Infrared spectroscopy of Sakurai s object

Size: px

Start display at page:

Download "Infrared spectroscopy of Sakurai s object"

Laureen Harper
5 years ago
Views:

1 Mon. Not. R. Astron. Soc. 298, L37 L41 (1998) Infrared spectroscopy of Sakurai s object S. P. S. Eyres, 1 A. Evans, 1 T. R. Geballe, 2 A. Salama 3 and B. Smalley 1 1 Department of Physics, Keele University, Keele, Staffordshire ST5 5BG 2 Joint Astronomy Centre, 660 N. A ohōkū Place, University Park, Hilo, HI 96720, USA 3 ISO Science Operations Centre, ESA Astrophysics Division, PO Box 50727, E Madrid, Spain Accepted 1998 June 8. Received 1998 May 27; in original form 1998 February 27 1 INTRODUCTION Sakurai s object (V4334 Sgr) was discovered on 1996 February 20 and was originally reported as a possible slow nova (Nakano 1996), although it is clear that it began to brighten as early as 1995 January (Nakano 1996). Optical spectra obtained shortly after discovery (Cappellaro 1996) suggested a high-luminosity star, of spectral type early G, quite unlike that of a nova in eruption. The slow rise to maximum, over a period of 14 months, coupled with the absence of emission lines, argues against the nova interpretation. Moreover, shortly after discovery, Duerbeck & Pollacco (1996) reported a planetary nebula (PN) centred on Sakurai s object; the spectrum of the PN is typical of old, low- to moderate-excitation PNe (Duerbeck & Pollacco 1996). Radio observations reported by Eyres et al. (1998) add further support to this scenario. The visual light curve, the low-excitation spectrum and the PN led Duerbeck & Pollacco (1996) to suggest that this object may be undergoing a final helium flash. If this is indeed the case then Sakurai s object presents an unprecedented opportunity to follow the evolution of the star during such an event. We have obtained ground-based near-infrared (IR) spectra on three occasions in 1996 and 1997, primarily after the dramatic changes that occurred after 1996 October (Asplund et al. 1997a; Kerber, Gratl & Roth 1997), and we present these here along with an Infrared Space Observatory (ISO) 2 10 m spectrum. 2 OBSERVATIONS IR spectroscopy was obtained with the cooled grating spectrometer CGS4 on the United Kingdom Infrared Telescope on three occasions; 1996 April 25, 1997 April 21 and 1997 July 13. Wavelength ranges covered, together with standard stars, are listed in Table 1; ABSTRACT We present near (ground-based) and far (ISO) infrared spectroscopy of Sakurai s object. As in the case of the optical spectrum, between 1996 and 1997 April the near-infrared spectrum underwent a dramatic change to later spectral type, and there is some evidence that the spectrum continued to evolve during Molecular features of carbon-bearing molecules (CN, C 2, CO) corresponding to those seen in cool carbon stars are now prominent in the 1 2:5 m range, and the 12 C/ 13 C ratio is low. The ISO data demonstrate the presence of hot circumstellar dust at a temperature of 680 K. If the dust shell is optically thin, the dust mass is 2:8 10 ¹8 M. Key words: circumstellar matter stars: evolution stars: individual: Sakurai s object (V4334 Sgr) ISM: molecules. flux calibration is good to 20 per cent. Wavelength calibration was obtained from the spectrum of an argon arc lamp and is accurate to better than m. The temperatures assumed for the standard stars were 6300 K for BS6496 (F7V), 6600 K for BS6012 and BS6310 (F4V). In 1997 July, we also observed the H-deficient carbon star HD (spectral type C1,2; Stephenson 1973) in addition to Sakurai s object, to serve as an aid to interpreting the data on Sakurai s object. We are also obtaining spectra with ISO (see Kessler et al. 1996). We present here an ISO observation obtained with the Short Wavelength Spectrometer (SWS) using the Astronomical Observational Template (AOT) SWS-01; the observation time was 2124 s. See de Graauw et al. (1996) for a description of the instrument and its operating modes, and Salama et al. (1997) for an overview of its calibration and aspects of data analysis. A summary of observations is given in Table 1. 3 THE NEAR-IR SPECTRA The evolution of the spectrum in the HK bands is shown in Fig. 1; all spectra are rather noisy in the m region because of the effect of telluric water vapour. The data in this and all subsequent figures have been dereddened by EðB ¹ VÞ ¼1:15 (Eyres et al. 1998). This is larger than the value obtained by Duerbeck & Benetti (1996) but the higher value is more consistent with the radio data and the interstellar reddening in the direction of Sakurai s object (see Eyres et al. for detailed arguments). In 1996 April, the spectrum contained only weak absorption lines, mostly shortward of 2.2 m, but subsequent observations show dramatic changes in the IR spectrum, which mirror the spectroscopic changes weakening of the H lines and the appearance of the C 2 Swan bands that occurred in the optical (Kerber, Gratl & Roth 1997; Asplund et al RAS

