- Email: [email protected]
The Hipparcos distance determination of the Wolf-Rayet system γ2 Velorum (WC8+O) and its ramifications
NEW ASTRONOMY New Astronomy 2 (1997) 245-250
ELSEVIEK
The Hipparcos distance determination of the Wolf-Rayet y 2 Velorum (WC8 + 0) and its ramifications*
system
K.A. van der Hucht”“, H. Schrijve?‘*, B. Stenholmb”, I. Lundstrijmb,4, A.F.J. Moffat”5, S.V. Marchenkoc’6, W. Seggewissd3’, D.Y.A. Setia Gunawane28, W. Sutantyo’.“, E.P.J. van den Heuvelg' lo, J.-P. De Cuyperh3”, A.E. G6mez'. ” “Space Research
Organization
Netherlands,
hLund Observatory, ‘Departement
de Physique,
Universite
de Montreal.
“Universitiitssternwarte ‘Kapteyn ‘Observatorium
Astronomical
Bosscha and Jurusan
‘Astronomical
Institute
Insiitute,
‘Koninklijke ‘Observatoire
2, NL-3584
CA Utrecht.
C.P. 6128, Succ.
"Centre-We“,
Bonn, Auf dem Huge1 71, D-53121
Astronomi,
Anton
Sorhonnelaan
P.O. Box 800. NL-9700 Instinct
Pannekoek. Sterrenwacht,
de Paris-Meudon,
The Netherlands
P.O. Box 43, S-221 00 Lund, Sweden
Teknologi
Kruislrun
Montreal,
AV Groningen,
Bandung,
J&n
Ringlaan
3, B- I180 F-92195
3J7. Cunada
The Netherlands
Ganesha
403. NL- 1098 SJ Amsterdam.
D.A.S.G.A.L..
Qc, H3C
Bonn, Germany
Brussel.
IO, Bandung
40132, Indonesia
The Netherlands
Belgium
Meudon
Cedex. France
Received 24 April 1997; accepted 28 May 1997 Communicated by J. Leonard Culhane
Abstract Hipparcos parallax measurements give a distance to the Wolf-Rayet WC8 + 0 spectroscopic binary y2 Vel of d = 258 ‘:) pc and a distance to the 04I(n)f star 5 Pup of d = 429’::” pc. Adopting for y2 Vel an interstellar extinction of AU = 0.06 mag, this implies an absolute magnitude MU = - 5.4 mag for the WC8 +0 binary system. Given that the binary components have a magnitude difference Am = 1.4 mag, we derive M,(WC8) = - 3.7 and M,(O) = - 5.0 mag. The latter indicates an 08.5111 rather than an 091 companion, as was adopted during the last 25 years. Apparently y’ Vel is not a member of, but a ‘Based on data from the ESA Hipparcos ‘[email protected] ‘[email protected] ‘[email protected] ‘[email protected] ‘[email protected] [email protected] ‘[email protected] [email protected] ‘[email protected] ‘“[email protected] ’ ‘[email protected] “[email protected] 1384-1076/97/$17.00 0 Sl384-1076(97)00018-3
PfI
astrometry
satellite.
1997 Elsevier Science B.V. All rights reserved.
246
K.A. van der Hucht et al. I New Astronomy
foreground object before the open cluster Cr 173 and the association Gum Nebula, y* Vel is still one of its ionizing sources, while 5 Pup Consequences of the Hipparcos distance determination of y’ Vel wavelengths, and, briefly, its association with the Gum Nebula, are
2 (1997) 24552.50
Ve10B2. Given a re-assessment of the distance of the appears to be located at the back of the Gum Nebula. for its mass, mass loss rate, luminosities at various discussed. 0 1997 Elsevier Science B.V.
PACS: 95.10.Jk; 97.10.Nf; 97.1O.Vm; 97.30.Eh; 97.8O.m Keywords: Binaries: spectroscopic; Stars: distances; Stars: fundamental parameters; Stars: individual: yz Vel; Stars: mass
loss; Stars: Wolf-Rayet
1. Introduction Wolf-Rayet (WR) stars are generally accepted to represent an evolved phase of initially massive (M, > 25M,) stars (Maeder, 1996). This explains their relatively small number - about 200 have been detected in the Galaxy so far (van der Hucht, 1996). Nevertheless, because of their emission lines originating in their hot and fast expanding stellar winds (terminal velocities u, = 1000-2500 km s-l, driving mass loss rates k = [0.7-lo] X 10p5Ma yr-‘), their characteristic spectra are easily detectable to large distances, making WR stars tracers of galactic structure. For reviews on WR stars, see van der Hucht ( 1992), Maeder & Conti ( 1994) and papers in Vreux et al. (1996). The apparently brightest WR star, discovered by Respighi (1872), y ’ Vel (= y Argus = HD 68273 = HR 3207 = WR 11 (van der Hucht et al., 1981) = HIP39953) has always been considered the nearest of its kind. In most previous distance determinations it has been assumed that y2 Vel is located at the same distance as its visual companion y ’ Vel (= HR 3206, BlIV, Hiltner et al. (1969)) and surrounding stars (members of the open cluster Cr 173 and the association Vel 0B2) with the same colour excess (Gum (1955): d = 250 pc; Smith (1968b): 460 pc; Baschek & Scholz (1971): 300 pc; Conti & Smith (1972): 460 pc; Abt et al. (1976): 480 pc; Eggen (1980), (1983): 450 pc; Lundstrijm & Stenholm (1984): 460 PC). In the past 25 years the estimate of about 450 pc for this distance has not been questioned any more and was quoted, e.g. in the WR galactic distribution study by van der Hucht et al. (1988). Since the study of Smith (1955) y’ Vel is known to be a WC+0 spectroscopic binary, with an orbital period of P = 78.5002 days (Niemela & Sahade, 1980; Moffat et al., 1986). There has been
one study involving a direct, interferometric observation by Hanbury Brown et al. (1970), who measured an angular semi-major axis a = 4.320.5 mas for the binary system y2 Vel. This, combined with an a value derived from the orbital period and estimated binary companion masses as far as known at the time, yielded a distance d = 350?50 pc.
