International Journal of Astronomy and Astrophysics, 2019, 9, 154-171
http://www.scirp.org/journal/ijaa
ISSN Online: 2161-4725
ISSN Print: 2161-4717

Spectroscopic Survey of Hα Emission Line Stars
Associated with Bright Rimmed Clouds
Kensuke Hosoya1*, Yoichi Itoh1, Yumiko Oasa2, Ranjan Gupta3, Asoke Kumar Sen4
Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, Kobe, Japan
Faculty of Education, Saitama University, Saitama, Japan
3
Inter University Center for Astronomy and Astrophysics (IUCAA), Pune, India
4
Department of Physics, Assam University, Silchar, India
1
2

How to cite this paper: Hosoya, K., Itoh,Y., Oasa, Y., Gupta, R. and Sen, A.K. (2019)
Spectroscopic Survey of Hα Emission LineStars Associated with Bright Rimmed
Clouds. International Journal of Astronomy and Astrophysics, 9, 154-171.
https://doi.org/10.4236/ijaa.2019.92012
Received: May 12, 2019
Accepted: June 18, 2019
Published: June 21, 2019
Copyright © 2019 by author(s) and
Scientific Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License (CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access

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 andCamera (IFOSC). Hα emission line was detected from 173 objects. Amongthem 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 Hαemission 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 toa weak-line T Tauri star. Those might be weak-line T Tauri stars with a flareddisk 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
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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 Hαemission line, but its equivalent width is less than 10 Å. [2] investigatednear-infrared colors of TTSs in the Taurus molecular cloud. They estimated theamount of the interstellar extinction from E ( R − I ) 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 ( J − H ) − ( H − K )color-color diagram. The range is well fitted by a linear line with an intersection
with the reddening vector from an M6 dwarf at AV = 1.9 mag (hereaftertermed as the dereddened CTTS line). Dereddened colors of approximately halfof 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 theradiation 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 theexcess depends on the optical thickness of the disk, geometric structure of thedisk, 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 informationfor 72 TTSs. They plotted them on the HR diagram and then estimated theirages 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 withthe 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 OBstars ionizes molecular cloud materials through shock and excites hydrogenatoms at the surface of the cloud, making diffuse Hα emission. [7] and [8] presented comprehensive catalogs of BRCs, in which 89 BRCs are listed. Propertiesof the clouds and young stellar objects (YSOs) associated with the clouds havebeen extensively investigated. [9] carried out millimeter mapping observationsof 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
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Hα grism spectroscopy and optical narrow-band imaging observations of 28
BRCs. They detected 460 Hα emission line stars. [11] carried out wide-fieldnear-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 andthose with redder colors were closer to the IRAS source. [13] presented anear-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 themolecular 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 of2011 January and 2012 January with IFOSC (IUCAA Faint Optical Spectrographand 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, theseOB 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 eachobject 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 theemission 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
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Table 1. Properties of the observed bright rimmed clouds.
Cloud

RA
[J2000]DEC[J2000]

HII region

Spectral typeof the exciting starDistance
obs. fields[pc]

BRC 1505 23 30.1

+33 11 54

S 236

O5V, O6V

3600

6

BRC 1605 19 48.9

−05 52 05

S 276

O7V, O8.5II, O9.5I

4002

BRC 1705 31 28.1

+12 06 24

S 264

O5V

450

3

BRC 1805 44 29.8

+09 08 54

S 264

O5V

450

4

BRC 1905 34 30.7

−02 38 12

S 277

O9.5V

400

1

BRC 2105 39 41.3

−03 37 12

S 277

O9.5V

400

1

BRC 2306 22 58.7

+23 09 58

S 249

O9V

1900

1

BRC 2406 34 55.4

+04 25 14

S 275

O5V

1400

9

BRC 2506 41 03.3

+10 21 59

S 273

O7V

913

3

BRC 2607 03 47.2

−11 45 47

S 296

O7III

990

2

BRC 2707 03 58.7

−11 23 19

S 296

B0V

990

2

BRC 2807 04 43.4

−10 21 59

S 296

B0IV

990

2

BRC 2907 04 52.4

−12 09 26

S 296

O7.5V

990

1

BRC 4507 18 23.7

−22 06 13

RCW 14

O8V

1930

2

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 propertiesof 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 thenear-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 lessthan 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 oneither 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 diagramwith 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 theisochrone on this diagram, it is possible to roughly estimate the age of the object.
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Table 2. Hα emission line stars.
ID