2 L38 S. P. S. Eyres et al. Table 1. Log of observations. Object Date Facility Wavelength range ( m) Standard stars (mag) Sakurai 1996 April 25 UKIRT BS6496 (K ¼ 5:13); HD (K ¼ 7:02) Sakurai 1997 April 21 UKIRT , BS6012 (J ¼ 5:4; H ¼ 5:50); BS6496 (K ¼ 5:13) HD July 21 UKIRT , , Sakurai 1997 April 14 ISO Sakurai 1997 July 13 UKIRT , , BS6310 (H ¼ 4:83); BS6496 (J ¼ 5:40; K ¼ 5:13; L ¼ 5:10) Figure 1. HK-band spectrum of Sakurai s object on the dates indicated, in the wavelength range m. The spectrum of HD has been displaced vertically for clarity. Locations of principal molecular features are indicated. The feature at 2.0 m in the spectrum of HD is probably due to telluric CO 2 absorption. 1997a). In particular, the CO first overtone bands, which were absent in 1996 April, are strong in the 1997 spectra. The locations of the molecular features are identified in Fig. 1. The HK spectra obtained in 1997 show the Ballik Ramsay C 2 Rbandheads [ m (2,1), m (3,2); m (0,0), m (1,1)]. Further, in view of the fact that CN Dv ¼ 0is very prominent at 1.1 m (see below), it is possible that CN [ m (0,1), m (1,2), m (2,3)] also contributes. Note that the Ballick Ramsay C 2 (1,0) m bandhead and the m CN bandhead fall just outside our wavelength coverage, but we see the long-wavelength portion of the band. The first overtone CO features are seen in absorption, with both 12 CO and 13 CO present. There are also features around 1.6 m, which may be second overtone CO but C 2 (Dv ¼¹1) may also contribute (see above). The spectrum of the carbon star HD is also shown in Fig. 1 for comparison. We note that the 13 CO features are absent in HD137613; however this is not unexpected in view of the large ( 500) 12 C/ 13 C ratio for this star (Tsuji et al. 1991). A further point of interest is that the v ¼ 0 2 and 1 3 features in HD are weaker relative to those arising in higher v states, whereas in Sakurai s object they are of comparable, or even greater, strength. We finally note that a feature due to HCN is sometimes seen at 2 m in the spectra of carbon stars (see e.g. Goebel et al. 1980). There is clearly no such feature in the spectrum of Sakurai s object, although this is not entirely surprising in view of its hydrogendeficient nature (the apparent feature in the spectrum of HD also hydrogen-deficent at this wavelength is likely due to strong telluric CO 2 absorption at this wavelength). Optical data (Duerbeck et al. 1997) show that the spectral type of Sakurai s object was changing from mid F to early G just prior to our 1997 April observations and indeed, Fig. 1 suggests a distinct change in the continuum slope. We have determined the spectral index n, defined by f l l n, over the wavelength range m (where there are no major absorption features) and find that n changes from ¹1:54 (1977 April) to ¹0:75 (1997 July): the spectrum does indeed become redder, consistent with the change in spectral type seen in optical data. While we should regard this conclusion with a modicum of caution in view of the fact that different standard stars were used in 1997 April and July (see Section 2), the small differences in their temperatures are very unlikely to give rise to such large changes in the continuum. Spectra of Sakurai s object and of HD in the m range are shown in Fig. 2. There are several molecular absorption features. CN is prominent and the data clearly show the presence of the triple-headed bands Dv ¼ 0R 2 (bandhead at m), R 1 ( m) and Q 1 ( m), together with the weaker band s R 21 ( m). The C 2 Ballik Ramsay R (0,1) feature at m is clearly present in the spectrum of Sakurai s object in 1997 April, but seems to be absent in 1997 July although the actual bandhead lies outside our data range; it is also present in the spectrum of HD The Ballik Ramsay R bandheads at m (2,0) and m (3,1) are weak, but present, in

