Interstellar Extinction in the Area of the Serpens Cauda Molecular Cloud

Baltic Astronomy, vol. 5, 125-147, 1996.

INTERSTELLAR EXTINCTION IN THE AREA OF THE SERPENS CAUDA MOLECULAR CLOUD

V. Straizys1, K. Cernis1 and S. Bartasiute1'2 1 Institute of Theoretical Physics and Astronomy, Gostauto 12, Vilnius 2600, Lithuania 2 Astronomical Observatory of the Vilnius University, Ciurlionio 29, Vilnius 2009, Lithuania Received August 1, 1995.

Abstract. Vilnius seven-color photometry has been obtained for 105 stars down to 12.9 mag in the area of the Serpens Cauda dark cloud. Photometric spectral classes, luminosity classes, absolute magnitudes, interstellar reddening, extinctions and distances have been determined. 23 heavily reddened stars with Ay > 1.0 mag were found; 18 of them are embedded in the dark cloud, others are background objects. The "extinction versus distance" diagram (with a correc- tion to the distance modulus of the Hyades of 3.3 mag) shows that the dark cloud is at a distance of 259 pc. Consequently, we confirm the small distance to the cloud found by the Mexican astronomers in 1988-1991. Some stars are found to have peculiar spectral energy distributions and should be investigated by a spectroscopic method. Key words: methods: observational - techniques: photometric: Vilnius photometric system - stars: fundamental parameters - interstellar medium: dust, extinction

1. INTRODUCTION

A large concentration of dark clouds extends through the Aquila, Serpens Cauda and Ophiuchus constellations. One of the darkest clouds in it is near the Serpens Cauda and Ophiuchus border line at the celestial equator. On the Palomar Sky Survey E copies (Fig. 1) the Serpens Cauda cloud appears as a very dark dust lane having a 126 V. Straizys, K. Cemis and S. Bartasiutè width of about 1/4 deg and extending from north to south between right ascensions 18h29m.O and 18h30m.5 and declinations +1°.6 and —0°.5 (in this paper we use the coordinates for 2000.0). From the west the lane has a more or less regular border with an area of higher surface density of stars. However, from the east several dark protubérants extending up to 18h34m are seen. On O-copies of the Palomar Sky Survey, as well as in the Lick Observatory Sky Atlas (1959) (Fig. 2), the background star density is less variable and the whole investigated area seems to be obscured more uniformly. The Serpens Cauda group of dust clouds may be also approx- imated as a sphere of the diameter of ~ 2° with several relatively transparent windows inside it. The northern part of the group is occupied by a molecular cloud, which is one of the most active star forming regions in the solar vicinity. The core of the molecular cloud is at lâ^O111 and +1°14'.5. Investigations of the Serpens molecular cloud recently have been described by Eiroa (1991). The cloud contains some far infrared sources (Nordh et al. 1982, Harvey et al. 1984, Zhang et al. 1988a,b, Rodriguez et al. 1989, Eiroa & Casali 1989), many near infrared sources (Strom et al. 1974, 1976, Churchwell & Koornneef 1986, Eiroa et al. 1987, Eiroa k Leinert 1987, Gomez de Castro et al. 1988, de Lara et al. 1991, Eiroa k Casali 1989, 1992), several small reflection and emission nebulae and HH objects (Sharpless 1959, Dorschner k Gurtler 1963, Worden k Grasdalen 1974, Strom et al. 1974, Hartigan k Lada 1985, Warren-Smith et al. 1987, Chavarria et al. 1988, Gomez de Castro et al. 1988, Reipurth k Eiroa 1992). Most of these objects are associated with pre-main-sequence stars. The molecular cloud has been observed in the lines of CO, H2CO, NH3 and H20 by Blair et al. (1975, 1978), Rodriguez et al. (1978, 1980, 1989), Loren et al. (1979, 1981), Loren k Wootten (1980), Dinger k Dickinson (1980), Ho k Barrett (1980), Little et al. (1980), Loren (1981), Bally k Lada (1983), Ungerechts k Gusten (1984), Snell k Bally (1986), Takano (1986), Clark k Turner (1987), Mirabel et al. (1987), Palla & Giovanardi (1989), Torrelles et al. (1992), Felli et al. (1992) and White et al. (1995) and in the far infrared and submm by Nordh et al. (1982), Harvey et al. (1984), Zhang et al. (1988a,b), Clark (1991) and Casali et al. (1993). The molecular cloud also contains some Ha emission stars and the Herbig Ae-type variable, VV Ser. The most massive stars in the star forming region are of spectral class B3. A polarimetric study in the area has been done by Voshchinnikov & Marchenko Interstellar extinction in the Serpens Cauda cloud 127

Fig. 1. The investigated area shown on a copy of the Palomar Sky Survey E map. 128 V. Straizys, K. Ôernis and S. Bartasiutè

' . i.- ••;. Ñ," ' ••• VrA-r-ÄT** Interstellar extinction in the Serpens Cauda cloud 129

