7. Optical spectra: Ca II, H, and lithium
In this section we investigate some youth and activity indicators for P1724, namely lithium absorption as well as H and Ca II H & K emission.
Two high-resolution spectra were obtained at the European Southern Observatory (ESO) on 29 and 30 Jan 1996 using the 3.6m telescope and CASPEC (Pasquini 1993 and references therein), covering the wavelength range from Å to Å . The 31.6 lines echelle grating was used together with the red cross-disperser (158 lines ) and the long camera (focal length , f/3). The above combination, together with ESO CCD #37 (TK1024AB, with pixels2 of ) and a slit aperture of ( on the sky), resulted in a nominal resolving power of . The slit height was set to , giving enough inter-order spacing to allow the subtraction of scattered light. Data reduction was performed using the echelle reduction package available within the Munich Image Data Analysis System (MIDAS, version Nov95), with the addition of some specially developed procedures making use of the algorithms prescribed by Verschueren & Hensberge (1990) for background subtraction and optimal order extraction. The reduction included tracing of the echelle orders, fitting and subtraction of the background from all frames, fitting of the blaze function and normalization to the continuum, extraction of echelle orders, wavelength calibration using thorium calibration lamp exposures taken at the same telescope position of each individual science frame, and merging of the orders.
Additional high-resolution spectra were obtained on 22 and 24 Jan 1997 using the KPNO 4.0m echelle 8. We used the long red camera and the T2KB CCD. Observations were taken through the ( 1.2 arc sec) slit. The first spectrum was made through thin cirrus, with about 1.5 arc sec seeing; on 24 Jan, the seeing was sub-arc second. Both sets of observations consisted of three 500 sec integrations. The spectra cover the range 4275 to 7385Å in 54 orders, at a resolution of . We obtained projector flat images to flatten the spectra. A Th-Ar comparison source was observed before and after each telescope slew. Initial reductions were undertaken at KPNO, using the IRAF doecslit package. We corrected for bias, extracted the orders, divided by the flats, and solved for the dispersion. The data were rebinned to a linear wavelength scale in each order and further reduced using IDL. The spectra were flattened in each order to remove any residual curvature left from the original flat division. We trimmed 510 points from the ends of the orders, leaving 1534 points per order. We then filtered the three individual spectra of each night to remove cosmic rays and co-added the spectra. The resulting spectra have S/N in excess of 100 per pixel near 6700Å , but the S/N is considerably less towards the blue. No attempt was made to reduce the counts to flux.
In addition to these relatively high-resolution spectra, we obtained three low-resolution spectra on 21, 25, and 26 July 1996 at the European Southern Observatory (ESO) using the 1.52m telescope equipped with a Boller & Chivens spectrograph. A 900 grooves/mm (ESO # 5) grating and the CCD FORD of pixels were used. With this set-up a mean resolution of Å (FWHM) in the 4400 to 7000 Å spectral range was achieved. The reduction of these spectra was carried out using the MIDAS package. Bias and dark subtraction was first performed on each frame. The 2-D frames were then divided by a mean flat-field and then calibrated in wavelength. Finally, sky subtraction was performed.
Another low-resolution spectrum was obtained with the Calar Alto Faint Object Spectrograph (CAFOS) at the 2.2m telescope of the Calar Alto Observatory, Spain, on 20 Dec 1996. We used the red grating G2, giving a mean resolution of Å (FWHM) in the spectral range of 3400 to 6300 Å . The reduction was carried out with the MIDAS package: bias and dark subtraction, dividing by a mean flat-field, wavelength calibration, and sky subtraction.
According to Soderblom et al. (1993), the presence of dark spots inhomogeneously distributed on the stellar surface can produce rotational modulation of the lithium 6708Å line strength since this line is quite temperature sensitive. Under extreme conditions, the abundance can appear to be larger by up to several tenths of a dex. We can check this with our data: The Li line should appear to be stronger when the spot is in front. For checking this, we use the (Li) values obtained with our high-resolution spectra at McDonald, ESO 3.6m CASPEC (see Table 3), and KPNO ( (Li) Å on 22 Jan 1997 and (Li) Å on 24 Jan 1997). Since the Li strength may be overestimated in low-resolution spectra (Covino et al. 1997), we do not use our low-resolution spectra for this particular investigation.