2. Hipparcos
data and reduction
The Hipparcos data are published in The Hipparcos Catalogue (ESA, 1997), where details on data reduction are given in Volume 1: Introduction and Guide to the Data. The objects discussed here were part of Proposal #00030 (KAvdH, WS, JPdC, EPJvdH 1982), Proposal # 00049 (BS, IL 1982), and Proposal # 00078 (AFJM, WS, AG, SVM 1982) for The Hipparcos Input Catalogue (Turon et al., 1992). The trigonometric parallaxes given in The Hipparcos Catalogue have been obtained by adjusting an astrometric model for stellar motion (with five parameters for stars assumed single) to a set of one-dimensional measurements performed in a number of transits through the Hipparcos field of view between 1989 and 1993. Systematic errors in the astrometry have been estimated to be less than 0.1 mas (Arenou et al., 1995; Lindegren, 1995). The ratio of the external errors to the standard errors quoted in the catalogue has been inferred to lie between 1.0 and 1.2. The Hipparcos parallaxes and proper motions of the WR object y2 Vel and the O-type star 5 Pup are given in Table 1. The Hipparcos p.m. values for y2 Vel are close to those given in The Hipparcos Input
247
K.A. van der Hucht et al. I New Astronomy 2 (1997) 245-2_fO Table 1 Hipparcos WR
II
data for 7’ Vel and C Pup name
type
y’ Vel i PUP
wcs+o 04I(n)f
HD
68273 6681 I
HIP
39953 39429
Catalogue (Turon et al., 1992). In this paper we concentrate on the Hipparcos parallax measurement (7~) of the spectroscopic binary y ’ Vel. Unfortunately, Hipparcos did not measure its visual companion y ’ Vel (Am = 2.6 mag, projected separation 42.“5). The other significant result (4.5g), the distance to < Pup is in good agreement with, e.g. Bieging et al. (1989) who gave d = 0.45 kpc. Among the Hipparcos data available to us are results for four other WR stars with 2.3 5 (T 5 3.6, as can be expected in a large sample. We will not discuss those data here. The quality of the Hipparcos results for individual stars can be judged from the number of rejected observations and the goodness of fit quoted in the catalogue. For the two stars considered here, these quantities lie within acceptable limits: the goodness of fit is negative in all cases, indicating an adjustment result that is better than the average expectation; for 5 Pup, 1% of the measurements (i.e. one measurement) was rejected in the adjustment process, for y ’ Vel no rejections were necessary. This indicates that the results for the two stars can be used with confidence. The correspondence in size between the semimajor axis of the binary orbit of y ’ Vel and its parallax is a coincidence. We have considered the influence of binary motion on the parallax determnation and found it not to be important, given that the period is much shorter than a year.
3. Absolute Since to be a velocity (1967).
magnitudes,
spectral types, masses
the study of Smith ( 1955) y* Vel is known WC +0 spectroscopic binary. A first radial solution was presented by Ganesh & Bappu An orbital period of P = 78.5002 days has
rr (mas) 3.88k0.53 2.33 kO.5
d
&COSS (mas yr-‘) - 5.93kO.54
I
- 30.82-cO.44
(PC) 9.90+0.43 16.7710.4
I
258’:; 4291;;”
been derived by Niemela & Sahade (1980) and confirmed by Moffat et al. (1986), while P = 78.5 19 days has been derived by Pike et al. (1983). The spectral type of the WC component is WC8 in the classification system of Smith (1968a), which has been quantified by Smith et al. (1990). The spectral classification of the 0 star is less certain, varying in the literature from 06 (Smith, 1955) via 07.5 (Ganesh & Bappu, 1967), 07 (Smith, 1968a) and 08 (Baschek & Scholz, 1971) to 091 (Conti & Smith, 1972). The 09 type is based on the O-star classification scheme of Conti and co-workers, as recently reviewed by van der Hucht (1996). The supergiant luminosity class, however, is not so certain: Conti & Smith (1972) could not measure the luminosity lineratio Si iv 4089/He i4143, because h4143 could not be seen with certainty. Therefore they estimated the luminosity class I by visual comparison with other O-type stellar spectra. They found from comparison of emission line strength between y2 Vel and the single WC8 star HD 192103 (WR 135) a magnitude difference between the WC8 and O-type