A&A 408, 689-692 (2003)
DOI: 10.1051/0004-6361:20031089

Research Note

A WN4 companion to BD +62$^\circ$2296 in Cas OB5

I. Negueruela^1,2

1 - Dpto. de Física, Ing. de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
2 - Observatoire de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France

Received 19 March 2003 / Accepted 7 July 2003

Abstract
I report observations of the triple system BD +62$^\circ$2296 showing that all its components are early-type stars, most likely physically related. The faintest component BD +62$^\circ$2296B is a hitherto uncatalogued Wolf-Rayet star. The brightest component, star A, is shown to be a seemingly normal B2.5Ia supergiant. Long-slit spectroscopy of BD +62$^\circ$2296B shows it to be a narrow-lined WN4 star. Given the spatial separation, the two objects are unlikely to form a physical binary. Spectra of the third visual component, BD +62$^\circ$2296C, allow its classification as a B0III star. Such concentration of massive stars strongly suggests that BD +62$^\circ$2296 is in reality a very compact young open cluster in the area of the OB association Cas OB5.

Key words: stars: early-type - stars: distances - stars: Wolf-Rayet - stars: individual: BD +62$^\circ$2296 - stars: binaries: close - stars: binaries: spectroscopic

1 Introduction

During a recent survey in search of distant OB stars (Negueruela & Marco 2003), intermediate-resolution spectroscopy was obtained with the Aurélie spectrograph on the 1.52-m telescope at the Observatoire de Haute Provence (OHP) for a number of luminous stars in the region of the association Cas OB5. As part of this programme, a classification spectrum of BD +62$^\circ$2296A was taken. On site examination of the raw spectrum of BD +62$^\circ$2296A revealed the presence of a relatively strong and very broad emission line at the wavelength of He II  $\lambda 4686$ Å, completely unprecedented at this spectral type. A reference to BD +62$^\circ$2296B being a Wolf-Rayet star appears in Batarya et al. (1994), but no Wolf-Rayet star is catalogued at this position. As Aurélie offers no spatial resolution and the seeing during the observations was rather poor, further observations of the components of BD +62$^\circ$2296 have been taken with long-slit spectrographs. These observations confirm that BD +62$^\circ$2296B is a Wolf-Rayet star and suggest that BD +62$^\circ$2296 is a compact cluster of young stars.

In spite of its brightness, BD +62$^\circ$2296 has not been intensively studied, the only spectral classification for BD +62$^\circ$2296A (B3Ia?) dating back to Morgan et al. (1955). BD +62$^\circ$2296A does not appear to be photometrically very variable: Haug (1970) gives V=8.64, (B-V)=1.07, while Hiltner (1956) gives V=8.65, (B-V)=1.09. Its radial velocity, averaging $v=-60.3\:{\rm km}~{\rm s}^{-1}$, was found to be variable by Abt et al. (1972). BD +62$^\circ$2296 is catalogued as a visual triple system (Dembowski 1883). BD +62$^\circ$2296C, also catalogued as CSI +62$^\circ$2296 2, is clearly separated, situated approximately $11\hbox{$^{\prime\prime}$ }$ to the SE (see Fig. 1), while BD +62$^\circ$2296B is separated by less than 2 $^{\prime\prime}$. With such small separation, photoelectric measurements must contain both stars. The WDS catalogue gives magnitudes $V_{\rm T}=8.93$ and $V_{\rm T}=11.20$ from Tycho (Fabricius et al. 2002) for components A and B. These are untransformed Tycho magnitudes (see Negueruela & Marco 2003 for a discussion on the danger of using transformed magnitudes for reddened early-type stars).

2 Observations

Spectroscopy of BD +62$^\circ$2296A was obtained with the Aurélie spectrograph on the 1.52-m telescope at the Observatoire de Haute Provence (OHP) in January 2002. The spectrograph was equipped with grating #3 and the Horizon 2000 EEV CCD camera (see Negueruela & Marco 2003 for details). This configuration gives a dispersion of $\sim$0.22 Å/pixel, covering a wavelength range of $\approx$440 Å. BD +62$^\circ$2296A was observed in four spectral regions, centred on $\lambda=4175$ Å (Jan. 18), $\lambda=4680$ Å (Jan. 19), $\lambda=5850$ Å and $\lambda=6600$ Å (both on Jan. 20).

Table 1: Tycho positions (Fabricius et al. 2002) for the three components of BD +62$^\circ$2296.

Further spectroscopy was taken with the Carelec spectrograph on the OHP 1.93-m on the nights of 6th and 7th July, 2002. I used grating #1 (1200 ln mm^-1), which gives a nominal dispersion $\sim$0.45 Å/pixel in the blue range (resolution element $\sim$1.5 Å). During the first night, the central wavelength was set to $\lambda4420$ Å. In the second night, two settings, centred at $\lambda4450$ Å and $\lambda5450$ Å were used.

