTTS at the age of 4 × 10⁶ yr. [6] deduced the surface gravity of TTSs in the Taurus molecular cloud from high resolution spectroscopy and then estimated their ages by plotting them on the HR diagram. The fraction of the objects with the intrinsic near-infrared excess decreases with the age. They concluded that the dissipation timescale of the circumstellar disk is 3 - 4 × 10⁶ yr. The general concept that a low-mass star evolves from a CTTS to a WTTS is well established by observational studies of nearby low-mass star forming regions.

Bright rimmed clouds (BRCs) are molecular clouds harboring an IRAS source and are located at periphery of an HII region. Ultra-violet radiation from OB stars ionizes molecular cloud materials through shock and excites hydrogen atoms at the surface of the cloud, making diffuse Hα emission. [7] and [8] presented comprehensive catalogs of BRCs, in which 89 BRCs are listed. Properties of the clouds and young stellar objects (YSOs) associated with the clouds have been extensively investigated. [9] carried out millimeter mapping observations of molecular clouds associated with the W5 HII region. They noticed that molecular clouds facing the HII region show a steep density gradient toward the HII region due to the compression of the HII region to the clouds. [10] conducted Hα grism spectroscopy and optical narrow-band imaging observations of 28 BRCs. They detected 460 Hα emission line stars. [11] carried out wide-field near-infrared imaging observations of 32 BRCs. They identified 2099 objects as YSO candidates. These observational studies confirmed the formation of low-mass stars in BRCs. [12] carried out a near-infrared survey of 44 BRCs. They indicated that stars with bluer colors are located closer to the OB star and those with redder colors were closer to the IRAS source. [13] presented a near-infrared image of BRC 14. They revealed that the fraction of YSO candidates to all sources, the extinction of all sources, and the near-infrared excess of the YSO candidates increased from the outside of the rim to the center of the molecular cloud. These results indicated that star formation proceeds from outside to the center of the cloud.

We conducted slit-less optical spectroscopy of 14 BRCs to identify Hα emission line stars. We discuss evolution process of a low-mass star in a massive-star forming region based on the relationship between the Hα emission line and its intrinsic near-infrared excess.

2. Observations and Data Reduction

Spectroscopic survey of Hα emission line stars were carried out on six nights of 2011 January and 2012 January with IFOSC (IUCAA Faint Optical Spectrograph and Camera) mounted on the IUCAA 2-m telescope at Giravali near Pune, India. We used the IFOSC5 grism and the wide Hα filter, and we did not use any slits. IFOSC has a 2048 × 2048 CCD with the field of view of 10.5’ × 10.5’. We had spectra centered at 6563 Å with a width of 80 Å and spectral resolution of ~ 9 Å. 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 0.3 Å.

3. Results

Emission lines of Hα were identified from 173 objects (Table 2). Their equivalent widths range from 0.3 Å to 132 Å. 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 10 Å 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 $\left(I,I-J\right)$ 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.

[CDP2009]: [17] ; [CMP2012]: [18] ; [DM99]: [19] ; [BNM2013]: [20] ; [MSB2007]: [21] ; [OSP2002]: [10] ; [PSM2011]: [22] ; [WFT2009]: [23] ; [WTF2008]: [24] .

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 $\left(J,J-H\right)$ color-magnitude diagram. It is revealed that most of the objects have mass between 0.5 M_e and 2 M_e.

4. 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 has 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 10 Å and does not show the intrinsic near-infrared excess. Type 2 object has the Hα emission line with its equivalent width less than 10 Å and shows the intrinsic near-infrared excess. Type 3 object has the Hα

emission line with its equivalent width larger than 10 Å and does not show the intrinsic near-infrared excess. Type 4 object has the Hα emission line with its equivalent width larger than 10 Å and shows the intrinsic near-infrared excess. For the Hα emission line stars in the observed BRCs, 30% stars are classified into Type 1, 21% into Type 2, 25% into Type 3, and 24% into Type 4. We do not find mass 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. [31] observed 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 star 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,

$H\left(r\right)=H_0{\left(\frac{r}{R_0}\right)}^{\beta },$ (1)

where $H_0$ is the disk half-thickness, $R_0$ is the radius of the central star, and $\beta$ is the flaring parameter [37] . $\beta$ is deduced to be between 1.00 and 1.20 for all objects. The average $\beta$ are $1.067\pm 0.008$ , $1.095\pm 0.015$ , and $1.106\pm 0.009$ for Type 1, 3, and 4 objects, respectively, while that of Type 2 objects is as high as $1.125\pm 0.011$ . Large $\beta$ of Type 2 objects supports the idea that those are WTTSs with a highly flared disk, although the difference in $\beta$ between Type 2 and Type 4 is less than 2σ 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 near-infrared 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 ~90 μG, stronger than that far inside, ~30 μG, thereby suggesting that the magnetic field inside the rim is enhanced by the UV-radiation-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 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.

5. Conclusion

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.

Acknowledgements

We thank the telescope staff members and operators at the IUCAA 2 m Telescope. This study was partly supported by the JSPS-DST collaboration.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Hosoya, K., Itoh, Y., Oasa, Y., Gupta, R. and Sen, A.K. (2019) Spectroscopic Survey of Hα Emission Line Stars Associated with Bright Rimmed Clouds. International Journal of Astronomy and Astrophysics, 9, 154-171. https://doi.org/10.4236/ijaa.2019.92012

References