companions of yZ Vel of Am = 1.4&O. 1 mag, the O-type companion being the brighter one. Their WC8 +091 classification was generally adopted during the last 25 years. As presented above, Hipparcos measured the distance to the spectroscopic binary y2 Vel as d = 25gf4’ _ 3, pc. We regard this as the best estimate of the true distance to y ’ Vel, although one must realize that the limits only refer to the standard (la) errors in the parallax determination. If one, pessimistically, would prefer to attach 3cr errors to the Hipparcos distances of y2 Vel and 5 Pup, then they could still be at the same distance. In order to relate the Hipparcos distance and the apparent magnitude u = 1.76 (Conti & Vacca, 1990) to an absolute magnitude for the y* Vel system, we
248
K.A. van der Hucht et al. I New Astronomy
need to know the interstellar extinction toward y2 Vel. No direct measure of the extinction, e.g. by 2200 A nulling, is available in the literature. Eggen (1980) gave reddening values for stars in the y ’ Vel group as well as for foreground and background stars. We adopt Eggen’s reddening value for the foreground stars Eh_” = 0.01420.010 mag as valid for y2 Vel, and convert this with Eh_,, = 0.74E,_, for O-type stars (Crawford, 1975). and A, = 3.39E,_, (Lundstrom & Stenholm, 1984) to A, = 0.06+0.05 mag. This implies an absolute magnitude M, = - 5.4 mag for the y2 Vel system, and with Am = 1.4 mag, we find M,(WC8) = - 3.7 and M,(O) = - 5.0 mag. M, = - 3.720.3 mag for a WC8 star is a value one magnitude fainter than was derived from cluster and association membership of WC8 stars in van der Hucht et al. (1988), but more in line with the absolute visual magnitudes of WCE stars therein. M,(O) = - 5.020.3 mag indicates, in the O-star spectral type - M, relation of Conti et al. (1983), an 08.5111 (M, = - 5.OkO.5 mag) rather than an 091 (M, = - 6.1kO.8 mag) companion. We do not exclude, given the uncertainties in equivalent width measurements in WR + 0 composite spectra, that the line measurements of Conti & Smith (1972) could be reconciled with an 08.5111 companion. However, in the published illustrations (Conti & Smith, 1972; Niemela & Sahade, 1980) the 0 star looks like a supergiant. The lower luminosity of the 0 star may be an effect of its high rotation as implied from the width of the absorption lines. Given the lower luminosity of the O-type companion than thought before, we also have to favour lower binary companion masses, and therefore the radial velocity solution of Pike et al. (1983), rather than that of Niemela & Sahade (1980). Given the Hipparcos distance, the measured angular semimajor axis a = 4.3 mas (Hanbury Brown et al., 1970), and the binary period P = 78.519 days and mass ratio q = 0.35 (Pike et al., 1983), the total mass of the binary system is M(WC8 + 08.5111) = 30 M,, with M(O8.5111) = 22M,, and M(WC8) = 8 M,. These masses are lower than the minimum masses derived from the radial velocity solution by Niemela & Sahade (1980): 32 and 17 M, respectively, but
Table 2 Measured
2 (1997) 245-250
and derived parameters
of y * Vel
Parameter
Value
Ref.
d (PC) = l/n
258:;; 1.76 0.06kO.05 - 5.420.3 1.4t0.1 - 3.720.3 - 5.OkO.3 78.519LO.006 0.35 4.320.5 30216 22212 8+4 1550240 [3.5?0.73 x 1o-5
111
v (mag)
AU(mag) M, system (mag) Am system (map)
M”(WC8) (ma& M,(O8.5111) (mag) P (days) Y a (mas) M(WC8 + 08.5111) (M,) M(08.5111) (M,) M(WC8) u,(WCI) ni(WC8)
(M,) (kms-‘) (Moyr-‘)
121 131 r41
[51 [51 [61
[71 r81
References: [I] The Hipparcos Catalogue (ESA, 1997). [2] v from Conti & Vacca (1990), in the narrow-band photometric system of Smith (1968a) and Lundstrom & Stenholm (1984), with A, = 1.I I A,. [3] Eh_,, = 0.014?0.010 mag for foreground stars toward the y’ Vel group (Eggen, 1980), converted with E,_\ = 0.74E,_, for O-type stars (Crawford, 1975) and A, = 3.39&_, (Lundstrom & Stenholm, 1984). [4] Conti & Smith (1972). [5] Pike et al. (1983). [6] Hanbury Brown et al. (1970). [7] van der Hucht et al. (1996). [8] ‘Radio’ mass loss rate from Prinja et al. (1990). (1991), scaled to adopted velocity and Hipparcos distance.
the total mass of 32 M, is in good agreement with that found from the radial velocity study by Pike et al. (1983). Table 2 summarizes measured and derived parameters.