RA [J2000]hms

DEC [J2000]
˚ ’ ”

EW(Hα)
Å

J

H

K

mag

mag

mag

Identification

BRC 151

5 23 32.21

33 27 43.6

16.8

14.36

13.29

12.41

[PSM2011] 107

2

5 23 15.81

33 27 43.5

3.8

15.17

14.25

13.96

[PSM2011] 135, [CMP2012] 975

3

5 22 54.07

33 26 33.0

1.2

14.35

13.55

13.25

[PSM2011] 82, [CMP2012] 559

4

5 22 51.19

33 26 32.9

11.4

15.96

15.19

14.59

[MSB2007]37, [PSM2011] 851, [CMP2012] 465

5

5 22 53.82

33 25 41.5

13.9

15.93

14.89

14.12

[MSB2007]38, [PSM2011] 883, [CMP2012] 548

6

5 22 52.30

33 24 7.4

1.7

13.58

12.71

12.25

[PSM2011] 61, [CMP2012] 496

7

5 22 51.90

33 23 59.3

1.1

14.61

14.21

13.65

[PSM2011] 56

8

5 22 51.04

33 25 47.1

2.5

13.08

12.34

11.75

[MSB2007] 41, [PSM2011] 847, [CMP2012] 461

95 22 54.15

33 24 58.0

2.7

15.15

14.46

14.20

[PSM2011] 1168, [CMP2012] 563

10

5 22 45.78

33 28 16.2

60

15.05

14.04

13.54

[PSM2011] 177, [CMP2012] 294

11

5 22 18.44

33 28 21.2

6.3

11.13

11.04

10.98

NGC1893-256, [PSM2011] 1072, [CMP2012] 11

12

5 22 51.04

33 25 47.1

1.1

13.08

12.34

11.75

[MSB2007] 41, [PSM2011] 847, [CMP2012] 461

13

5 22 43.02

33 25 5.4

24.5

11.65

11.31

10.92

NGC1893-35, [MSB2007] 47, [PSM2011] 12

14

5 22 43.78

33 25 25.8

40.1

12.38

10.77

9.42

[MSB2007] 46, [PSM2011] 92, [CMP2012] 241

15

5 22 38.78

33 22 5.4

8.6

14.68

13.70

13.08

[PSM2011] 718, [CMP2012] 151

16

5 22 46.84

33 29 27.9

9.6

14.83

13.9

13.36

[MSB2007] 40, [PSM2011] 131, [CMP2012] 329

17

5 22 49.35

33 29 2.1

7.1

14.99

14.06

13.57

[PSM2011] 112, [CMP2012] 410

18

5 22 49.57

33 30 1.5

28.8

14.42

13.45

12.95

[PSM2011] 137, [CMP2012] 414

19

5 23 9.33

33 30 2.3

35.7

10.79

10.41

10.01

GGA 333, [PSM2011] 520

5 23 6.32

33 31 1.7

6.4

14.74

13.82

13.24

[PSM2011] 97, [CMP2012] 873

21

5 23 4.96

33 31 50.1

1.5

15.14

14.38

13.89

[PSM2011] 105, [CMP2012] 848

22

5 22 52.23

33 29 58.0

6.8

12.68

11.91

11.35

[MSB2007] 34, [PSM2011] 31, [CMP2012] 494

23

5 23 3.32

33 29 25.1

1.7

15.46

14.41

13.72

[MSB2007] 13, [PSM2011] 113, [CMP2012] 806

24

5 23 2.76

33 29 40.2

8.8

15.47

14.60

13.90

[MSB2007] 17, [PSM2011] 175, [CMP2012] 795

25

5 23 0.07

33 30 39.0

21.7

14.34

13.42

12.97

[PSM2011] 99, [CMP2012] 718

26

5 22 58.11

33 30 41.0

9.3

13.84

13.30

12.63

[MSB2007] 27, [PSM2011] 33

27

5 23 13.59

33 29 44.0

5.2

15.34

14.48

13.93

[PSM2011] 144, [CMP2012] 961

28

5 23 1.07

33 29 25.1

30

15.97

14.89

14.19

[MSB2007] 21, [PSM2011] 253, [CMP2012] 743

29

5 23 9.95

33 29 8.8

12.6

15.40

14.11

13.20

[MSB2007] 2, [PSM2011] 140, [CMP2012] 924

19

5 23 9.33

33 30 2.3

35.7

10.79

10.41

10.01

GGA 333, [PSM2011] 520

5 23 6.32

33 31 1.7

6.4

14.74

13.82

13.24

[PSM2011] 97, [CMP2012] 873

21

5 23 4.96

33 31 50.1

