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Messina 1R. Santallo 2T. Elliott 4 ,5G. Feiden 6A. Mauas 7R. Petrucci 8 ,9 and E. Sofia, 78 Catania, Italy e-mail: sergio. Received: 15 March Accepted: 13 December There are a variety of different techniques available to estimate the ages of pre-main-sequence stars.
Components of physical pairs, thanks to their strict coevality and the mass difference, such as the binary system analyzed in this paper, are best suited to test the effectiveness of these different techniques. We aim to investigate which age-dating technique provides the best agreement between the age of the system and that of the association. We have retrieved their Li equivalent widths from the literature and measured their effective temperatures and luminosities. We find that the rotation periods and the Li contents of both stars are consistent with the distribution of other bona fide members of the cluster.
We explore the origin of the discrepant age inferred from isochronal fitting, including the possibilities that either the two components may be unresolved binaries or that the basic stellar parameters of both components are altered by enhanced magnetic activity. The latter is found to be the more reasonable cause, suggesting that age estimates based on Li content are more reliable than isochronal fitting for pre-main-sequence stars with pronounced magnetic activity.
The circumstance in which both components of the system are coeval but with different basic properties offers the advantage of putting further constraints on the age determination techniques. Song et al. Measurements of the projected rotational velocity see Table 1 indicate that both stars are fast rotators. Fast rotation, enhanced magnetic activity, high Li abundance in the case of TX Psa all suggest that these are very young stars.
In fact, the isochronal age inferred by Song et al. Table 2 Properties of the selected comparison stars. To use age-dating tecnhiques based on rotation and Li EW, which is also affected by rotation, and to compare the calculated ages with the above mentioned estimates, we have carried out a photometric monitoring campaign. Our photometric monitoring was carried out from September 19 to November 2, for a total of 21 nights. On each night we collected on average three consecutive frames in the V filter for a total of 57 frames using an integration time of sec per frame.
We used tasks within IRAF 1 for bias correction and flat fielding, and the technique of aperture photometry to extract magnitude time series for the targets and for other stars detected in the frames, that were selected as candidate comparison stars. We identified two stars that were found to be non-variable and were used to build an ensemble comparison C1 and C2 in Table 2. After averaging the three consecutive differential magnitudes obtained on each night collected within a time interval of about 15 minwe obtained 21 average V -band differential magnitudes for Red vs blue dating psa component of the binary system for the subsequent analysis.
We used the BVR Red vs blue dating psa, and I filters and collected a total of 37 frames per filter.
Data Red vs blue dating psa and magnitude extraction were carried out as outlined in Sect. Top-middle panel : Lomb-Scargle periodogram solid line. Top-right panel : clean periodogram. Bottom panel : light curve phased with the rotation period.
The uncertainty associated with each point is smaller than the symbol size. The image scale obtained is 1. Observations were collected with the R c filter using s integration time. Observations were carried out from July 14 to September 7, for a total of 9 nights during which we achieved a total of frames. After averaging consecutive magnitudes collected within a time interval of about 30 min we obtained 48 average R -band magnitudes for the subsequent analysis.
Differential magnitudes of the targets were obtained using an ensemble comparison consisting of C1, C2, and C3 see Table 2. Use of two independent approaches allows us to be more confident in the interpretation of the of the periodogram analysis. The FAP was estimated using a Monte-Carlo method, that is, by generating artificial light curves obtained from the real one, keeping the date but permuting the magnitude values. In the top-right panel, we plot the Clean periodogram where the power peak arising from the light rotational modulation dominates, whereas all secondary peaks, arising from the aliasing, are effectively removed.
Following the prescription of Lamm et al. In the bottom panel, we plot the light curve phased with the rotation period. In Fig. The small difference between the two rotation periods may arise either from the effect of active regions growth and decay or from presence of surface differential rotation. Bottom panel : RV curve phased with the rotation period. The red solid line represents the sinusoidal fit with the rotation period. The phased light curve from PEST shows that Red vs blue dating psa is not available at some parts of Red vs blue dating psa phase. Moreover, we note that the amplitude of spot induced photometric variability in the V -band is larger than that in the R -band.