The data are displayed in Fig. 4f, which clearly shows rotational modulation of (Li): It appears to be lowest near phase 0.4, i.e. when the star is brightest, i.e. when the spot is on the back side, as expected. Rotational modulation of (Li) has not been seen before, except very recently in the wTTS V410 Tau: Fernández & Miranda (1998) have studied their own spectroscopic monitoring data of V410 Tau obtained almost simultaneously with published photometric monitoring by Petrov et al. (1994). Fernández & Miranda found a peak-to-peak variability in V of 0.6 mag and a peak-to-peak variability in (Li) of 0.12Å clearly in phase with the well-known rotation period of V410 Tau. Neither such a large variation in (Li) nor its rotational modulation has been seen in V410 Tau before, although the star had been observed often. Whenever other authors (Patterer et al. 1993, Welty & Ramsey 1995) monitored this star in V or (Li), its variation in both V and (Li) were significantly lower - except Strassmeier et al. (1997), who found a variation in V of 0.65Å . Also, a number of Pleiades stars monitored by Soderblom et al. (1993) that do not exhibit any rotational modulation of (Li) show much lower variation in V, namely below 0.1 mag. Hence, the larger the V mag variation, the stronger the rotational modulation of (Li) is, which is consistent with our results on P1724, where we see a large V -band variation together with rotationally modulated Li 6708Å line strength.
Other absorption lines also appear to be variable, namely the Fe I 6431 and Ca I 6439 lines used in the DI analysis (see Table 3). However, these lines are slightly weaker than the Li I 6708 line making it more difficult to detect rotational modulation with confidence. Furthermore, these lines are not as temperature sensitive as the lithium line. We would thus expect a smaller amplitude variation. Rotational modulation is therefore difficult to quantify for these lines with the given S/N.
To obtain the proper lithium abundance, we must use the (Li) value near phase 0.4, when it is not affected by the spot. This gives (Li) Å . From the NLTE curves of growth from Pavlenko & Magazzù (1996), and using the measured gravity, effective temperature, and the above lithium equivalent width, and considering the errors, we obtain the lithium abundance of N(Li) , in the customary scale where (H) . This is consistent with the primordial value, to be expected for this star with its very young age and its relatively high mass 9.
An additional timeseries of the chromospheric lines is shown in Fig. 11, from the observations listed in Table 3. In Figs. 10 and 11, we see a wide variety of profile characteristics: broad and narrow emission line components, asymmetries, double-peaked profiles, multiple absorption components, and variable flux. We seek to determine whether this variability is rotationally modulated and if so, whether it is correlated with the photospheric active regions mapped above.
The profile of an inactive MK standard of the appropriate spectral type (K0) is rotationally broadened and subtracted from the H profile at each phase in the timeseries for the purpose of measuring the chromospheric line flux. The true underlying photospheric absorption, however, is most certainly influenced by the temperature inhomogeneities on the stellar disk as are other absorption lines. We proceed under the assumption that the disk integrated equivalent width of the photospheric absorption is, to first order, preserved as the star rotates. This assumption is valid for nearly all of the photospheric lines and deviations from this should be small.
The residual line flux is then integrated within the window bounded by the dotted vertical lines in Fig. 11. We cannot directly compare these quantities since each was computed relative to a different photospheric flux. We instead convert the residual line flux to an absolute line flux, , by estimating the absolute stellar flux in the continuum at 6562.808Å at each phase in the timeseries. This is done in the following manner: Differential photometry (Sect. 2) and the image reconstruction give us an estimate of the stellar flux at 6562.808Å relative to an arbitrary zero-point. This relative flux is expressed as a fraction of the mean (or zero-point). The zero-point flux is then estimated using the Kurucz stellar atmosphere models (Kurucz 1994), interpolated to give the closest match to P1724. The Kurucz models have been found to be quite adequate spectrophotometric proxies of young cluster stars down to (Clampitt & Burstein 1997).