1.5

15.14

14.38

13.89

[PSM2011] 105, [CMP2012] 848

22

5 22 52.23

33 29 58.0

6.8

12.68

11.91

11.35

[MSB2007] 34, [PSM2011] 31, [CMP2012] 494

23

5 23 3.32

33 29 25.1

1.7

15.46

14.41

13.72

[MSB2007] 13, [PSM2011] 113, [CMP2012] 806

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Continued
ID

RA [J2000]hms

DEC [J2000]
˚ ’ ”

EW(Hα)
Å

J mag
mag

H mag
mag

K mag
mag

Identification24

5 23 2.76

33 29 40.2

8.8

15.47

14.60

13.90

[MSB2007] 17, [PSM2011] 175, [CMP2012] 795

25

5 23 0.07

33 30 39.0

21.7

14.34

13.42

12.97

[PSM2011] 99, [CMP2012] 718

26

5 22 58.11

33 30 41.0

9.3

13.84

13.30

12.63

[MSB2007] 27, [PSM2011] 33

27

5 23 13.59

33 29 44.0

5.2

15.34

14.48

13.93

[PSM2011] 144, [CMP2012] 961

28

5 23 1.07

33 29 25.1

30

15.97

14.89

14.19

[MSB2007] 21, [PSM2011] 253, [CMP2012] 743

29

5 23 9.95

33 29 8.8

12.6

15.40

14.11

13.20

[MSB2007] 2, [PSM2011] 140, [CMP2012] 924

30

5 23 3.82

33 29 24.2

2

14.80

13.90

13.30

[MSB2007] 12, [PSM2011] 114, [CMP2012] 819

31

5 23 4.05

33 29 48.4

26.3

15.03

13.95

13.15

[MSB2007] 16, [PSM2011] 201, [CMP2012] 825

BRC 171

5 31 23.12

12 10 33.7

6.9

13.17

12.46

12.02

2

5 31 26.94

12 10 20.5

2.7

11.96

11.25

10.89

3

5 31 23.59

12 9 43.9

5.2

10.42

9.62

9.19

HI Ori, HBC 93, [DM99] 143

45 31 28.05

12 9 10.3

16.8

9.41

8.31

7.34

HK Ori, HBC 94

55 31 15.51

12 11 23.7

2.5

12.10

11.21

10.72

[DM99] 136

6

5 31 19.46

12 9 15.3

2.2

11.99

10.78

10.09

7

5 30 51.70

12 8 36.7

26.5

11.64

10.87

10.45

8

5 31 12.49

12 7 54.5

2

13.29

12.52

12.21

9

5 31 21.91

11 54 56.7

2.2

13.60

12.96

12.69

V4480 Ori, HBC 91, [DM99]125

BRC 181

5 44 19.40

9 16 19.3

3.5

13.12

12.39

12.03

2

5 44 22.89

9 10 35.3

2.5

12.45

11.72

11.45

[DM99] 247

3

5 44 23.21

9 12 3.9

1.4

10.00

9.36

8.95

[DM99] 248

4

5 44 37.32

9 11 59.1

3.8

12.95

12.33

12.05

5

5 44 37.02

9 13 20.0

15.4

12.33

11.66

11.33

6

5 44 30.23

9 12 23.8

8.1

14.13

13.36

12.91

7

5 44 17.35

9 10 59.2

17

10.79

10.04

9.62

8

5 44 25.73

9 12 1.3

1.9

13.07

12.35

12.09

9

5 44 10.04

9 8 40.8

43.2

13.85

13.12

12.64

10

5 44 14.86

9 8 8.2

9.4

12.67

11.97

11.67

[DM99] 243

11

5 44 7.26

9 6 38.1

41.9

11.66

10.94

10.68

V629 Ori, [DM99] 240

12

5 44 28.66

9 6 18.6

8.8

14.43

13.81

13.46

13

5 44 8.99

9 9 14.8

3.4

11.10

10.03

9.25

14

5 44 12.77

9 5 10.5

10.5

13.26

12.59

12.24

15

5 43 56.59

9 18 16.9

26.8

12.85

12.04

11.67

16

5 43 31.22

9 17 46.3

10.8

12.88

12.30

11.97

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V630 Ori, [DM99] 244

QR Ori, [DM99] 241[DM99] 238

International Journal of Astronomy and Astrophysics

K. Hosoya et al.
Continued
ID

RA [J2000]hms

DEC [J2000]
˚ ’ ”