(1982), King et al. (1983), Pavlova et al. (1984), Warren-Smith et al. (1987), Gomez de Castro et al. (1988) and Kardopolov et al. (1991). However, the area is not investigated photometrically. Only some stars have been observed in the UBV system. The heavily reddened and pre-main-sequence objects have been observed in the near infrared JHKLM passbands as well as by the IRAS satellite far infrared passbands. Spectral energy distributions for nine pre-main- sequence stars were investigated by Cohen & Kuhi (1979). According to White et al. (1995), the lower mass limit of the molecular cloud is 1450 M®. The mass of stars associated with the cloud has been estimated by Eiroa Sz Casali (1992) to be ~ 37M®. This leads to an upper limit of the star forming efficiency of ~ 2 — 5 per cent which is comparable with that estimated in other dark cloud complexes. Despite numerous investigations of the Serpens Cauda star forming region, its distance is poorly known. The first estimate of its distance was made by Strom et al. (1974) who considered the star HD 170634 to be associated with the reflection nebulosity S68. Taking its spectral type as AO (V:), V = 9.2 mag and EB-V w 0.3 mag, they obtained a distance of 436 pc. This crude distance estimate has been in use for many years. According to our results, obtained in the present paper, Strom et al. used much too small interstellar reddening to the star. The next distance estimation was made by Chavarria et al. (1987, 1988) using uvby/3 and JHKLM photometry and MK spectral classes of four early-type stars physically related with the dust cloud. These stars were: HD 170634 (Al V), HD 170739 (B4 V), HD 170784 (B3 V) and HD 171491 (B3 V). Their physical relation with the dark cloud is made evident by the reflection nebulae seen around these stars: the S68 or DG 152 nebula around HD 170634, DG 153 around HD 170739 and HD 170784 and an IRAS infrared nebula around HD 171491 (Zhang et al. 1988a). The authors have found a larger than normal total-to-selective interstellar extinction ratio RBV — 3.7 ± 0.3 and an average distance to these four stars of (245 ± 30) pc. In the same year, Zhang et al. (1988a) revised the distance determination of HD 170634, HD 170739 and HD 170784, taking for them spectral types B8/9 III, B5 II/III and B2/3 III respectively, estimated by N. Houk (unpublished). Accepting the ratio R = 3.1, Zhang et al. have obtained distances of 550 pc, 820 pc and 950 pc for 130 V. Straizys, K. Cernis and S. Bartasiute the three stars, the mean value being (750±200) pc. If the anomalous value of RBV — 3.7 is taken, the mean distance is (650 ± 180) pc. Later on, de Lara & Chavarria (1989) and de Lara et al. (1991) reclassified the spectra of these stars and confirmed their earlier results that all the stars belong to the main sequence. Excluding HD 170634 and adding two new stars, BD-2°4607 and Chavarria 7, the authors have obtained a mean distance to the five stars to be (311 ± 38) pc. A mean value of RBV = 3.3 ± 0.3 has been found using the JHKLM photometry of eight heavily reddened stars. To solve the distance problem, we decided to obtain observations in the Vilnius photometric system of stars investigated in the mentioned papers as well as of other stars in the Serpens Cauda molecular cloud area. The Vilnius photometric system allows one to determine spectral classes and absolute magnitudes of stars of all spectral types photometrically, and this is a completely independent method of their distance determination.

2. OBSERVATIONS

Photometry of stars in the Vilnius system was obtained in 1994 and 1995 with the 1 meter telescope at the Maidanak Observatory in Uzbekistan. Some of the stars were observed in 1995 with the 1.65 meter telescope of the Moletai Observatory in Lithuania. In an area of about 6.5 square degrees, centered on HD 170634, most stars down to V = 12 mag have been measured. Near the center of the molecular cloud the limiting magnitude is about 12.9 mag. The standard star was HD 165401 (BD+4°3589) of spectral type Gl V and V = 6.79 mag from the Aquila Standard Region (Zdanavicius et al. 1969). The identification chart for the area is shown in Fig. 3. The results of the photometry are given in Table 1. Thirteen stars num- bered in Fig. 3 have not been observed and they are not present in Table 1. The usual accuracy of magnitudes and color indices is of the order of ±0.01 mag. The values, for which the rms error is 0.02- 0.03 mag, are marked by a colon. For some stars the errors of the ultraviolet color indices were found to be > 0.03 mag - these values do not appear in the table. Some of them may be variables. Interstellar extinction in the Serpens Cauda cloud 131

23« .

. . • ' • • -92 . . 66 35 113- 106 96 '.-94 77" >3* ' -M ' * • 104 '"M • 85 64- xjg -31 •«•MJ.-«- 36-. 7 • • 100 81— 57' ,46 • 112 97 88"83'' 76 3l 30 20 8 • 3 -1- 114 103 49- I .••110 70 !'75 .33-: » 4 ' .. \'74 60 • / 22 13— 118 28 • 2 • 109 90. \ 47 41 72-, i M [ 7 • 1 " 116 • /* 108 87- 70 .40 •' rl7 I •99 78 48' «38 19- li)5 65 43 15 12- V» ur 3f 14—' 89- * 82 58. -44 • / / • 11" 107 84 - i —53 . 24 IS • • 9 I . 37 : 115 102^, 95 63 " ' • oaV' I* 93 71 • " 42 4 -21- 101. 80- 68 50-« * 45 26 29.. 16 ' 39- 27 25 / »117

Fig. 3. Identification chart of the investigated area. A copy from Chavarria et al. (1988).

3. DETERMINATION OF SPECTRAL CLASSES, ABSOLUTE MAGNITUDES, COLOR EXCESSES, EXTINCTIONS AND DISTANCES

The spectral classes and absolute magnitudes of these stars were determined by the same three methods described by Straizys et al. (1993): 1. The reddening-free diagrams QUPY, QPYV', QUPY, QXYV; QUXY, QUPYV] QUPY, Qxzs and Qxzs, QXYZ calibrated in terms of spectral class and absolute magnitude by Straizys et al. (1982); 132 V. Straizys, K. Cernis and S. Bartasiute