The resulting line fluxes at each rotational phase are listed in Table 3 and plotted in Fig. 12. While the chromospheric emission, as measured in , does appear to be rotationally modulated, the peak of the emission does not coincide with the phase of spot transit. Since no IR excess emission has been detected in the star's SED (Fig. 6), we assume that there is no disk accretion contributing to the variability in . More likely, the emission is chromospheric in origin and the variability should be explained in that context. The average line flux is typical of wTTS.
Using the solar analogy, we expect that plages in the upper atmosphere, seen as bright regions in , might contribute to the rotational modulation of the line flux. Solar plages are found to be roughly co-spatial with underlying photospheric spots. If indeed plage regions are modulating the flux in P1724, they are not precisely co-spatial with photospheric active regions. However, we note that (a) the timeseries was taken over a period of seven stellar rotations; longer-term variations could be contaminating the flux measurements; (b) the variations seen during the third stellar rotation cannot be explained by measurement uncertainties alone; it appears that smaller amplitude flux variations do occur on timescales less than one stellar rotation; and (c) the profiles consist of both a narrow and broad emission component; the latter could be a consequence of the Stark effect and may indicate microflaring events (cf. Montes et al. 1997). Resolving the various contributions to the line flux and determining their spatial relation to photospheric active regions will require better time resolution and an independent analysis of the broad and narrow line contributions.
In the case of V410 Tau, Fernández & Miranda (1998) detected variability clearly modulated by rotation. They found very strong evidence from the line profiles, that this modulation is due to rotation. However, in the past, this modulation has not always been detected, sometimes the modulation is present, sometimes it is not detectable, although the spot appears to be stable over several years. Hence, while a spot may be stable for many years, plages are not, as discussed in Fernández & Miranda (1998). It might be possible to detect rotationally modulated emission in P1724 in the future, if it were to form sufficiently large plages.
In most of the high-resolution spectra which cover the 6000 to 6800Å region of the spectrum (Table 3; Figs. 10 e-h and Å , Å , and in the core of . We identify the first two features as [NII] 6583.6 Å and [OI] 6300.3Å lines. The [OI] line falls at a constant heliocentric wavelength and is incompletely subtracted night sky emission (this line is not seen in the sky-subtracted ESO-3.6m CASPEC spectra). However, the [NII] and the narrow emission components, after heliocentric correction, are consistent with the rest velocity of P1724. These lines are too narrow to originate in the stellar atmosphere, but must be physically or dynamically related to P1724.
In the KPNO echelle slit spectra, the narrow component and the [NII] line are both spatially extended, but neither fill the 4 arcsec decker. The stellar continuum has a spatial FWHM of 0.91 arcsec near [NII], while the [NII] line has a FWHM of 1.01 arcsec. The line was not as well focussed, with spatial widths of 0.96 and 1.12 arcsec, respectively. The broad emission is no more extended than the stellar continuum. Uncertainties in the spatial width are about arcsec. This suggests that these narrow emission lines arise in an extended region about half an arcsec in extent. There is a small nebulosity projected around P1724 (see, e.g. the POSS image in Fig. 3 of Preibisch et al. 1995); the narrow emission lines could arise in a portion of the nebulosity adjacent to the star. However, this emission cannot be wind, as P1724 is not that massive. Also, P1724 is not hot enough to excite nebula emission. The apparent Å variation in the equivalent width of the [NII] line could be a combination of rotational modulations in the stellar continuum flux (which can account for only Å variation in the equivalent width) and varying degrees of success in background subtraction.
Finally, we obtained an additional spectrum in the blue wavelength range on 20 Dec 1996 with the CAFOS at the Calar Alto 2.2m telescope, with a similar set-up (except using the blue grating G1) and reduction procedures as above. Fig. 13 shows this spectrum with Ca II H&K emission filling in the absorption lines. We estimate the equivalent widths of these lines to be Å and Å , rather typical for active young stars.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998