	Figure 1: Close-up of the area surrounding BD +62$^\circ$2296 (u-band image taken with the 2.2-m telescope at Calar Alto on October 24th, 2002, using the BUSCA camera). The components of BD +62$^\circ$2296 are marked as A, B and C. Stars #1 & #2 are B-type stars. Positions for the components are listed in Table 1.
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Finally, spectroscopy was taken on July 22nd-25th 2002, using the 2.5-m Isaac Newton Telescope (INT) at La Palma, Spain. The telescope was equipped with the Intermediate Dispersion Spectrograph (IDS) and the 235-mm camera. On the night of the 22nd, the detector used was an EEV10 CCD, while on the three other nights it was the Tek#5 CCD. The slit width was set to $1\hbox{$.\!\!^{\prime\prime}$ }2$ and the R1200Y grating was used. This configuration gives a nominal dispersion of $\approx$0.48 Å/pixel with the EEV and $\approx$0.8 Å/pixel with the Tek#5. During these nights, observations were performed with seeing slightly better than  $1\hbox{$^{\prime\prime}$ }$. Under these conditions, components A and B were clearly resolved. Exposures of all three components were obtained at different wavelengths, taking care that the other components were left outside the slit.

Table 2: Log of spectroscopic observations of the three components of BD +62$^\circ$2296. The brighter component A is a B2.5Ia supergiant; component C is B0III, while component B is WN4. Observations marked A+B were performed under seeing conditions not allowing the separation of the two components. All dates refer to 2002.

The complete log of observations is given in Table 2. All the data have been reduced with the Starlink packages CCDPACK (Draper et al. 2000) and FIGARO (Shortridge et al. 1997) and analyzed using FIGARO and DIPSO (Howarth et al. 1997).

Figure 2: Blue spectra of BD +62$^\circ$2296A (top) and BD +62$^\circ$2296C (bottom), taken on July 22nd, 2002, with the INT. The strength of N II lines and Mg II  $\lambda 4481$ Å in the spectrum of BD +62$^\circ$2296A identify the star as a supergiant. The spectrum of BD +62$^\circ$2296A taken with the 1.52-m OHP on Jan 19th, 2002, is superimposed as a dotted line. The emission feature at $\lambda 4686$ Å, completely unexpected at this spectral type, arises from the unresolved companion.

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3 Results

The spectrum of BD +62$^\circ$2296A obtained with Aurélie can be seen in Fig. 2, as a dotted line. The most striking feature is clearly the prominent broad emission line centred on the wavelength of He II $\lambda$4686 Å. This line is not seen at all at this spectral type, while its presence in emission is an indication of strong mass loss in bright O-type stars and Wolf-Rayet stars.

Unlike Aurélie, the long-slit spectrograph Carelec produces spatially resolved spectra. In the spectrum taken on July 7th 2002, with a seeing $\approx$ $2\hbox{$^{\prime\prime}$ }$ (estimated from the FWHM of the spectra of stars #1 and #2), components A and B are not entirely resolved. By extracting 5 pixels on the Northern side of the spectrum, centred 4 pixels away from the peak in the spatial direction, I obtained a spectrum of component B, displayed in Fig. 3.

Figure 2 shows a classification spectrum of BD +62$^\circ$2296A, taken with the INT, without any contribution from the WR star. The presence of moderately strong Mg II  $\lambda 4481$ Å confirms that the star is a mid-range B supergiant. The strength of the O II spectrum and the relative weakness of Mg II  $\lambda 4481$ Å support a spectral type around B3Ia. At B2Ia, Si III  $\lambda4552$ Å is almost as strong as He I  $\lambda4471$ Å and C II  $\lambda4267$ Å is still weak compared to the O II lines. BD +62$^\circ$2296A is therefore later and a spectral type B2.5Ia seems adequate. A lower luminosity class is precluded by the strength of the Si II  $\lambda\lambda4128{-}30$ Å doublet in comparison to the neighbouring He I lines (for classification criteria, see Walborn & Fitzpatrick 1990 and Lennon et al. 1990).

The INT spectrum of BD +62$^\circ$2296B is shown in Fig. 3. It is typical of a WN4 star. N III $\lambda$4640 Å is hardly seen, while N IV $\lambda$4057 Å and N V $\lambda$4604 Å have comparable strengths. All the other line ratios are compatible with this spectral type. The line widths are typical of a narrow-lined Wolf-Rayet. The red spectrum of BD +62$^\circ$2296B taken on July 25th displays broad strong emission lines corresponding to He II $\lambda$6560 Å and the N IV $\lambda$7103-7128 Å complex (the latter just on the spectrum edge). Very weak lines corresponding to He II  $\lambda\lambda$6406 & 6683 Å are likely present, as well. The presence of all these lines is in perfect accord with the spectral classification derived from the blue. Both the blue and red spectra show features attributable to component A, in spite of the orientation of the slit intending to leave it out.