Table 1. Results of photometry

No. BD RA(2000) DEC(2000) V U-V P-V X-V Y-V Z-V V-S n h m s oi

1* +1°3665 18 25 17 + 1 14.9 9.42 2.36 1.83 1.22 0.51 0.19 0.48 2 2. +1°3666 18 25 21 + 1 20.8 9.56 3.92 3.27 2.29 0.93 0.34 0.89 2 3. +1°3667 18 25 35 + 1 35.3 9.97 2.20 1.66 1.02 0.43 0.16 0.41 2 4* +1°3668 18 25 48 + 1 21.3 9.20 3.93 3.30 2.31 0.93 0.34 0.89 3 5. +1°3669 18 25 50 + 2 01.3 9.05 3.71 1.39 0.58 1.33 2 6. +0°3930 18 25 58 + 0 52.6 10.55 2.71 1.98 1.22 0.56 0.22 0.50 1 7.* +1°3670 18 26 00 + 1 41.9 8.93 2.23 1.70 1.10 0.46 0.17 0.46 2 8. +1°3671 18 26 02 + 1 35.3 9.01 4.01: 1.53 0.63 1.45 2 9.* +0°3931 18 26 03 + 0 46.8 6.83 2.33 1.67 0.88 0.34 0.14 0.31 1 10. 18 26 07 + 1 00.9 11.47 2.62 2.11 1.46 0.64 0.23 0.59 2 11. 18 26 10 + 0 54.8 11.77 3.21: 2.80: 1.91 0.71 0.36 0.74 2 12. 18 26 17 + 1 02.7 10.65 2.66: 2.06 1.28 0.56 0.19 0.49 2 13.* +1°3672 18 26 49 + 1 22.6 10.34 2.76 2.13 1.26 0.63 0.20 0.52 2 14. 18 27 08 + 0 57.8 10.96 3.48: 2.63: 1.62 0.84 0.27 0.72 2 15. 18 27 09 + 0 58.9 10.73 2.26 1.78 1.20 0.51 0.19 0.49 2 16.* +0°3936 18 27 11 + 0 11.4 5.18 2.60 1.98 1.14 0.50 0.20 0.52 1 17. 18 27 13 + 1 07.1 9.03 2.26 1.75 1.12 0.46 0.16 0.45 2 19. 18 27 27 + 1 04.2 10.27 2.29 1.81 1.22 0.52 0.19 0.51 2 20* +1°3678 18 27 29 + 1 31.1 8.82 2.23 1.67 0.98 0.40 0.15 0.38 2 21. +0°3939 18 27 33 + 0 25.9 11.00 2.73 2.30 1.55 0.63 0.25 0.61 1 22. +1°3679 18 27 36 + 1 26.9 10.15 2.40 1.82 1.15 0.49 0.17 0.48 2 24. 18 27 45 + 0 53.3 11.97 2.67: 2.24: 1.56 0.63 0.24 0.65 2 26. 18 27 48 + 0 17.1 11.38 2.35 1.86 1.24 0.56 0.21 0.56 1 27* +0°3940 18 27 50 + 0 12.0 7.74 2.12 1.46 0.62 0.24 0.10 0.19 2 28. +1°3680 18 27 53 + 1 23.9 10.55 4.02: 3.57: 2.39: 0.84 0.50 0.87 2 29. +0°3942 18 27 55 + 0 15.8 10.75 2.59 1.95 1.18 0.54 0.19 0.51 1 30. +1°3681 18 27 57 + 1 29.3 10.79 3.24: 2.84 1.91 0.70 0.34 0.72 2 31. 18 28 06 + 1 47.9 12.53 2.81: 2.19: 1.55 0.70 0.25 0.68 2 32. 18 28 08 + 1 31.5 11.22 2.36 1.88 1.29 0.55 0.20 0.54 2 33. +1°3682 18 28 09 + 1 27.8 9.91 4.42: 3.74 2.53 0.95 0.42 0.89 2 34. 18 28 11 + 0 53.7 11.77 3.46: 2.99: 2.01 0.74 0.38 0.72 2 35. +1°3683 18 28 12 + 2 00.5 8.81 3.81: 1.43 0.58 1.41 2 36. +1°3684 18 28 29 + 1 45.3 10.58 2.38 1.89 1.24 0.54 0.21 0.53 2 37* +0°3943 18 28 35 + 0 47.8 8.05 2.61 2.13 1.45 0.57 0.22 0.59 1 38. 18 28 42 + 1 04.3 11.19 2.84 2.33 1.63 0.64 0.27 0.66 2 39* VV Ser 18 28 48 + 0 08.7 11.71 3.33 2.52 1.66 0.84 0.31 1.06 1 11.84 3.39 2.55 1.66 0.83 0.26 1.17 1 11.91 3.41 2.59 1.72 0.87 0.28 1.14 1 11.91 3.38 2.55 1.69 0.80 0.24 1.17 1 40. 18 28 56 + 1 06.5 11.34 2.94 2.40 1.67 0.71 0.27 0.68 2 Interstellar extinction in the Serpens Cauda cloud 133

Table 1 ( continued)