4. Mass loss rate, X-ray and y-ray luminosities Prinja et al. (1990), (1991) derived for y2 Vel a stellar wind terminal velocity u, = 1415 kms-‘, driving a ‘radio’ mass loss rate M = 8.1 X 10m5M, yr-‘, using distance, radio data, and intrinsic parameters given by Abbott et al. (1986). Scaling this mass loss rate to u, = 1550 kms-’ (from ISOSWS spectra of forbidden lines (van der Hucht et al., 1996)) and d = 258 pc measured by Hipparcos, we find for y* Vel a ‘radio’ mass loss rate &i = 3.5 X 10m5Ma yr-‘. This is in good agreement with the
K.A.
van der Hucht
et al. I New Astronomy
recent determination of the value ni = [2-31 X 10-5M, yrr’ by Stevens et al. (1996) from spectral fitting of ASCA X-ray spectra of y* Vel. The new ‘radio’ mass loss rate could be further reduced by a factor three or so due to possible clumping of the WR wind (Moffat & Robert, 1994), bringing it closer to the ‘polarization’ mass loss rate of y2 Vel derived by St-Louis et al. (1988). With the Hipparcos distance for y2 Vel, X-ray and y-ray luminosities derived from measured fluxes have to be scaled down by a factor (258/450)2 = 0.33, while theoretical estimates of observable fluxes have to be scaled up with that factor. Doing so for the ROSAT-PSPC data (Pollock, 1995; Willis et al., 1995) one finds Lx/L,= 0.002,not a very conspicuous value among the observed WR X-ray luminosities in Pollock (1995). It is the X-ray flux variability, however, caused by the colliding WC8 and 08.5111 stellar winds, which makes the X-ray observations of y * Vel of great importance (Willis et al., 1995; Stevens et al., 1996). COMPTEL ‘6A1 1.809 MeV y-ray images of the Vela region (Oberlack et al., 1994; Diehl et al., 1995) encompass the positions of the Vela supernova remnant and y2 Vel. The Hipparcos distance of y * Vel and its lower WC8 mass compared to most previous studies, indicating an even more evolved state, do increase the possibility of a sizable y-ray contribution from y* Vel. It is of interest to note that the distance to the center of the Gum Nebula, with an angular diameter of 36”, has been re-assessed at d = 290-+30pc with a radius of 902 10 pc (France, 1990). The Hipparcos measurements put y* Vel close to the center of the Gum Nebula and 5 Pup to the back of the Gum Nebula. With the ROSAT determination of the Vela SNR distance d= 400+200 pc (Aschenbach et al., 1995), theVela SNR could be in the center or at the back back of the Gum Nebula. For discussions on contents and origin of the Gum Nebula, we refer to Franc0 (1990) and Oberlack et al. (1994).
Acknowledgments We are very grateful to the staff of the Hipparcos
2 (1997)
245-250
249
operations team and the Hipparcos data-reduction consortia. We thank Peter Conti, Philippe Eenens, Arnout Van Genderen, Ian Howarth, Virpi Niemela, Chris Sterken, and Perry Williams for constructive comments.
References Abbott, D.C.. Bieging, J.H., Churchwell. ApJ, 303, 239.
E., & Torres. A.V., 1986, \
Abt, H.A., Landolt, A.U., Levy, S.G., & Mochnacki, 81, 541.
S.. 1976, AJ,
Arenou, F., Lindegren, L., Froeschlt, M., Gomez, A.E., Turon, C., & Perryman, M.A.C., 1995, A&A, 304, 52. Aschenbach, B., Egger, R., & Triimper. J.. 1995, Natur, 373, 587. Baschek, B. & Scholz, M., 1971, A&A, 11, 83. Bieging, J.H., Abbott, DC., & Churchwell. E.B., 1989, ApJ, 340, 518. Conti, P.S. & Smith, L.J., 1972, ApJ, 172, 623. Conti, P.S., Garmany, CD., de Loore, C., & Vanbeveren, D.. 1983, ApJ, 274, 302. Conti, P.S. & Vacca, W.D., 1990, AJ, 100, 431. Crawford, D.L., 1975, PASP, 87, 481. Diehl. R., Bennett, K., B&men, H., Dupraz, C., Hermsen, W., Knodlseder, J., Lichti, G.G., Morris, D., Oberlack, U., Ryan, J., Schonfelder, V., Steinle, H.. Varendorff, M.. & Winkler, C., 1995, A&A. 298, L25. Eggen, O.J., 1980, ApJ, 238, 627. Eggen. O.J., 1983, AJ, 88, 197. ESA 1997, The Hipparcos Catalogue, ESA SP-1200. France, G.A.P., 1990, A&A, 227, 499. Ganesh, K.S. & Bappu, M.K.V., 1967, Kodaikanal Obs. Bull., Series A, No. 183, p. A77. Gum, C.S., 1955, MmRAS, 67, 155. Hanbury Brown. R., Davis, J., Herbison-Evans, D., & Allan, L.R., 1970, MNRAS, 148, 103. Hiltner. W.A., Garrison, R.F., & Schild, R.E., 1969, ApJ, 157, 313. Lindegren, L., 1995, A&A. 304, 61. Lundstrom, I. & Stenholm, B., 1984, A&AS, 58, 163. Maeder, A. & Conti, P.S., 1994, ARA&A, 32, 227. Maeder. A. & Meynet, G., 1994, A&A, 287, 803. Maeder, A., 1996, in: J.-M. Vreux, A. Detal, D. Fraipont-Caro, E. Gosset & G. Rauw (eds.), Wolf-Rayet Stars in the Framework of Stellar Evolution, Proc. 33rd Liege Int. Astroph. Coll. 1996 (Liege: Univ. of Liege) p, 39. Moffat, A.F.J., Vogt, H., Paquin. G., Lamontagne, R., & Barrera, L.H., 1986, AJ, 91, 1386. Moffat. A.F.J. & Robert. C., 1994, ApJ, 421, 310. Niemela, V.S. & Sahade. J., 1980, ApJ, 238, 244. Oberlack, U., Diehl, R., Montmerle, T.. Prantzos, N., & Von Ballmoos, P., 1994, ApJS, 92, 433. Pike, CD., Stickland, D.J., & Willis, A.J., 1983, Ohs., 103. 154.