Figure 3: Blue spectra of BD +62$^\circ$2296B. The bottom spectrum was taken on July 7th 2002 with the 1.93-m OHP telescope and represents the extraction of a 5-pixel-wide strip centred 4 pixels away from the spectrum peak and therefore includes an important contribution from the spectrum of BD +62$^\circ$2296B, which was not resolved in relatively poor seeing. The top spectrum was taken on July 25th 2002 with the INT and the slit was set on top of the WR star (which was resolved) and perpendicular to the axis joining the two stars. Though all emission lines are stronger, a small light contribution from the supergiant is evident (note also the lower spectral resolution).

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The only spectrum including BD +62$^\circ$2296B in the yellow region is that of July 7th. Unfortunately, it is less well resolved than the blue one from the same date, perhaps reflecting the larger difference in magnitude between the supergiant and the Wolf-Rayet star towards longer wavelengths. In spite of this, broad He II $\lambda$5411 Å and C IV $\lambda$5808 Å emission lines are seen, with comparable strength. He I $\lambda$5875 Å is weakly in emission, though its strength is difficult to assess due to its nearness to the edge of the spectrum. All these lines must be contributed by the WN4 star.

4 Discussion

The close companion to the B2.5Ia supergiant BD +62$^\circ$2296A, BD +62$^\circ$2296B, is a WN4 Wolf-Rayet star. Its Wolf-Rayet number will be WR 159 (van der Hucht, priv. comm.) BD +62$^\circ$2296 appears listed as an emission line star in the catalogue of Wackerling (1970). The H$\alpha$ spectrum of BD +62$^\circ$2296A is displayed in Fig. 4. There is a narrow P-Cygni profile, with an edge velocity of $v_{\rm edge} \approx 500\:{\rm km}~{\rm s}^{-1}$, superimposed on a broad emission bump. Such features are typical of blue supergiants of high luminosity and indicate the presence of a relatively slow radiative wind. From comparison of the OHP and INT spectra, there does not seem to be a significant contribution from the WR star to this emission feature.

	Figure 4: H$\alpha$ spectrum of BD +62$^\circ$2296A, taken on Jan. 20, 2002. Stellar features are marked, except for H$\alpha$, which clearly has two components, a narrow P-Cygni profile superimposed on a much broader base.
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From its spectrum (see Fig. 2), BD +62$^\circ$2296C is a B0III star. Its $V_{\rm T}$ magnitude is 10.74 from Tycho. The difference in magnitude with component A is compatible with their being at the same distance. For BD +62$^\circ$2296A, assuming the intrinsic colour of a B2.5Ia star (B-V)₀=-0.14 (Wegner 1994), a dereddened distance modulus DM=12.0 for Cas OB5 (Humphreys 1978) and a standard reddening law (R=3.1), we find M_V=-7.1 using the photometry from Haug (1970) (though this measurement very likely includes component B). This luminosity is typical of a bright supergiant of this spectral type.

The absorption lines in the spectrum of BD +62$^\circ$2296B appear to be due to light contamination from component A (for example, the characteristic C II doublet is seen in absorption close to He II $\lambda$6560 Å). This does not rule out the possibility that component C is itself a close binary. As a matter of fact, if it is at the same distance as component A, and its Tycho magnitude is correct, it would be rather more luminous than corresponds to its spectral type. The average absolute magnitude of a WN4 star is M_V=-3.5 (van der Hucht 2001).

At a distance in excess of 2 kpc, an angular separation of $\approx$ $2\hbox{$^{\prime\prime}$ }$ implies a distance between components A and B ($\approx$$5000\:$AU) far too large to be consistent with their being an interacting binary. The composite spectrum shown by BD +62$^\circ$2296A under moderate seeing serves as a cautionary tale when considering observations of more distant massive stars. Only because the system is relatively nearby are we able to resolve components A and B, in spite of their rather large spatial separation.

The fact that the three components of BD +62$^\circ$2296 are early-type massive stars can hardly be due to a chance alignment. As a matter of fact, two other objects which fell along the slit (marked #1 & #2 in Fig. 1) are also B-type stars. BD +62$^\circ$2296 is therefore likely a previously unrecognised compact young open cluster. A photometric study of this area will be presented in a future paper.

Acknowledgements

The author would like to thank all the staff at the Observatoire de Haute Provence for their support and friendliness. The INT is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias. The July 22nd observations were obtained as part of the ING service scheme.

The author would very much like to thank Dr. Karel van der Hucht for his many important remarks. The image shown in Fig. 1 was kindly provided by Dr. A. Marco. Many thanks to S. Clark, P. Crowther, A. Herrero and M. Urbaneja for their comments.

This research has made use of the Simbad data base, operated at CDS, Strasbourg, France. This research has made use of the Washington Double Star Catalog maintained at the US Naval Observatory.

During part of this work, IN has been a researcher of the programme Ramón y Cajal, funded by the Spanish Ministerio de Ciencia y Tecnología and the University of Alicante. This research is partially supported by the Spanish Ministerio de Ciencia y Tecnología under grant AYA2002-00814.

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