No. BD RA(2000) DEC(2000) V U-V P-V X-V Y-V Z-V V-S n h m s oi

41. 18 28 58 + 1 11.0 11.16 2.98 2.35 1.55 0.69 0.23 0.68 2 42.* +0°3945 18 29 05 + 0 26.4 8.95 3.24 2.77 1.87 0.68 0.33 0.72 2 43. 18 29 08 + 1 05.4 11.64 2.78 2.24 1.54 0.67 0.23 0.65 2 44. 18 29 14 + 0 53.0 10.16 2.74 2.14 1.48 0.65 0.23 0.65 2 45* +0°3947 18 29 16 + 0 19.4 7.96 3.89 3.26 2.23 0.86 0.34 0.83 2 46. +1°3685 18 29 24 + 1 38.1 10.62 2.53 2.03 1.38 0.58 0.21 0.56 2 47. +1°3686 18 29 28 + 1 13.0 11.02 2.61 2.04 1.39 0.59 0.22 0.59 2 48. +1°3687 18 29 32 + 1 08.3 11.33 2.42 1.94 1.32 0.56 0.20 0.56 2 49. 18 29 39 + 1 32.8 12.89 2.35: 1.65: 0.67 0.27 0.71 2 50* +0°3948 18 29 45 + 0 18.6 9.01 2.25 1.71 1.06 0.46 0.17 0.44 2 51. 18 29 49 + 1 45.5 11.81 3.35: 2.96: 1.98 0.72 0.34 0.77 2 52. 18 29 53 + 0 45.7 11.68 4.20: 3.55: 2.45 0.97 0.39 0.95 2 53. 18 29 54 + 0 48.6 10.89 2.70 2.10 1.36 0.70 0.23 0.62 2 54. +1°3688 18 29 54 + 1 43.8 9.85 2.38 1.93 1.31 0.55 0.21 0.54 2 55. +1°3690 18 29 56 + 1 58.8 9.78 0.89: 0.37 0.14 0.28 2 56.» +1°3689 18 29 58 + 1 10.9 8.51 1.92 1.41 0.64 0.27 0.08 0.22 2 57. +1°3692 18 30 00 + 1 40.9 10.15 2.34 1.88 1.28 0.54 0.20 0.53 2 58. +0°3950 18 30 07 + 0 48.4 9.97 2.48 2.00 1.35 0.56 0.22 0.54 3 59. 18 30 10 + 1 54.6 12.23 2.57: 2.06: 1.44 0.59 0.22 0.62 2 60. +1°3693 18 30 10 + 1 19.6 10.54 2.77 2.01 1.19 0.52 0.19 0.48 1 61* +1°3694 18 30 24 + 1 13.6 9.86 2.47 1.92 1.20 0.60 0.20 0.55 2 62. 18 30 24 + 1 48.1 12.02 2.56 2.04 1.45 0.61 0.22 0.62 2 63* +0°3951 18 30 27 + 0 42.7 9.63 2.34 1.85 1.26 0.53 0.22 0.50 1 64. +1°3695 18 30 28 + 1 54.3 10.33 3.02: 1.28: 0.47 1.19 2 65. 18 30 32 + 1 04.7 12.27 3.04: 2.40: 1.64 0.73 0.24 0.73 2 66. +1°3696 18 30 33 + 2 01.5 10.42 2.33 1.75 0.86 0.38 0.10 0.30 2 67. 18 30 35 + 1 20.4 12.15 1.45: 0.64 0.26 0.58 2 68* +0°3952 18 30 37 + 0 22.8 8.35 2.25 1.70 1.08 0.46 0.17 0.45 2 70. 18 30 54 + 1 12.9 11.99 1.61: 0.74 0.29 0.74 2 71.* +0°3953 18 30 55 + 0 25.8 8.71 2.19 1.69 1.12 0.47 0.17 0.46 1 72. 18 30 56 + 1 14.3 12.42 2.92: 2.33: 1.69: 0.77 0.27 0.74 3 73. +1°3697 18 30 56 4- 2 01.3 11.03 3.71 2.87 1.85 0.90 0.33 0.78 2 74.* +1°3698 18 30 58 + 1 23.5 8.40 2.00 1.57 1.03 0.51 0.16 0.47 2 75. 18 31 00 + 1 27.6 12.86 1.95: 0.80 0.30 0.76 2 76. 18 31 00 + 1 30.5 12.48 2.09: 0.82 0.30 0.82 2 77. 18 31 06 + 2 00.7 11.56 2.59 2.08 1.44 0.60 0.22 0.60 2 78. 18 31 08 + 1 09.1 10.94 2.81: 2.10 1.25 0.53 0.18 0.51 2 79.* +1°3700 18 31 09 + 1 27.0 8.53 1.65 1.31 0.90 0.46 0.14 0.41 2 80.* +0°3954 18 31 18 + 0 21.3 10.01 2.63 1.91 1.11 0.53 0.18 0.44 2 81. 18 31 20 + 1 40.9 10.36 2.32 1.76 1.14 0.51 0.18 0.50 2 134 V. Straizys, K. Cernis and S. Bartasiute

Table 1 ( continued)

No. BD RA(2000) DEC(2000) V U-V P-V X-V Y-V Z-V V-S n h m s o i

82. 18 31 25 + 0 55.4 12.05 3.56: 2.74: 1.73 0.84 0.30 0.74 2 83. +1° 3702 18 31 26 + 1 36.7 10.11 2.20 1.70 1.07 0.46 0.16 0.46 2 84. 18 31 35 + 0 53.9 11.79 2.54: 2.07 1.44 0.60 0.22 0.62 2 85.* +1° 3703 18 31 39 + 1 53.8 9.32 3.95: 3.28 2.31 0.98 0.35 0.95 2 86. 18 31 40 + 1 17.4 11.85 3.26: 2.61: 1.86: 0.81 0.27 0.88 2 87. 18 31 52 + 1 12.0 11.45 2.61 2.06 1.40 0.57 0.19 0.59 1 88. 18 31 53 + 1 37.4 11.88 3.15 2.42 1.45 0.63 0.20 0.63 2 90. +1° 3704 18 31 57 + 1 17.8 10.10 2.25:1.75 1.18 0.51 0.18 0.51 2 91. +1° 3705 18 32 18 + 1 52.5 10.61 2.38: 1.81 1.20 0.55 0.18 0.56 2 92. +1° 3706 18 32 32 + 2 05.6 9.94 4.61: 3.95: 2.72 1.09 0.42 1.02 2 96* +1° 3708 18 32 46 + 2 03.3 9.71 2.59:1.87 0.95 0.43 0.12 0.39 2 97. +1° 3709 18 32 53 + 1 35.9 9.97 2.23 1.72 1.13 0.49 0.17 0.50 2 99. 18 32 58 + 1 05.6 11.36 2.69 2.16 1.50 0.66 0.23 0.64 3 100. +1° 3710 18 33 03 + 1 40.8 10.70 2.28 1.75 1.16 0.50 0.18 0.51 2 103. 18 33 11 + 1 35.2 10.75 3.44: 2.60 1.56 0.70 0.24 0.64 2 104* +1° 3711 18 33 15 + 1 50.8 8.42 3.78 3.17 2.18 0.85 0.32 0.81 2 105. 18 33 29 + 1 06.8 11.45 2.51 2.00 1.44 0.64 0.23 0.62 3 106* +1° 3712 18 33 32 + 1 54.2 7.95 2.21 1.60 0.80 0.31 0.11 0.27 2 108. +1° 3713 18 33 47 + 1 06.4 10.58 2.26 1.79 1.22 0.52 0.18 0.55 5 109* +1° 3714 18 33 59 + 1 17.4 8.78 2.28 1.82 1.26 0.52 0.19 0.52 2 110. +1° 3715 18 34 10 + 1 27.9 10.82 2.21 1.77 1.20 0.51 0.19 0.56 2 111. +0° 3967 18 34 10 + 0 56.8 10.37 2.36 1.84 1.27 0.55 0.19 0.54 2 112. +1° 3717 18 34 20 + 1 35.3 10.38 2.42: 1.87: 1.25 0.54 0.19 0.54 2 113* +1° 3718 18 34 35 + 2 04.7 8.45 2.53 2.06 1.39 0.55 0.21 0.56 2 114. +1° 3719 18 34 57 + 1 28.4 10.00 2.78: 2.22: 1.53 0.63 0.24 0.62 2 116. +1° 3720 18 35 04 + 1 17.7 10.51 2.63 2.19 1.48 0.60 0.23 0.62 3 117.* -0° 3513 18 35 08 + 0 02.6 7.94 2.20 1.75 1.32 0.66 0.21 0.64 3 118.* +1° 3723 18 35 45 + 1 15.9 9.88 2.57 2.12 1.43 0.58 0.22 0.58 2

