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Astron. Astrophys. 334, 873-894 (1998)

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4. Long-term photometric variations

There is a considerable body of photometric observations for P1724 that goes back nearly 30 years. We have collected the absolute photometry in Table 2, where we also list the observations by Warren & Hesser (1977) and Cutispoto et al. (1996), in addition to our own CTIO observations in the BVRI bands (Sect. 2) converted to the standard system, for use in the following section. The individual measurements by Cutispoto et al. (1996) have not been published previously, and were kindly provided by G. Cutispoto (priv. comm.). The 5.7-day periodicity in P1724 is evident in each separate data set over the entire span of the available observations, although strict coherence is probably not maintained. Indications of this from the radial velocity observations were already described above. Nevertheless, the spectroscopic data suggest that the spot characteristics are more or less stable for at least [FORMULA] rotation periods (1.5 years), which is the duration of those observations.


[TABLE]

Table 2. UBVRIJHKL absolute photometric observations of P1724


The situation over longer time scales is illustrated in Fig. 3. In addition to the absolute photometry in the visual band from Warren & Hesser (1977) and Cutispoto et al. (1996), we show all the differential photometry in the same band (MTSB, CTIO, and RW ; Sect. 2). Based on the discussions in previous sections, the period we have adopted for this comparison is [FORMULA] days. We fold and plot all the data throughout the remainder of the paper with the same period and epoch. The origin adopted for the phases is the date of the first of our Doppler imaging observations (see Sect. 6). The five light curves appear to be roughly in phase, although the uncertainty in the period is such that the relative phasing over such long intervals is difficult to establish precisely. For example, a difference of only 0.0005 days in P leads to a phase shift of [FORMULA] between the Warren & Hesser (1977) data (epoch [FORMULA]) and the more recent photometry, which we cannot rule out. Nevertheless, considering the long interval represented in fig. 3 ([FORMULA] 30 years), the phase coherence displayed is quite remarkable.

[FIGURE] Fig. 3. The 5.7-day periodicity in the brightness and the radial velocity of P1724 over 30 years. We show all the available photometry in the V band that covers a significant fraction of the cycle: a Warren & Hesser (1977); b Cutispoto et al. (1996); c MTSB; d CTIO; e  RW. Also shown in f are the radial velocities (RV) from CfA (open circles) and from our Doppler imaging observations (filled circles). All observations have been folded with the same period [FORMULA] days, and the vertical scale on the top five panels is the same to facilitate the comparison; the same origin is adopted here as in the Doppler imaging analysis below, namely, JD 2450067.70257; the mean epoch of the observations is shown in each data set

Fig. 4 shows a sample of the color information available for P1724, folded in the same way as Fig. 3. From Figs. 3 and 4, we see that the color of the star is bluer when it is brighter, consistent with rotational modulation due to spots.

[FIGURE] Fig. 4. Rotational modulation in the color indices. We show, from top to bottom, a [FORMULA] from Warren & Hesser (1977), b [FORMULA] from Cutispoto et al. (1996), c [FORMULA] from our MTSB data, d [FORMULA] from our CTIO data, e [FORMULA] from RW, and f equivalent width of the lithium 6708Å line (see Sect. 7 for details); ephemeris as in the previous figure

The relative phasing between the radial velocities (shown in the bottom panel of Fig. 3) and the recent photometry, which cover essentially the same time interval, is entirely consistent with this picture, as we show in more detail in Sect. 6. The minima/maxima in the RV curve (Fig. 3f) are shifted by [FORMULA] in phase compared to the photometry (Fig. 3a-e). This is exactly what one expects from a spot on the star: The max/min RV occurs not when the spot is crossing the center of the disk (when the photometry is at a minimum), but rather when the spot is between the disk center and the limb.

Because the Doppler shifts we measure are actually produced by the presence of surface features, it is difficult to determine the velocity of the center of mass of the star accurately. In principle the most representative values are those when the spot(s) are distributed symmetrically on the disk, either in front or behind, which corresponds to minimum or maximum brightness, respectively. Under this assumption we estimate from the curves in Fig. 3 a mean value [FORMULA].

The photometric observations for P1724 show some evidence for changes in the brightness at maximum light. Fig. 5 displays all the available data in the V band that are on the standard system. The dotted line drawn through the highest points suggests a dimming of roughly 0.15 mag over the past [FORMULA] years. In addition, the amplitude of the variation in the V band seems to have increased, as determined from sine curve fits to the data from Warren & Hesser (1977) ([FORMULA]), Cutispoto et al. (1996) ([FORMULA]), MTSB ([FORMULA]), CTIO ([FORMULA]), and RW ([FORMULA]). From our sine curve fits, we obtain [FORMULA] of 0.20, 0.20, 0.27, 0.36 and 0.34 mag, respectively, for the data sets listed above, with uncertainties [FORMULA] to 0.02 mag. Both of these changes - overall dimming and growing amplitude - are consistent with a gradual increase in the spot coverage and/or a decrease in spot temperature over time.

[FIGURE] Fig. 5. Long-term variations in the V band. The figure shows all available observations in the standard system, as a function of date. The dotted line drawn through the highest points suggests a gradual decrease in brightness

Our CfA radial velocity monitoring covered [FORMULA] years, i.e. many rotation periods. As discussed above, we do see some indication of changes in the spot(s) in those data. However, it is difficult to investigate the details of any phase drifts over such a long period ([FORMULA] 30 years) from photometric data sets that are rather sparse and inhomogeneous. We can only conclude here that the spot(s) seems remarkably stable, but we cannot rule out small phase shifts with the data available.

If the photometric and RV variability are indeed due to spot(s), then one would perhaps expect to see a much faster evolution in phase of the variable quantities, as suggested by the solar analogy, mainly due to spot migration. We point out, however, that P1724 is a very young object quite different from the Sun, and one should therefore not necessarily expect rapid spot evolution as in older stars. In fact, in the case of the wTTS V410 Tau, the spot distribution is dominated by a large de-centered polar spot that appears to be very long-lived - certainly lasting several years, and possibly more than a decade (Hatzes 1995). There are many other documented cases of long-lived spots lasting a decade, and even longer with very little change in the apparent rotational period (e.g., Hall & Busby 1990, Oláh et al. 1997).

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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