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Pollock, A.M.T., 1995, in: K.A. van der Hucht & P.M. Williams (eds.), Wolf-Rayet Stars: Binaries, Colliding Winds, Evolution, Proc. IAU Symp. 163 (Dordrecht: Kluwer) p. 429. Prinja, R.K., Barlow, M.J., & Howarth, I.D., 1990, ApJ, 361, 607. An Erratum to this article was published in Prinja et al. (1991) Prinja, R.K., Barlow, M.J., & Howarth, I.D., 1991, ApJ, 383, 466. Erratum to Prinja et al. (1990) Respighi, L., 1872, CR, 74, 514. Smith, H.J., 1955, Southern Wolf-Rayet Stars, Thesis Harvard University, pp. 69-79. Smith, L.F., 1968a, MNRAS, 138, 109. Smith, L.F., 1968b. MNRAS, 141, 317. Smith, L.F., Shara, M.M., & Moffat, A.F.J., 1990, ApJ, 358, 229. Stevens, I.R., Corcoran, M.F., Willis, A.J., Skinner, S.L., Pollock, A.M.T., Nagase, F., & Koyama, K., 1996, MNRAS, 283, 589. St-Louis, N., Moffat, A.F.J., D&en, L., Bastien, P., & Robert, C., 1988, ApJ, 330, 286. Turon, C., Crezi, M., Egret, D., et al., 1992, The Hipparcos Input Catalogue, ESA SP- 1136.
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van der Hucht, K.A., Conti, P.S., Lundstrom, I., & Stenholm, B., 1981, SSRv, 28, 227. van der Hucht, K.A., Hidayat, B., Admiranto, A.G., Supelli, K.R., & Doom, C., 1988, A&A, 199, 217. van der Hucht, K.A., 1992, A&ARv. 4, 123. van der Hucht, K.A., 1996, in: J.-M. Vreux, A. Detal, D., FraipontCaro, E. Gosset & G. Rauw (eds.), Wolf-Rayet Stars in the Framework of Stellar Evolution, Proc. 33rd Liege Int. Astroph. Coll. (Liege: Univ. of Liege) p. I. van der Hucht, K.A., Morris, PW.., Williams, P.M., Setia Gunawan, D.Y.A., et al., 1996, A&A, 315. Ll93. Vreux, J.-M., Detal, A., Fraipont-Caro, D., Gosset, E., & Rauw, G. (eds.), 1996, Wolf-Rayet Stars in the Framework of Stellar Evolution, Proc. 33rd Liege Int. Astroph. Coll. 1996 (Liege: Univ. of Liege). Willis, A.J., Schild, H., & Stevens, I.R., 1995, A&A. 298, 549.
ELSEVIEK
The Hipparcos distance determination of the Wolf-Rayet y 2 Velorum (WC8 + 0) and its ramifications*
system
K.A. van der Hucht”“, H. Schrijve?‘*, B. Stenholmb”, I. Lundstrijmb,4, A.F.J. Moffat”5, S.V. Marchenkoc’6, W. Seggewissd3’, D.Y.A. Setia Gunawane28, W. Sutantyo’.“, E.P.J. van den Heuvelg' lo, J.-P. De Cuyperh3”, A.E. G6mez'. ” “Space Research
Organization
Netherlands,
hLund Observatory, ‘Departement
de Physique,
Universite
de Montreal.
“Universitiitssternwarte ‘Kapteyn ‘Observatorium
Astronomical
Bosscha and Jurusan
‘Astronomical
Institute
Insiitute,
‘Koninklijke ‘Observatoire
2, NL-3584
CA Utrecht.
C.P. 6128, Succ.
"Centre-We“,
Bonn, Auf dem Huge1 71, D-53121
Astronomi,
Anton
Sorhonnelaan
P.O. Box 800. NL-9700 Instinct
Pannekoek. Sterrenwacht,
de Paris-Meudon,
The Netherlands
P.O. Box 43, S-221 00 Lund, Sweden
Teknologi
Kruislrun
Montreal,
AV Groningen,
Bandung,
J&n
Ringlaan
3, B- I180 F-92195
3J7. Cunada
The Netherlands
Ganesha
403. NL- 1098 SJ Amsterdam.
D.A.S.G.A.L..
Qc, H3C
Bonn, Germany
Brussel.