Notes: No. 1 = HD 169579 (GO); No. 4 = HD 169670 (K0); No. 7 = HD 169724 (F5); No. 9 = HD 169725 (A3) = ADS 11339 AB = IDS 18210+0043, sep = 0.3", Am = 0.3 mag; No. 13 = IDS 18217+0118, sep = 3.1", Am = 0.3 mag; No. 16 = 59 Ser, HD 169985 (AO) = ADS 11353 AB = IDS 18221+0008, sep = 3.8", GO III + A6 V; Am = 2.3 mag, A is a triple SB; Interstellar extinction in the Serpens Cauda cloud 135

No. 20 = HD 170056 (A3); No. 27 = HD 170115 (AO); No. 37 = HD 170294 (G5); No. 39 = VV Ser, JDs of observations: 2449577.326, 2449599.240, 2449602.204, 2449604.244; No. 42 = HD 170377 (K2); No. 45 = HD 170413 (K2); No. 50 = HD 170492 (F2); No. 56 = HD 170545 (AO) = ADS 11396 A = IDS 18249+0107, sep. 6.8", Am = 1.6 mag; No. 61 = HD 170634 (AO); No. 63 = HD 170635 (F8), ADS 11401 A = IDS 18254+0039, sep. 5.9", Am = 2.0 mag; No. 68 = HD 170652 (F2); No. 71 = HD 170713 (F2); No. 74 = HD 170739 (B8) = ADS 11410 AB = IDS 18259+0119, sep. 2.4", Am = 0.3 mag; No. 79 = HD 170784 (B8); No. 80 = HD 170794 (AO); No. 85 = HD 170882 (K2); No. 96 = HD 171091 (AO); No. 104 = HD 171183 (K0); No. 106 = HD 171234 (A3); No. 109 = HD 171288 (F8); No. 113 = HD 171410 (GO); No. 117 = HD 171491 (B5) = IDS 18300-0002 AB = LSIV+00 2; sep = 0.3", Am = 0.0 mag; No. 118 = HD 171609 (G5).

2. The

EY-V = (Y-V)-(Y-V)0, (1) 5 log r = V - My + 5 - Ay. (2) Here Ay = RyyEy-Y = RBVEB-V (3) and RBV/RW — Ey-y I EB-V- (4)

For early-type stars this ratio is 0.755, i.e. Ryy = 1.32RBV- If we take the normal value of RBV = 3.15 (Straizys 1992), it corresponds Interstellar extinction in the Serpens Cauda cloud 137 to RYV = 4.16. This value of RYV was used for the reddened stars outside the molecular cloud. Chavarria et al. (1988) and de Lara et al. (1991) have found a larger than normal ratio R in the Serpens cloud. If we take their value of RBV = 3.3, then RYV — 4.4. This value of RYV has been used by us for the reddened stars embedded in the molecular cloud or located behind it. For three stars, HD 170634, HD 170739 and HD 170784, the values of RBv equal to 3.6, 4.3 and 4.1, respectively, have been taken from de Lara et al. (1991), see discussion below. These values give RYV equal to 4.8, 5.7 and 5.4. The results of the classification, determination of color excesses, extinctions and distances of the stars are given in Table 2. The expected errors are: ±0.03 mag for EY-V, ±0.1 mag for AY and ±25 per cent for distance. In Table 2, the distances r > 200 pc are rounded to the nearest number multiple to 10. The last column gives the accuracy of classification: a means the standard devia- tion of color indices of the program star with respect to the selected standard.

4. INTERSTELLAR EXTINCTION

Fig. 4 shows a plot of extinction versus distance for the molecular cloud area. One can notice at once that the extinction shows a sudden rise at about 200 pc distance. Due to errors in the distance determination we have to expect a scatter of stars which actually reside in the same dust cloud and have the same distance (if we neglect a possible radial extent of the cloud which is expected to be of the order of 10 pc). At 200 pc, the expected rms error is about ± 50 pc. Consequently, 18 stars between 190 pc and 305 pc (including the components of the binary stars HD 170739 and HD 171491) with AY > 1.0 mag may be considered as a part of the dark cloud population. Their mean distance is (247 ± 37) pc which may be considered as the distance of the dark cloud. Other confirmation of this distance comes from the unreddened stars which in Fig. 4 disappear at about the same r. On the other hand, both groups of authors did not notice that two stars (HD 170739 and HD 171491) are close binaries, and their distances have been determined incorrectly (see further in the text). In Table 1 there are four stars used for distance determination of the dark cloud by Chavarria et al. (1987, 1988), Zhang et al. (1988a) and de Lara et al. (1991): HD 170634 (No. 61 in Table 1), 138 V. Straizys, K. Cernis and S. Bartasiute