3 Infrared spectroscopy of Sakurai s object L39 Figure 2. The spectrum of Sakurai s object in the wavelength range m; the spectrum for 1997 July has been displaced upwards by 10 per cent for clarity, that of HD downwards by 10 per cent. Short tick marks indicate the locations of the Dv ¼ 0R 2,R 1,Q 1 and s R 21 bands of CN. Also indicated are the Ballik Ramsay C 2 R bandheads. Figure 3. The spectrum of Sakurai s object in the wavelength range m obtained on 1997 July 13; the spectrum of HD was obtained on 1997 April 21. both objects. A preliminary attempt at spectral synthesis (see below) shows that the feature at m is primarily Sr ii (l ¼ 1:0327 m), consistent with the overabundance of s-process elements reported by Asplund et al. (1997a); the feature at m is a blend of several C i lines. The spectrum in the m range is smooth (see Fig. 3) and we note in particular that there is no evidence for (i) emission in the unidentified infrared (UIR) bands (although in view of the spectral type of the central star, this is not unexpected), or (ii) any features that might correspond to molecules such as HCN (3.1 m). Our data suggest therefore that the absence of evidence for H-bearing species (e.g. HCN, UIR emission) indicates that the H deficiency of Sakurai s object is not a consequence of H being locked up in molecular (or particulate) form. We note however that there seems to be broad, weak emission over the wavelength range m and centred at 3:89 ð 0:01Þ m, which is not instrumental and for which we have no ready identification. 4 DISCUSSION 4.1 Spectral synthesis Shortly after discovery, the optical spectrum of Sakurai s object revealed that it is C-rich and H-poor (Benetti et al. 1996). Observations by Duerbeck & Benetti (1996) indicated a spectral type F2 I, and confirmed an overabundance of O and C with an

4 L40 S. P. S. Eyres et al. Figure 4. The synthesized spectrum in the wavelength range m (solid line); features are identified as in Fig. 2. The dotted curve is the spectrum of Sakurai s object for 1997 July. See text for details. Figure 5. IR flux distribution of Sakurai s object over the wavelength range m with the stellar contribution subtracted; data shortward of 2.5 m are from Figs 1 and 2, those longward of 2.4 m are from the ISO SWS. The significantly negative dust flux for wavelengths 2mm is a consequence of oversubtraction of the stellar flux as a result of uncertainties in the stellar effective temperature. The curve is the best fit to the ISO data of the function n b Bðn; T d Þ, with b ¼ 2:0 and T d ¼ 680 K. ISO data have been binned for display purposes into m bins. underabundance of H. Their data also suggested a possible IR excess, which they tentatively attributed to emission by dust at 3000 K, although this is uncomfortably hot for any common dust material. Major changes in the optical spectrum after 1996 October were reported by Asplund et al. (1997a); our observations indicate that corresponding changes were occurring in the IR. Asplund et al. (1997a) have carried out an analysis of the optical spectrum, and concluded that Sakurai s object is indeed hydrogendeficient, carbon-rich, and enriched in light s-process elements. They also noted that the chemical composition of Sakurai s object is rather similar to that of the RCB stars, and that the 12 C/ 13 C ratio in 1996 October was low. As a preliminary attempt to investigate the nature of the IR spectrum we have calculated some synthetic spectra. We used synthe (Kurucz 1993) to generate spectra covering the m region. We have used an effective temperature based on the optical photometry of Duerbeck et al. (1997), whose data cover the period 1994 April These authors conclude that Sakurai s object was continually cooling while increasing slightly in bolometric luminosity. They estimate that the effective temperature in 1997 April was 6000 K. We therefore adopted T eff ¼ 6000 K and log g ¼ 0:5 for our synthesis and assumed a microturbulence of y turb ¼ 5kms ¹1. Sakurai s object is extremely hydrogen-deficient and carbon-rich, and appropriate model atmospheres were discussed