IO, Bandung
40132, Indonesia
The Netherlands
Belgium
Meudon
Cedex. France
Received 24 April 1997; accepted 28 May 1997 Communicated by J. Leonard Culhane
Abstract Hipparcos parallax measurements give a distance to the Wolf-Rayet WC8 + 0 spectroscopic binary y2 Vel of d = 258 ‘:) pc and a distance to the 04I(n)f star 5 Pup of d = 429’::” pc. Adopting for y2 Vel an interstellar extinction of AU = 0.06 mag, this implies an absolute magnitude MU = - 5.4 mag for the WC8 +0 binary system. Given that the binary components have a magnitude difference Am = 1.4 mag, we derive M,(WC8) = - 3.7 and M,(O) = - 5.0 mag. The latter indicates an 08.5111 rather than an 091 companion, as was adopted during the last 25 years. Apparently y’ Vel is not a member of, but a ‘Based on data from the ESA Hipparcos ‘[email protected] ‘[email protected] ‘[email protected] ‘[email protected] ‘[email protected] [email protected] ‘[email protected] [email protected] ‘[email protected] ‘“[email protected] ’ ‘[email protected] “[email protected] 1384-1076/97/$17.00 0 Sl384-1076(97)00018-3
PfI
astrometry
satellite.
1997 Elsevier Science B.V. All rights reserved.
246
K.A. van der Hucht et al. I New Astronomy
foreground object before the open cluster Cr 173 and the association Gum Nebula, y* Vel is still one of its ionizing sources, while 5 Pup Consequences of the Hipparcos distance determination of y’ Vel wavelengths, and, briefly, its association with the Gum Nebula, are
2 (1997) 24552.50
Ve10B2. Given a re-assessment of the distance of the appears to be located at the back of the Gum Nebula. for its mass, mass loss rate, luminosities at various discussed. 0 1997 Elsevier Science B.V.
PACS: 95.10.Jk; 97.10.Nf; 97.1O.Vm; 97.30.Eh; 97.8O.m Keywords: Binaries: spectroscopic; Stars: distances; Stars: fundamental parameters; Stars: individual: yz Vel; Stars: mass
loss; Stars: Wolf-Rayet
1. Introduction Wolf-Rayet (WR) stars are generally accepted to represent an evolved phase of initially massive (M, > 25M,) stars (Maeder, 1996). This explains their relatively small number - about 200 have been detected in the Galaxy so far (van der Hucht, 1996). Nevertheless, because of their emission lines originating in their hot and fast expanding stellar winds (terminal velocities u, = 1000-2500 km s-l, driving mass loss rates k = [0.7-lo] X 10p5Ma yr-‘), their characteristic spectra are easily detectable to large distances, making WR stars tracers of galactic structure. For reviews on WR stars, see van der Hucht ( 1992), Maeder & Conti ( 1994) and papers in Vreux et al. (1996). The apparently brightest WR star, discovered by Respighi (1872), y ’ Vel (= y Argus = HD 68273 = HR 3207 = WR 11 (van der Hucht et al., 1981) = HIP39953) has always been considered the nearest of its kind. In most previous distance determinations it has been assumed that y2 Vel is located at the same distance as its visual companion y ’ Vel (= HR 3206, BlIV, Hiltner et al. (1969)) and surrounding stars (members of the open cluster Cr 173 and the association Vel 0B2) with the same colour excess (Gum (1955): d = 250 pc; Smith (1968b): 460 pc; Baschek & Scholz (1971): 300 pc; Conti & Smith (1972): 460 pc; Abt et al. (1976): 480 pc; Eggen (1980), (1983): 450 pc; Lundstrijm & Stenholm (1984): 460 PC). In the past 25 years the estimate of about 450 pc for this distance has not been questioned any more and was quoted, e.g. in the WR galactic distribution study by van der Hucht et al. (1988). Since the study of Smith (1955) y’ Vel is known to be a WC+0 spectroscopic binary, with an orbital period of P = 78.5002 days (Niemela & Sahade, 1980; Moffat et al., 1986). There has been
one study involving a direct, interferometric observation by Hanbury Brown et al. (1970), who measured an angular semi-major axis a = 4.320.5 mas for the binary system y2 Vel. This, combined with an a value derived from the orbital period and estimated binary companion masses as far as known at the time, yielded a distance d = 350?50 pc.
2. Hipparcos
data and reduction
The Hipparcos data are published in The Hipparcos Catalogue (ESA, 1997), where details on data reduction are given in Volume 1: Introduction and Guide to the Data. The objects discussed here were part of Proposal #00030 (KAvdH, WS, JPdC, EPJvdH 1982), Proposal # 00049 (BS, IL 1982), and Proposal # 00078 (AFJM, WS, AG, SVM 1982) for The Hipparcos Input Catalogue (Turon et al., 1992). The trigonometric parallaxes given in The Hipparcos Catalogue have been obtained by adjusting an astrometric model for stellar motion (with five parameters for stars assumed single) to a set of one-dimensional measurements performed in a number of transits through the Hipparcos field of view between 1989 and 1993. Systematic errors in the astrometry have been estimated to be less than 0.1 mas (Arenou et al., 1995; Lindegren, 1995). The ratio of the external errors to the standard errors quoted in the catalogue has been inferred to lie between 1.0 and 1.2. The Hipparcos parallaxes and proper motions of the WR object y2 Vel and the O-type star 5 Pup are given in Table 1. The Hipparcos p.m. values for y2 Vel are close to those given in The Hipparcos Input
247
K.A. van der Hucht et al. I New Astronomy 2 (1997) 245-2_fO Table 1 Hipparcos WR
II
data for 7’ Vel and C Pup name
type
y’ Vel i PUP
wcs+o 04I(n)f
HD
68273 6681 I
HIP
39953 39429
Catalogue (Turon et al., 1992). In this paper we concentrate on the Hipparcos parallax measurement (7~) of the spectroscopic binary y ’ Vel. Unfortunately, Hipparcos did not measure its visual companion y ’ Vel (Am = 2.6 mag, projected separation 42.“5). The other significant result (4.5g), the distance to < Pup is in good agreement with, e.g. Bieging et al. (1989) who gave d = 0.45 kpc. Among the Hipparcos data available to us are results for four other WR stars with 2.3 5 (T 5 3.6, as can be expected in a large sample. We will not discuss those data here. The quality of the Hipparcos results for individual stars can be judged from the number of rejected observations and the goodness of fit quoted in the catalogue. For the two stars considered here, these quantities lie within acceptable limits: the goodness of fit is negative in all cases, indicating an adjustment result that is better than the average expectation; for 5 Pup, 1% of the measurements (i.e. one measurement) was rejected in the adjustment process, for y ’ Vel no rejections were necessary. This indicates that the results for the two stars can be used with confidence. The correspondence in size between the semimajor axis of the binary orbit of y ’ Vel and its parallax is a coincidence. We have considered the influence of binary motion on the parallax determnation and found it not to be important, given that the period is much shorter than a year.