EW(Hα)
Å

J mag
mag

H mag
mag

K mag
mag

17

5 43 57.00

9 16 24.0

38.8

12.34

11.57

11.17

18

5 43 50.67

9 12 25.0

10.3

13.33

12.37

11.82

19

5 43 39.59

9 10 40.8

4.9

13.34

12.66

12.35

20

5 43 51.36

9 13 40.6

66.9

14.62

13.98

13.74

21

5 43 32.20

9 9 24.0

7.2

11.29

10.55

10.19

22

5 43 44.50

9 8 1.2

20.8

14.57

12.62

11.09

23

5 43 53.50

9 7 11.8

42

13.09

12.23

11.71

24

5 43 20.92

9 6 7.1

5

10.07

9.13

8.41

25

5 43 52.20

9 6 51.4

1.8

14.45

13.82

13.56

Identification

V626 Ori, [DM99]231

V625 Ori, [DM99]227

BRC 191

5 34 34.24−2 58 16.6

55

12.77

11.67

11.03

Haro 5-83

2

5 34 20.38−2 57 46.9

3.1

13.75

12.27

11.27

V1945 Ori

[WFT2009] RMCX 324

BRC 241

6 34 25.74

4 28 15.8

20.6

14.57

13.55

12.76

2

6 34 41.04

4 27 9.9

15.9

14.82

13.50

12.68

3

6 33 50.40

4 32 55.9

3.5

13.11

12.42

11.98

4

6 34 2.21

4 30 7.6

9.9

14.43

13.49

13.00

5

6 34 17.07

4 27 35.3

22.5

14.72

13.53

12.69

6

6 34 4.22

4 34 34.1

9.9

14.45

13.44

12.86

[WFT2009] RMCX 199

7

6 34 3.41

4 34 8.8

22

13.24

12.36

11.72

[WFT2009] RMCX 195

8

6 33 58.01

4 33 31.3

20.2

14.75

13.71

12.97

[WFT2009] RMCX 179

9

6 33 55.73

4 28 23.4

14.4

15.48

14.41

13.85

10

6 33 54.60

4 29 41.6

6

15.35

13.87

12.88

11

6 34 8.90

4 29 38.5

13.7

15.15

14.13

13.64

12

6 33 45.75

4 31 3.5

25.4

15.24

14.08

13.56

13

6 33 56.09

4 31 19.3

32.1

14.65

13.65

13.11

[WFT2009] RMCX 172

14

6 34 2.85

4 34 51.0

2.4

13.55

12.68

12.20

[WFT2009] RMCX 190

15

6 33 41.16

4 37 5.1

3.8

14.01

12.94

12.17

[WFT2009] RMCX 149

16

6 32 35.75

4 46 30.4

23

14.01

13.10

12.56

[BNM2013] 66.01 15

4

6 34 2.21

4 30 7.6

9.9

14.43

13.49

13.00

[WFT2009] RMCX 189

5

6 34 17.07

4 27 35.3

22.5

14.72

13.53

12.69

6

6 34 4.22

4 34 34.1

9.9

14.45

13.44

12.86

[WFT2009] RMCX 199

7

6 34 3.41

4 34 8.8

22

13.24

12.36

11.72

[WFT2009] RMCX 195

8

6 33 58.01

4 33 31.3

20.2

14.75

13.71

12.97

[WFT2009] RMCX 179

9

6 33 55.73

4 28 23.4

14.4

15.48

14.41

13.85

DOI: 10.4236/ijaa.2019.92012

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K. Hosoya et al.
Continued
ID

RA [J2000]hms

DEC [J2000]
˚ ’ ”