5 Infrared spectroscopy of Sakurai s object L41 by Asplund et al. (1997b). A model atmosphere temperature structure with our adopted parameters was kindly made available in synthe format by Dr M. Asplund (private communication). In the synthesis, the elemental abundances were adjusted to agree with those given by Asplund et al. (1997a). The version of synthe available to us contains some rather dated continuous opacity sources, which meant that we could not get good quantitative agreement. However, the synthetic spectra generated did enable us to perform qualitative investigations of our data, and aided identification of features. Fig. 4 shows the synthesized spectrum in the m region (comparison with the actual spectrum longward of 1.4 m is not straightforward as dust emission dominates longward of 2mm; see Fig. 5). The synthesis shows that there are practically no regions in the spectrum where the emergent flux reaches the continuum level. This is mainly due to the significant amount of blanketing from carbon lines and carbon-bearing molecules (e.g. C 2, CO and CN). There is, however, a broad general agreement with the observations, although the details differ (see Figs 2, 4). The synthesis has enabled us to identify several features in the spectra, including C i lines around 1.07 m (Multiplet No. 1 in Moore 1972: 3s 3 P o 3p 3 D) and Sr ii at m (Multiplet No. 2 in Moore 1972: 4 2 D 5 2 P o ), along with several molecular bands. The strength of these lines is in agreement with that expected from the enhanced abundances of these elements (Asplund et al. 1997a). We will present a detailed analysis of the complex IR spectra of this object in a future paper. 4.2 The dust continuum After removing the photospheric flux, the flux distribution rises longward of 1.4 m, suggesting emission by hot dust (see Fig. 5). The star contributes significantly to the total flux shortward of 2:4 m and we have removed the stellar contribution from the ISO data using the photometry of Duerbeck et al. (1997). We have attempted to fit the resulting ISO data with a function of the form n b Bðn; T d ), where T d is the dust temperature and b is the b index for the dust, defined in the usual way in terms of the dust emissivity e n n b. The best fit is obtained with b ¼ 2:0 0:2 suggesting that the carbon is graphitic and T d ¼ K, where the uncertainty is 1j (see Fig. 5). If the dust shell is optically thin at the wavelengths of interest, we can estimate the mass of dust. Since Sakurai s object shows evidence for being carbon-rich, we assume the emissivity law for carbon used by Evans et al. (1997), which is based on the results of Martin & Rogers (1987). We find a dust mass of 1:9 10 ¹9 D 2 kpc M, where D kpc is the distance in kpc. Thus for a distance of 3.8 kpc (Eyres et al. 1998) the dust mass is 2:8 10 ¹8 M. We note that the dust mass associated with Sakurai s object is significantly greater than that associated with the similar object FG Sge ( 3:3 10 ¹9 M ; Woodward et al. 1993). 5 CONCLUDING REMARKS The observations of Sakurai s object presented here are consistent with the optical observations, namely a carbon star that is rapidly evolving. Absorption in the first overtone CO bands appeared sometime between 1996 April and 1997 April, and the spectrum became redder over the period 1997 April 1997 July. The ISO data indicate the presence of graphitic dust at temperature 680 K. Further observations of this unique object are highly desirable. Our IR observations of Sakurai s object are continuing with UKIRT and with ISO; details, together with a full analysis of the data and spectral syntheses, will be presented elsewhere. ACKNOWLEDGMENTS We thank Dr M. Asplund for helpful comments on an earlier version of this paper and for providing us with a model atmosphere for the hydrogen-deficient carbon-rich case. UKIRT is operated by the Joint Astronomy Centre on behalf of the UK Particle Physics and Astronomy Research Council (PPARC). Some of the observations described here were obtained as part of the UKIRT Service Programme. ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NAS. We acknowledge the efforts of the many individuals who have made ISO such an outstanding success. SPSE is supported by PPARC. The data were analysed using facilities provided for the Keele Starlink node. REFERENCES Asplund M., Gustafsson B., Lambert D. L., Kameswara Rao N., 1997a, A&A, 321, L17 Asplund M., Gustafsson B., Kiselman D., Eriksson K., 1997b, A&A, 323, 286 Benetti S., Duerbeck H. W., Seitter W. C., Harrison T., 1996, IAUC 6325 Cappellaro E., 1996, IAUC 6322 de Graauw Th. et al., 1996, A&A, 315, L49 Duerbeck H. W., Benetti S., 1996, ApJ, 468, L111 Duerbeck H. W., Pollaco D., 1996, IAUC 6328 Duerbeck H. W., Benetti S., Gautschy A., vangenderen A. M., Kemper C., Liller W., Thomas T., 1997, AJ, 114, 1657 Evans A., Geballe T. R., Rawlings J. M. C., Eyres S. P. S., Davies J. K., 1997, MNRAS, 292, 192 Eyres S. P. S., Richards A. M. S., Evans A., Bode M. F., 1998, MNRAS, 297, 905 Goebel J. H., et al. 1980, ApJ, 235, 104 Kerber F., Gratl H., Roth M., 1997, IAUC 6601 Kessler M. F. et al., 1996, A&A, 315, L27 Kurucz R. L., 1993, Kurucz CD-ROM 18: SYNTHE Spectrum Synthesis Programs and Line Data, SAO Martin P. G., Rogers C., 1987, ApJ, 322, 374 Moore C. E., 1972, A Multiplet Table of Astrophysical Interest, NSRDS- NBS 40 Nakano S., 1996, IAUC 6322 Salama A. et al., 1997, Proceedings of the First ISO workshop on analytical spectroscopy. ESA SP-419, p. 17 Stephenson C. B., 1973, Publ. Warner & Swasey Observatory, 1, 4 Tsuji T., Iye M., Tomioka K., Okada T., Sato H., 1991, A&A, 252, L1 Woodward C. E., Lawrence G. F., Gehrz R. D., Jones T. J., Kobulnicky H. A., Cole J., Hodge T., Thronson H. A., 1993, ApJ, 408, L37 This paper has been typeset from a T E X=L A T E X file prepared by the author.

Strong helium A Ê absorption in Sakurai's object (V4334 Sgr)

Strong helium A Ê absorption in Sakurai's object (V4334 Sgr) Mon. Not. R. Astron. Soc. 307, L11±L15 (1999) Strong helium 10 830-A Ê absorption in Sakurai's object (V4334 Sgr) S. P. S. Eyres, 1,2 B. Smalley, 2 T. R. Geballe, 3 A. Evans, 2 M. Asplund 4 and V. H. Tyne