Table 2. Results of photometric classification, determination of color excesses, extinction and distances of the stars

No. Photom. PPM Mv Ey-v Ay r a sp. type sp. type* pc mag

1. F8 IV GO +3.0 0.00 0.00 192 0.01 2. G8 III K0 +0.7 0.19 0.79 410 0.01 3. F0 V F2 +2.8 0.07 0.29 240 0.01 4. G9 III K0 +0.5 0.17 0.71 400 0.02 5. Ml III K5 -1.0 0.20 0.83 700 0.02 6. A8 III +1.2 0.26 1.08 450 0.02 7. F5 V F5 +3.1 0.00 0.00 146 0.01 8. Ml II-III K5 -1.8 0.34 1.42 760 0.01 9. A7 V A3 +2.3 0.06 0.25 72 0.01 10. F9 V +4.0 0.12 0.50 250 0.01 11. K2 V +6.3 0.03 0.13 117 0.02 12. F0: V +2.8 0.20 0.83 250 0.02 13. AO: V AO +1.6 0.48 2.00 220 0.04 14. Al V +1.6 0.66 2.90 196 0.01 15. F7 V +4.0 0.03 0.13 210 0.01 17. F6 V +3.6 0.00 0.00 122 0.01 19. F8 V +4.1 0.02 0.09 164 0.01 20. Fl V A3 +2.8 0.00 0.00 160 0.01 21. G6 V +5.2 0.05 0.21 131 0.02 22. F2 V F2 +3.1 0.07 0.29 220 0.01 24. G6 V +5.2 0.04 0.17 210 0.01 26. F5 V +3.6 0.11 0.46 290 0.01 27. A2 V AO +1.3 0.04 0.17 179 0.01 28. K4 V K5 +6.1 0.06 0.25 69 0.04 29. F0 IV-V +2.5 0.18 0.75 320 0.01 30. K2 V +5.6 0.00 0.00 109 0.02 31. F6 V +3.7 0.22 0.97 370 0.01 32. GO V +4.4 0.00 0.00 230 0.01 33. K2 III K +1.3 0.11 0.48 420 0.01 34. K2 V +5.2 0.03 0.12 195 0.03 35. Ml III M -1.0 0.24 1.00 580 0.05 36. F5 V +3.6 0.09 0.40 210 0.01 37. G5 IV G5 +3.1 0.00 0.00 98 0.01 38. G5 V +4.5 0.05 0.22 197 0.02 40. G2 IV-V +4.0 0.15 0.66 270 0.01 Interstellar extinction in the Serpens Cauda cloud 139

Table 2 (continued)

No. Photom. PPM MV Ey-V Ay r a sp. type sp. type pc mag

41. F5 V +3.7 0.24 1.06 190 0.01 42. K2 V K2 +6.5 0.00 0.00 31 0.02 43. GO V +4.4 0.13 0.57 220 0.01 44. F6 V +3.7 0.17 0.75 139 0.02 45. K0 III K2 +0.8 0.09 0.38 230 0.01 46. F9 V +4.2 0.06 0.26 171 0.01 47. F8 V F8 +4.1 0.09 0.40 200 0.01 48. F8 V +4.1 0.06 0.26 250 0.01 49. G7 V +5.6 0.07 0.31 250 0.01 50. F0 V F2 +2.8 0.10 0.42 144 0.01 51. K2 V +5.0 0.00 0.00 230 0.02 52. K0.5 III-IV +1.6 0.21 0.92 680 0.01 53. B8 V +1.0 0.58 2.55 290 0.03 54. GO V +4.4 0.00 0.00 123 0.01 55. A4 V +2.4 0.14 0.62 225 0.02 56. AO V AO +1.6 0.12 0.53 189 0.02 57. F8 V +4.1 0.04 0.18 149 0.01 58. GO V F8 +4.4 0.03 0.13 122 0.01 59. F9 V +4.2 0.07 0.31 350 0.01 60. FO III +1.5 0.17 0.75 460 0.02 i— t B8 V AO +1.1 0.48 2.30 196 0.03 62. GO V +4.4 0.07 0.31 290 0.02 63. F8 V F8 +4.1 0.02 0.09 122 0.01 64. G9.5 III +0.8 0.52 2.29 280 0.02 65. F8 III +2.0 0.22 0.97 720 0.02 66. A1 V A2 +1.7 0.21 0.92 360 0.04 67. F6 V +3.8 0.18 0.79 320 0.01 68. F2 V F2 +3.1 0.06 0.25 100 0.01 70. F6 V +3.8 0.28 1.23 250 0.01 71. F6 V F2 +3.7 0.00 0.00 100 0.01 72. F8 V +4.1 0.26 1.14 270 0.01 73.* A7 V KO +2.6 0.62 2.73 138 0.02 74a* B5 V B8 -0.1 0.43 2.45 215 0.02 74b* B6 V +0.2 0.43 2.45 215 0.02 75.* G 0.01 76. G9 III +1.6 0.12 0.53 1180 0.01 140 V. Straizys, K. Cernis and S. Bartaaiute

Table 2 (continued)