EW(Hα)
Å

J mag
mag

H mag
mag

K mag
mag

10

6 33 54.60

4 29 41.6

6

15.35

13.87

12.88

11

6 34 8.90

4 29 38.5

13.7

15.15

14.13

13.64

12

6 33 45.75

4 31 3.5

25.4

15.24

14.08

13.56

13

6 33 56.09

4 31 19.3

32.1

14.65

13.65

13.11

[WFT2009] RMCX 172

14

6 34 2.85

4 34 51.0

2.4

13.55

12.68

12.20

[WFT2009] RMCX 190

15

6 33 41.16

4 37 5.1

3.8

14.01

12.94

12.17

[WFT2009] RMCX 149

16

6 32 35.75

4 46 30.4

23

14.01

13.10

12.56

[BNM2013] 66.01 15

17

6 32 34.95

4 44 39.2

32.8

10.11

9.90

9.53

NGC2244 279

18

6 32 45.11

4 45 23.1

15.5

13.33

12.29

11.57

[WTF2008] Main 878

19

6 32 31.01

4 50 6.0

1.5

12.07

11.57

11.09

NGC2244 269

20

6 32 31.44

4 42 34.0

0.5

11.74

11.03

10.83

[WTF2008] Main 831

21

6 32 50.74

4 44 47.6

1.3

12.94

12.09

11.70

[WTF2008] Main 898

22

6 32 1.83

4 53 38.6

6.5

14.26

13.32

12.91

[WTF2008] Main 498

23

6 32 7.82

4 52 28.4

18.3

14.03

13.21

12.76

[WTF2008] Main 590

24

6 32 4.66

4 54 51.5

2.8

13.80

12.92

12.54

[WTF2008] Main 541

25

6 32 4.53

4 53 25.0

2.7

13.59

12.71

12.23

[WTF2008] Main 536

26

6 31 41.02

4 54 47.9

8.3

14.18

13.36

12.95

[WTF2008] Main 133

27

6 31 43.85

5 2 57.5

4

13.27

12.38

11.97

[WTF2008] Main 169

28

6 31 49.26

4 57 0.9

5.4

13.7

12.85

12.27

[WTF2008] Main 254

29

6 31 51.65

4 55 5.2

6.7

13.91

13.01

12.45

V547 Mon30

6 31 29.76

4 54 49.1

8.8

11.45

10.89

9.69

GGA 395, NGC2244 74

31

6 31 39.62

4 59 45.1

23.5

14.12

13.13

12.65

[WTF2008] Main 124

32

6 31 39.88

4 56 39.1

17.8

15.05

14.10

13.62

33

6 31 20.87

5 4 8.5

3.8

14.07

13.23

12.71

[BNM2013] 66.03 296

34

6 31 48.31

4 58 20.3

5.8

12.43

11.57

11.08

[WTF2008] Main 243

35

6 31 29.52

4 54 34.2

8.1

14.92

14.11

13.72

[WTF2008] Main 43

36

6 30 42.71

4 55 31.4

1.7

11.23

10.47

9.75

V539 Mon37

6 31 0.55

4 58 7.3

8.4

14.64

13.76

13.18

[WFT2010] 126

38

6 31 30.96

5 6 58.5

28.6

12.88

12.08

11.60

39

6 31 40.00

5 5 56.4

6.3

9.68

8.96

8.27

NGC2244 10640

6 31 2.88

5 3 49.3

8.9

13.74

12.84

12.38

[WFT2010] 134

41

6 31 12.63

5 5 2.6

3.6

14.42

13.30

12.62

Identification[WFT2009] RMCX 233

BRC 251

6 41 1.53

10 14 56.1

20.7

12.93

12.18

11.66

LkHα 462

6 41 7.48

10 15 4.5

3.3

13.24

12.47

12.15

ESO-Hα 493

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K. Hosoya et al.
Continued
ESO-Hα 504