We Red vs blue dating psa a series of high-precision radial velocity measurements of both components from Bailey et al. The measured dispersions have the same order of magnitude as those arising from magnetic activity jitters at infrared wavelengths Bailey et al. We performed Lomb-Scargle and Clean periodogram analyses of these series. When RV values are phased with this period see Fig.
The similarity between this period and the rotation period actually, in the case of Lomb-Scargle the RV and photometric periods are equalsuggests that the radial velocity variation is induced by the stellar activity. For both targets we find no evidence of IR excess, whereas a ificant far- and near- UV flux excess is observed. Owing to their excess, fluxes in the Unear- UVand far- UV bands were excluded from the fitting procedure.
To date, the fit of the LDB represents the most reliable absolute age-dating technique. Messina et al. Whereas in Messina et al. In the left panel of Fig. In the right panel of Fig. Then, to make the comparison with the observations, we transformed the model Li abundance into Li EW. For this purpose, we have used the curves of growth from Zapatero Osorio et al. A more detailed discussion on the modeling with the Feiden and the Baraffe et al.
We refer the reader to Messina et al. Circled symbols are stars hosting a debris disk. Solid and dotted lines represent polynomial fits to the median period distribution and to its Red vs blue dating psa and lower boundaries from Messina et al. This age inferred from LDB fitting is slightly older but still comparable within the uncertainties with the age of the whole association. However, it is worth noticing the extreme model dependency of such ages.
In fact, if one instead adopts non-magnetic models then a much younger age is derived see e. We note that rotation has a key role in the Li depletion mechanism. First, Soderblom et al. Similar have been recently found by Bouvier et al. The same Li depletion-rotation connection has been found by Messina et al. It is worth noting that contrary to the observational evidence, theoretical studies Eggenberger et al. Low-mass stars in young clusters and associations exhibit a distribution of rotation periods that primarily depends on age, mass, and among other factors on the initial rotation period and the disk lifetime.
Starting from the higher-mass F and G stars, the width of the period distribution progressively decreases as far as the age increases see, e. In contrast to the technique based on the LDB, owing to the width of period distribution at any given mass and the uncertainty associated with the age of the benchmark association and clusters, a precise absolute age measurement of WW Psa and TX Psa based on rotation period is not possible. Nonetheless, we can check whether their rotation periods fall within the distribution of all other bona-fide members or are outliers. However, since WW Psa and TX Psa also fall on the upper boundary of the distribution of close binary and multiple members, we may be dealing with two components that are themselves unresolved close binaries.
Therefore, the rotation periods of WW Psa and TX Psa are compatible with those of other bona fide members, although there is some hint of a possible unresolved binary nature of both components. Dashed lines are isochrones whereas blue solid lines are the evolutionary tracks. Models are from Baraffe et al. Luminosity and effective temperature are the last age dependent quantities that we fit with isochrones to infer the age of WW Psa and TX Psa. The observed magnitude, distance, and bolometric correction are used to derive the luminosity that, together with the effective temperature, allows us to compare the positions of both components on the Hertzsprung-Russell HR diagram with isochrones from different evolutionary models.
The inferred masses, as typed in Fig. The discrepancy is only partly mitigated in the Dartmouth models, showing that the magnetic fields play some role in these very active stars through inhibition of stellar convection, which in stars with larger radius and higher luminosity at a given age, with respect to non-active stars. This residual age discrepancy suggests that some other physics, such as starspots e. We note that both x - and y -scales are logarithmic. The radial velocities of both components have been monitored quite extensively. Elliott et al.
The spectrum is converted to a numerical mask so that the continuum has a value of 0 and the absorption lines have the value one at their peak. The mask is then convolved with the observed spectrum of desired target to create the cross correlation function. The quoted uncertainties are calculated from the standard deviation of individual radial velocity measurements, as opposed to direct measurement uncertainties. It is true that the associated uncertainties are larger than those presented in Elliott et al.
However, from the recalculation of the radial velocities using the M-type template there was more variability in the resultant values. This could be explained by a better match of the numerical mask with the observed spectra combined with the effect of star spots that can mimic radial velocity variance see Lagrange et al. Both standard deviations are larger than the uncertainties associated with the single measurements and, as discussed earlier, also larger than the expected RV variations due to jitters generated by the magnetic activity, which in the infrared are of the order of 0.
However, the possibility of Keplerian origin of the low-amplitude RV variations remains quite marginal.Red vs blue dating psa
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