3. Absolute Since to be a velocity (1967).
magnitudes,
spectral types, masses
the study of Smith ( 1955) y* Vel is known WC +0 spectroscopic binary. A first radial solution was presented by Ganesh & Bappu An orbital period of P = 78.5002 days has
rr (mas) 3.88k0.53 2.33 kO.5
d
&COSS (mas yr-‘) - 5.93kO.54
I
- 30.82-cO.44
(PC) 9.90+0.43 16.7710.4
I
258’:; 4291;;”
been derived by Niemela & Sahade (1980) and confirmed by Moffat et al. (1986), while P = 78.5 19 days has been derived by Pike et al. (1983). The spectral type of the WC component is WC8 in the classification system of Smith (1968a), which has been quantified by Smith et al. (1990). The spectral classification of the 0 star is less certain, varying in the literature from 06 (Smith, 1955) via 07.5 (Ganesh & Bappu, 1967), 07 (Smith, 1968a) and 08 (Baschek & Scholz, 1971) to 091 (Conti & Smith, 1972). The 09 type is based on the O-star classification scheme of Conti and co-workers, as recently reviewed by van der Hucht (1996). The supergiant luminosity class, however, is not so certain: Conti & Smith (1972) could not measure the luminosity lineratio Si iv 4089/He i4143, because h4143 could not be seen with certainty. Therefore they estimated the luminosity class I by visual comparison with other O-type stellar spectra. They found from comparison of emission line strength between y2 Vel and the single WC8 star HD 192103 (WR 135) a magnitude difference between the WC8 and O-type companions of yZ Vel of Am = 1.4&O. 1 mag, the O-type companion being the brighter one. Their WC8 +091 classification was generally adopted during the last 25 years. As presented above, Hipparcos measured the distance to the spectroscopic binary y2 Vel as d = 25gf4’ _ 3, pc. We regard this as the best estimate of the true distance to y ’ Vel, although one must realize that the limits only refer to the standard (la) errors in the parallax determination. If one, pessimistically, would prefer to attach 3cr errors to the Hipparcos distances of y2 Vel and 5 Pup, then they could still be at the same distance. In order to relate the Hipparcos distance and the apparent magnitude u = 1.76 (Conti & Vacca, 1990) to an absolute magnitude for the y* Vel system, we
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K.A. van der Hucht et al. I New Astronomy
need to know the interstellar extinction toward y2 Vel. No direct measure of the extinction, e.g. by 2200 A nulling, is available in the literature. Eggen (1980) gave reddening values for stars in the y ’ Vel group as well as for foreground and background stars. We adopt Eggen’s reddening value for the foreground stars Eh_” = 0.01420.010 mag as valid for y2 Vel, and convert this with Eh_,, = 0.74E,_, for O-type stars (Crawford, 1975). and A, = 3.39E,_, (Lundstrom & Stenholm, 1984) to A, = 0.06+0.05 mag. This implies an absolute magnitude M, = - 5.4 mag for the y2 Vel system, and with Am = 1.4 mag, we find M,(WC8) = - 3.7 and M,(O) = - 5.0 mag. M, = - 3.720.3 mag for a WC8 star is a value one magnitude fainter than was derived from cluster and association membership of WC8 stars in van der Hucht et al. (1988), but more in line with the absolute visual magnitudes of WCE stars therein. M,(O) = - 5.020.3 mag indicates, in the O-star spectral type - M, relation of Conti et al. (1983), an 08.5111 (M, = - 5.OkO.5 mag) rather than an 091 (M, = - 6.1kO.8 mag) companion. We do not exclude, given the uncertainties in equivalent width measurements in WR + 0 composite spectra, that the line measurements of Conti & Smith (1972) could be reconciled with an 08.5111 companion. However, in the published illustrations (Conti & Smith, 1972; Niemela & Sahade, 1980) the 0 star looks like a supergiant. The lower luminosity of the 0 star may be an effect of its high rotation as implied from the width of the absorption lines. Given the lower luminosity of the O-type companion than thought before, we also have to favour lower binary companion masses, and therefore the radial velocity solution of Pike et al. (1983), rather than that of Niemela & Sahade (1980). Given the Hipparcos distance, the measured angular semimajor axis a = 4.3 mas (Hanbury Brown et al., 1970), and the binary period P = 78.519 days and mass ratio q = 0.35 (Pike et al., 1983), the total mass of the binary system is M(WC8 + 08.5111) = 30 M,, with M(O8.5111) = 22M,, and M(WC8) = 8 M,. These masses are lower than the minimum masses derived from the radial velocity solution by Niemela & Sahade (1980): 32 and 17 M, respectively, but
Table 2 Measured
2 (1997) 245-250
and derived parameters
of y * Vel
Parameter
Value
Ref.