No. Photom. PPM MY EY-V AY r A sp. type sp. type pc mag

77. G1 V +4.6 0.05 0.22 220 0.01 78. A8 V +2.6 0.23 1.01 290 0.02 79* B3 V B8 -1.0 0.39 2.11 305 0.02 80* B-A AO 0.02 81. F2 V +3.1 0.09 0.40 240 0.01 82. A5 V +2.3 0.60 2.64 260 0.02 83. F3 V F2 +3.2 0.03 0.13 230 0.01 84. G2 V +4.7 0.04 0.18 240 0.01 85. G8 III K2 +1.5 0.28 1.23 210 0.02 86.* G 0.04 87* F 0.02 88. A7 Illm: +1.1 0.37 1.63 640 0.01 90. F5 V F8 +3.6 0.05 0.22 180 0.01 91. F5 V +3.6 0.09 0.40 210 0.02 92. K0 III -0.5 0.26 1.14 720 0.02 96. A3 V AO +1.7 0.21 0.92 260 0.02 97. F5 V F5 +3.6 0.04 0.18 173 0.01 99. F8 V +4.1 0.16 0.70 200 0.01 100. F5 V F8 +3.6 0.05 0.22 240 0.01 103. A7m: +1.9: 0.42 1.85 250 0.03 104. G9 III K0 +0.7 0.10 0.44 290 0.01 105. F9 V +4.2 0.12 0.53 220 0.01 106. A7 V A3 +1.9 0.00 0.00 162 0.01 108. F7 V +4.0 0.05 0.22 187 0.01 109. GO V F8 +4.4 0.00 0.00 75 0.01 110. F8 V +4.1 0.00 0.00 220 0.02 111. F7 V +4.0 0.07 0.31 163 0.01 112. F6 V F5 +3.7 0.09 0.38 182 0.01 113. G2 V GO +4.7 0.00 0.00 56 0.01 114. G GO 0.01 116. G5 V GO +5.1 0.03 0.13 114 0.02 117* B3+B3 V B5 -1.0,-1.0 0.59 2.46 280 0.03 118. G5 V G5 +4.7 0.00 0.00 109 0.01

Notes: PPM spectral type is from Roser & Bastian (1991); Nos. 16, 75, 80, 86, 87 and 114 - classification uncertain; Nos. 61, 73, 74, 79, 117 - see Table 3 and Section 5. Interstellar extinction in the Serpens Cauda cloud 141

3.0 -1—i—I—I—i—r- -1—i—(—r—i—i—i—i—i—i—i—r

Ay 0 O O O

2.0 -

1.0 -

O 0<3° CP 3 <5OoVffJ o 00 O ° 0.0 'o'd 0 eoo 0» 'o' ao' _J i J—i—i i i i.i i i —I 0 200 400 600 , , 800 r (pc) Fig. 4. Interstellar extinction Ay plotted against distance for the stars from Table 2. The arrow shows a shift of the position of star No. 73 if it is a binary with identical components.

HD 170739 (No. 74), HD 170784 (No. 79) and HD 171491 (No. 117). Their spectral classes and physical parameters, used by the previous authors, as well as determined in the present paper, are intercom- pared in Table 3. One can see that the photometric classification of these stars in the Vilnius system is much closer to the results of the Mexican astronomers (de Lara et al. 1988). Consequently, we confirm a small distance to the Serpens cloud. The distance of 750 pc, obtained by Zhang et al. (1988) seems to be unrealistic and is caused by the erroneous luminosity classes of the stars used in distance determination. The calibration of the Vilnius photometric system in terms of absolute magnitude is based on the distance modulus of the Hyades cluster V — My = 3.2 mag. The newest investigations show that the Hyades distance modulus is closer to 3.3 mag (Dzervitis & Paupers 1994). Thus, the Hyades are 1.047 times farther away. Consequently, the distances of the Serpens Cauda dark cloud, obtained from Fig. 4, should be multiplied by this factor, becoming 259 pc. The positions of the 18 reddened stars, used for determination of the distance of the dark cloud, seem to be not correlated with the 142 V. Straizys, K. Cernis and S. Bartasiutè

Table 3. Physical parameters of four stars used by Chavarria et al. (1987, 1988), Zhang et al. (1988a) and de Lara et al. (1991) for distance determination of the Serpens Cauda dust cloud.

No. 61 = HD 170634 = BD+1°3694 = Chavarria 10 Spectral type: B7 V [1], B7 V [2], Al V [3], B8/9 III [4], B8 V [5] Absolute magnitude My. +1.0 [3], -1.0 [4], +1.1 [5] Extinction Ay. 2.65 [3], 2.08 [4], 2.30 [5] Distance (pc): 180 [3], 550 [4], 196 [5]

No. 74 = HD 170739 = BD+1°3698 = Chavarria 12* Spectral type: B6 V [2], B4 V [3], B5 II-III [4], B5+B6 V [5] Absolute magnitude My: -1.4 [3], -3.0 [4], -0.1 and +0.2 [5] Extinction Ay. 2.95 [3], 1.83 [4], 2.45 [5] Distance (pc): 240 [3], 820 [4], 215 [5]

No. 79 = HD 170784 = BD+1°3700 = Chavarria 13 Spectral type: B5 V [2], B3 V [3], B2/3 III [4], B3 V [5] Absolute magnitude My: -1.6 [3], -3.1 [4], -1.0 [5] Extinction Ay. 2.56 [3], 1.70 [4], 2.11 [5] Distance (pc): 330 [3], 950 [4], 305 [5]

No. 117 = HD 171491 = BD-0°3513 = Chavarria 15** Spectral type: B5 V [2], B3 V [3], B3+B3 V [5] Absolute magnitude My. -1.6 [3], -1.0 and -1.0 [5] Extinction Ay. 2.76 [3], 2.46 [5] Distance (pc): 232 [3], 280 [5]