3

6 41 9.85

10 15 2.6

11.1

13.19

11.80

10.85

4

6 41 7.01

10 16 28.9

1.3

12.99

12.32

12.06

5

6 40 59.79

10 2 12.6

15.5

14.66

13.82

13.50

ESO-Hα 454

66 41 2.86

10 7 11.9

4.5

13.98

13.22

12.87

ESO-Hα 468

7

6 41 7.26

9 58 31.2

0.3

12.54

11.87

11.68

V609 Mon8

6 40 49.21

9 57 38.8

12.4

13.57

12.74

12.37

ESO-Hα 415

96 41 17.25

9 54 32.4

25.6

12.83

12.05

11.55

IQ Mon10

6 40 49.75

9 52 58.6

2.7

14.20

13.56

13.33

V343 Mon11

6 40 50.59

9 54 57.3

6.1

12.47

11.69

11.28

LkHα 3112

6 40 52.55

9 52 6.0

2

12.69

11.97

11.72

ESO-Hα 425

13

6 41 12.58

9 52 31.2

9.6

11.49

10.69

10.21

MM Mon14

6 40 59.45

9 59 45.5

4.9

12.78

12.09

11.69

V602 Mon15

6 40 54.88

9 53 12.4

5.5

13.92

13.09

12.40

ESO-Hα 435

16

6 40 46.95

9 52 40.6

42

14.75

14.06

13.41

ESO-Hα 405

17

6 40 54.11

9 52 24.6

28.9

15.34

14.88

14.33

ESO-Hα 432

18

6 40 54.20

9 55 52.0

32

13.70

12.94

12.76

NGC2264 SBL 219

BRC 261

7 3 40.75−11 46 16.6

22.7

15.24

14.06

13.21

2

7 3 48.22−11 41 46.0

5.2

13.61

12.84

12.55

3

7 4 7.18−11 40 32.7

3.9

14.08

13.87

13.73

4

7 3 49.91−11 31 17.0

8.2

14.03

13.20

12.95

5

7 3 54.00−11 32 47.9

5.2

12.34

11.61

11.18

6

7 3 54.03−11 32 37.2

63.1

14.57

13.7

13.29

7

7 3 58.35−11 33 34.6

19.1

14.92

13.93

13.52

8

7 4 1.33−11 36 17.5

26.3

14.77

13.93

13.42

9

7 4 0.68−11 37 49.3

12.8

13.98

13.23

12.93

10

7 3 38.52−11 31 51.9

22.8

14.22

13.28

12.75

11

7 3 43.68−11 36 56.6

33.3

14.89

13.92

13.66

12

7 4 4.70−11 31 15.9

24.5

15.57

14.66

14.38

13

7 3 58.06−11 32 39.9

3.4

14.10

13.23

12.93

14

7 3 50.34−11 32 51.4

16.3

15.81

14.92

14.30

[CPD2009] J070350.34-113251.415

7 4 0.41−11 33 59.6

2.4

12.10

10.92

10.19

[CPD2009] J070400.41-113359.5

16

7 3 56.71−11 35 9.6

23.3

14.34

12.95

12.11

[CPD2009] J070356.71-113509.6

17

7 4 3.94−11 35 57.1

11.6

14.51

13.63

13.21

18

7 4 5.53−11 39 52.4

28.2

15.62

14.72

14.20

19

7 3 48.98−11 37 19.4

20

15.42

14.58

14.24

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K. Hosoya et al.
Continued
ID

RA [J2000]hms

DEC [J2000]
˚ ’ ”