d (PC) = l/n
258:;; 1.76 0.06kO.05 - 5.420.3 1.4t0.1 - 3.720.3 - 5.OkO.3 78.519LO.006 0.35 4.320.5 30216 22212 8+4 1550240 [3.5?0.73 x 1o-5
111
v (mag)
AU(mag) M, system (mag) Am system (map)
M”(WC8) (ma& M,(O8.5111) (mag) P (days) Y a (mas) M(WC8 + 08.5111) (M,) M(08.5111) (M,) M(WC8) u,(WCI) ni(WC8)
(M,) (kms-‘) (Moyr-‘)
121 131 r41
[51 [51 [61
[71 r81
References: [I] The Hipparcos Catalogue (ESA, 1997). [2] v from Conti & Vacca (1990), in the narrow-band photometric system of Smith (1968a) and Lundstrom & Stenholm (1984), with A, = 1.I I A,. [3] Eh_,, = 0.014?0.010 mag for foreground stars toward the y’ Vel group (Eggen, 1980), converted with E,_\ = 0.74E,_, for O-type stars (Crawford, 1975) and A, = 3.39&_, (Lundstrom & Stenholm, 1984). [4] Conti & Smith (1972). [5] Pike et al. (1983). [6] Hanbury Brown et al. (1970). [7] van der Hucht et al. (1996). [8] ‘Radio’ mass loss rate from Prinja et al. (1990). (1991), scaled to adopted velocity and Hipparcos distance.
the total mass of 32 M, is in good agreement with that found from the radial velocity study by Pike et al. (1983). Table 2 summarizes measured and derived parameters.
4. Mass loss rate, X-ray and y-ray luminosities Prinja et al. (1990), (1991) derived for y2 Vel a stellar wind terminal velocity u, = 1415 kms-‘, driving a ‘radio’ mass loss rate M = 8.1 X 10m5M, yr-‘, using distance, radio data, and intrinsic parameters given by Abbott et al. (1986). Scaling this mass loss rate to u, = 1550 kms-’ (from ISOSWS spectra of forbidden lines (van der Hucht et al., 1996)) and d = 258 pc measured by Hipparcos, we find for y* Vel a ‘radio’ mass loss rate &i = 3.5 X 10m5Ma yr-‘. This is in good agreement with the
K.A.
van der Hucht
et al. I New Astronomy
recent determination of the value ni = [2-31 X 10-5M, yrr’ by Stevens et al. (1996) from spectral fitting of ASCA X-ray spectra of y* Vel. The new ‘radio’ mass loss rate could be further reduced by a factor three or so due to possible clumping of the WR wind (Moffat & Robert, 1994), bringing it closer to the ‘polarization’ mass loss rate of y2 Vel derived by St-Louis et al. (1988). With the Hipparcos distance for y2 Vel, X-ray and y-ray luminosities derived from measured fluxes have to be scaled down by a factor (258/450)2 = 0.33, while theoretical estimates of observable fluxes have to be scaled up with that factor. Doing so for the ROSAT-PSPC data (Pollock, 1995; Willis et al., 1995) one finds Lx/L,= 0.002,not a very conspicuous value among the observed WR X-ray luminosities in Pollock (1995). It is the X-ray flux variability, however, caused by the colliding WC8 and 08.5111 stellar winds, which makes the X-ray observations of y * Vel of great importance (Willis et al., 1995; Stevens et al., 1996). COMPTEL ‘6A1 1.809 MeV y-ray images of the Vela region (Oberlack et al., 1994; Diehl et al., 1995) encompass the positions of the Vela supernova remnant and y2 Vel. The Hipparcos distance of y * Vel and its lower WC8 mass compared to most previous studies, indicating an even more evolved state, do increase the possibility of a sizable y-ray contribution from y* Vel. It is of interest to note that the distance to the center of the Gum Nebula, with an angular diameter of 36”, has been re-assessed at d = 290-+30pc with a radius of 902 10 pc (France, 1990). The Hipparcos measurements put y* Vel close to the center of the Gum Nebula and 5 Pup to the back of the Gum Nebula. With the ROSAT determination of the Vela SNR distance d= 400+200 pc (Aschenbach et al., 1995), theVela SNR could be in the center or at the back back of the Gum Nebula. For discussions on contents and origin of the Gum Nebula, we refer to Franc0 (1990) and Oberlack et al. (1994).
Acknowledgments We are very grateful to the staff of the Hipparcos
2 (1997)
245-250
249
operations team and the Hipparcos data-reduction consortia. We thank Peter Conti, Philippe Eenens, Arnout Van Genderen, Ian Howarth, Virpi Niemela, Chris Sterken, and Perry Williams for constructive comments.
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