* Binary star ADS 11410 AB, d = 2.4" and AV = 0.3 mag. ** Binary star IDS 18300-0002 AB, d = 0.3" and AV = 0.0 mag. [1] Racine (1968), [2] Pavlova et al. (1986), [3] de Lara et al. (1988), [4] Zhang et al. (1988a), [5] This paper Interstellar extinction in the Serpens Cauda cloud 143 observable variations of the surface density of faint background stars seen on the Palomar Sky Survey red plate: heavily reddened stars even prefer the areas with relatively rich background. Such are the stars Nos. 13, 14, 64, 73, 74, 79 and 103 with Av > 1.8. On the other hand, the stars with low reddening seem to appear only in the areas with higher density of background stars, i.e. in more transparent areas. Such stars are: 2, 4, 5, 6, 8, 31, 33, 35, 59, 66 and 92. However, there is a curious case: the star No. 76, situated visibly near one of the luminous stars, HD 170784, illuminating the reflection nebula DG 153, is the most distant star in the investigated area! Also, no correlation is found between the positions of the most reddened stars and the distribution of the extended far infrared emission found by the astronomical satellite IRAS (Zhang et al. 1988a). All this means that the interstellar dust is distributed very un- evenly both in the Serpens molecular cloud and in more distant clouds: there are some very dense patches in the molecular cloud at 250 pc distance and many windows through which we see stars up to 1.2 kpc. Even the densest core of the molecular cloud is projected on the background with considerably differing star density: the faint stars are scarce to the right of the star HD 170634 (illuminating the DG 152 reflection nebula) and dense to the left of it. On the blue Palomar Atlas copy, the Serpens cloud shows much larger extinction and the background star density seems to be more uniform.

5. SOME INDIVIDUAL STARS

Some heavily reddened and peculiar stars of the investigated area will be discussed separately.

No. 39 = VV Ser

The Herbig Ae-type star VV Ser cannot be classified photometrically due to its very peculiar color indices. For this star, Herbig (1960) and Chavarria et al. (1988) found a spectral type A2e with Ha and H¡3 in emission. For an A2 star the observed color indices yield Ey-v ~ 0.65 mag or Ay = 2.9 mag if Ryv = 4.4. This value of extinction is in a reasonable agreement with other stars embedded into the dark cloud. If we take the ZAMS absolute magnitude of 1.8 144 V. Straizys, K. Cernis and S. Bartasiute mag for the star, its distance is 263 pc, which is sufficiently close to the derived distance of the cloud.

No. 73 = BD+1° 3697

This star is of spectral type A7 V (the Q, Q and aQ methods) or A6 V (the CLASS method) and is heavily reddened (Ay = 2.8 mag). Its distance contradicts the run of extinction with distance in Fig. 4. On the diagram QUXY, QUPYV, its absolute magnitude cannot be determined accurately, since the star lies outside the calibrated net- work, in the area of the ZAMS stars. If the star is of spectral type A7 and belongs to the ZAMS, its My should be +2.6 and the distance - 138 pc. It is unrealistic to find so heavily reddened star lying considerably in front of the dark cloud. Probably, the star is either abso- lutely brighter (however, this contradicts with its photometric classification) or it is a binary. If both components of the binary are of the same brightness, then their magnitudes are V = 11.03 + 0.75 = 11.78 mag and the distance of the pair is 195 pc, i.e. it falls into the rms error bar of the dark cloud. The PPM Star Catalogue (Vol. 2) gives right ascension 18h30m56s, declination +2°05'26" and a K0 spectral class for 1°3697. Both declination and spectral class show that the PPM identification is wrong.

No. = HD 1707S9 = BD+1°3698 = Chavarria 12

The quoted papers do not mention that the star HD 170739 is a visual binary, ADS 11410, with a separation of the components 2".4 and AF = 0.3 mag. The binarity of the star was clearly seen during our photometric observations. The magnitude difference of 0.3 mag and the integrated brightness 8.40 mag lead to brightnesses of the components 9.01 and 9.31 mag. The magnitude difference of 0.3 mag corresponds to one spectral subclass in the B4-B6 range. Consequently, the components can be either of B4 and B5 spectral classes or of B5 and B6 spectral classes. For RBV = 4.3 (this corresponds to RYV = 5.7) we get Ay = 2.39 mag and the distance 254 pc for a B4+B5 pair and Ay = 2.45 mag and 215 pc for a B5+B6 pair. In distance determination, we take My values for the ZAMS stars. Our photometric classification in the integrated light gives B6 V for the spectral type. This means that the second distance value is preferable. Interstellar extinction in the Serpens Cauda cloud 145

No. 79 = HD 170784 = BD+1°3700 = Chavarria 13

This star is found to be the most distant one among the four stars listed in Table 3. This is confirmed also by the results of de Lara et al. and Zhang et al. However, this star illuminates the reflection nebula DG 153, and it should belong to the molecular cloud complex. The visible extent of the molecular cloud is about 2°, i.e. about 9 pc at 250 pc distance. If the cloud is more or less spherical, its depth cannot be much larger than ~ 10 pc. Thus, HD 170784 actually is closer than its apparent magnitude shows. A circumstellar dust envelope of large grains, producing the neutral extinction, could be responsible for the apparent faintness of this star. However, the near infrared photometry of the star (Chavarria et al. 1988, de Lara et al. 1991) does not show any presence of a warm dust envelope around this star.

No. 117 = HD 171491 = BD-0° 3513 = Chavarria 15

This star is a close visual binary IDS 18300-0002 AB with components of similar brightness. If both components are of the spectral type B3 V, their V = 7.94 + 0.75 = 8.69 mag, Mv = -1.0 mag, Av = 2.46 mag and r = 279 pc. In determining the cloud distance, we took both components into account.

6. CONCLUSIONS

Our investigation of the interstellar extinction in the direction of the Serpens Cauda star forming region shows that the dark cloud with its small reflection and emission nebulae is at (259 ± 37) pc. 23 heavily reddened stars with Ay between 1.0 and 2.9 mag have been discovered. 18 of them probably are embedded into the dark cloud and others are the background objects. It is important to obtain for all of them JHK photometry and to estimate the ratio of total-to-selective extinction individually. Also, these stars can be used for investigation of the interstellar extinction law in the Serpens molecular cloud.

ACKNOWLEDGMENT. Two of us (V.S. and K.C.) acknowl- edge the support provided by the International Science Foundation under contract LE 9000. 146 V. Straizys, K. Cernis and S. Bartasiutè

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