EW(Hα)
Å

J mag

H mag

K mag

mag

mag

mag

Identification

BRC 271

7 4 8.04−11 23 54.7

16.9

13.12

12.44

12.20

[OSP2002] BRC 27 222

7 4 6.70−11 26 8.5

17

10.76

10.02

9.12

LkHα 2203

7 4 9.96−11 23 16.4

18

11.79

10.71

9.83

[OSP2002] BRC 27 254

7 3 53.04−11 29 35.3

10.9

12.36

11.54

10.95

LkHα 2195

7 3 54.03−11 32 37.2

88.3

14.57

13.7

13.29

6

7 3 49.9−11 31 17.0

10.1

14.03

13.20

12.95

7

7 3 45.76−11 23 14.9

5.8

13.59

12.59

12.19

8

7 4 4.27−11 23 55.7

100

15.01

14.02

13.54

[OSP2002] BRC 27 129

7 4 5.93−11 23 58.7

6.1

14.42

13.47

12.93

[OSP2002] BRC 27 1610

7 4 12.93−11 24 3.2

5.9

15.37

14.39

13.91

[OSP2002] BRC 27 2811

7 3 47.58−11 22 33.9

3.5

13.93

13.15

12.85

12

7 3 57.05−11 30 16.9

131.8

15.30

14.63

14.30

13

7 4 22.75−11 27 50.9

12.8

14.24

13.36

12.95

14

7 4 20.87−11 29 36.2

18.6

13.80

12.88

12.29

15

7 4 17.79−11 28 21.2

23

14.50

13.75

13.3

16

7 4 45.00−11 27 58.9

28

14.86

14.16

13.80

17

7 4 23.25−11 24 17.3

5.8

13.45

12.64

12.19

18

7 4 16.80−11 24 32.4

46.2

14.12

13.21

12.60

19

7 4 40.79−11 25 37.0

7

14.38

13.57

13.26

20

7 4 13.53−11 24 55.8

15.3

12.19

11.29

10.78

[OSP2002] BRC 27 29

21

7 4 26.26−11 31 20.7

17.8

12.47

11.60

11.15

LkHα 222[CPD2009] J070420.87-112936.2[CPD2009] 112

BRC 451

7 18 36.26−22 7 14.6

9.7

15.71

14.87

14.67

2

7 18 22.93−22 4 14.5

10.5

13.3

12.45

11.84

3

7 18 28.46−22 5 15

31.5

15.07

14.03

13.36

4

7 18 39.85−22 0 42.2

10.3

15.50

14.73

14.51

5

7 19 2.80−22 1 4.6

38.6

13.80

12.97

12.24

6

7 18 56.73−22 1 5.3

12.8

14.89

14.09

13.55

7

7 18 57.67−22 1 36.4

31.1

14.66

13.47

12.83

[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 astrong 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]DOI: 10.4236/ijaa.2019.92012

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K. Hosoya et al.

Figure 1. Near-infrared color-color diagram of the Hα emission line stars in the BRCs. A
certain portion of stars with a weak Hα emission line show intrinsic near-infrared excess.

on the ( J , J − H ) color-magnitude diagram. It is revealed that most of the objects have mass between 0.5 M and 2 M.

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 (Figure2). 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-infraredexcess is defined as the intersection of the near-infrared excess border and thedereddened 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-infraredexcess. 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-infraredexcess. 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
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K. Hosoya et al.

Figure 2. The intrinsic near-infrared excess and the equivalent widths of the Hα emission lineof the TTSs associated with the BRCs. Type 3 objects and Type 4 objects are identified as
CTTSs and Type 1 objects as WTTSs. Type 2 objects are newly identified with this study.

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 findmass 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-infraredmagnitudes 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 suchas 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.
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K. Hosoya et al.
Table 3. Classification of the Hα emission line stars.
RegionType 1

Type 2Type 3

Type 4

BRC (this study)

52 (30%)36 (21%)44 (25%)41 (24%)

BRC (Ikeda et al.)177 (36%)108 (22%)98 (20%)108 (22%)

IC 1396 (Nakano et al.)236 (37%)45 (7%)

249 (39%)109 (17%)Orion

167 (31%)

16 (3%)

221 (41%)135 (25%)

Taurus53 (42%)5 (4%)

25 (20%)42 (34%)

Chamaeleon

44 (63%)

3 (4%)

14 (18%)12 (15%)ρ Oph

185 (81%)

7 (3%)

21 (9%)

16 (7%)The general concept that a low-mass star evolves from a CTTS to a WTTS iswell established by many observational studies of nearby low-mass star forming
regions. In our definition, Type 3 and Type 4 objects correspond to CTTSs andType 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 numericalsimulations. [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 fromcircumstellar 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-infraredexcess. 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 areassociated with the BRCs. Photons from the nearby OB star ionize hydrogenatoms 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 thedisk is heated by the UV radiation then expands. We constructed SEDs of theHα emission line stars in IC 1396 cluster with the photometric data of GuideStar Catalog, 2MASS catalog, and WISE catalog, then fitted them with the SED
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K. Hosoya et al.

model of [36]. Surface height of the circumstellar disk is expressed as,β r H (r ) = H0   , R0 

(1)

where H 0 is the disk half-thickness, R0 is the radius of the central star, andβ 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 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 byphoto 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 ofmaterial 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 theType 2 region, or in Type 4 region near the border between Type 2 region andType 4 region in the near-infrared excess and Hα equivalent width diagram. Similarities between Type 2 objects and pre-transitional disk objects is also foundin the near-infrared and WISE colors. We plotted the TTSs in the BRCs, thetransitional 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 theirK- [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 thesimilar region in the diagram.
[40] carried out near-infrared polarimetry of BRC 74. They found that themagnetic field in the layer just behind the rim ran along the rim. The estimatedmagnetic field strength was ~90 μG, stronger than that far inside, ~30 μG, thereby suggesting that the magnetic field inside the rim is enhanced by theUV-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 andthe 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 inthe close vicinity of the central star under a strong magnetic field is to be investigated.
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K. Hosoya et al.

Figure 3. The near-infrared K-band and WISE 4.6 μm-, 12 μm-, and 22 μm-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.

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 Hαemission 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.
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