Free Access
Volume 575, March 2015
Article Number A4
Number of page(s) 17
Section Stellar atmospheres
Published online 10 February 2015

Online material

Table 5

Hβ equivalent widths and fluxes for the members of γ Vel and Cha I observed with UVES.

Table 6

Mass accretion rates derived in the literature with different methods.

Appendix A: Cha I: comparing with the literature

In Fig. A.1, we compare the mass accretion rates from the literature with those computed in this work from the Hα EW (see also Table 6).

Hartmann et al. (1998) derived mass accretion rates from intermediate-resolution spectrophotometry of the hot continuum emission. Ten accretors are shared with us. Our values and those obtained by these authors agree within the errors with the only exception of J11072825–7652118, for which our acc is lower than the Hartmann et al. (1998) value by ~1.7 dex, but it is close to the values reported by other authors (see Daemgen et al. 2013). Three accretors of our sample have also been observed by Kim et al. (2009), who measured acc through U-band photometry. Differences between these values and our determinations are within ~0.3 dex, on average. Recently, Espaillat et al. (2011) have measured acc with a similar method to the latter authors; for the four accretors in common with, us a mean difference of ~0.5 dex is found. Seven accreting objects are shared with Antoniucci et al. (2011), who measured acc through the Brγ line. Four stars show similar mass accretion rates, while the values for the three targets with the lowest acc are higher than ours. Similar differences have also been found by Biazzo et al. (2012) for low-mass stars in Chamaeleon II. Robberto et al. (2012) have derived acc from Hα photometry for five accretors of our sample. The mean difference in acc between theirs and our value is ~0.7 dex. Costigan et al. (2012) report acc measurements using three different diagnostics (EWHα, 10%WHα, and EWCa iiλ8662) for four accretors in common with us. The agreement between their results and ours is good, especially when we consider the acc derived from their EWHα. The case of J11075809–7742413 is emblematic because they measure the highest difference in acc derived through the three methods, but the value obtained with EWHα is very close to ours. This suggests that the discrepancies in the acc values are mostly due to the method used for deriving it rather than to the different instrumentation used or to an intrinsical variability of the source. Finally, Daemgen et al. (2013) have observed three accretors in common with us and have adopted the Brγ line as diagnostics. The agreement with our values is fairly good, with the exception of 10555973–7724399 for which they have derived log acc = −7.4 M yr-1 at odds with our value of −9.1 M yr-1, which is more similar, within the errors, to the values of −8.8 M yr-1 and −8.4 M yr-1 obtained by Robberto et al. (2012) and Hartmann et al. (1998), respectively.

In conclusion, we think that the comparison between our acc, as derived from the Hα luminosity, and the literature values is in general quite good. The differences/inconsistencies can be attributed to intrinsic short-term and long-term variability (as outlined in Sect. 4.4) and the different photometric/spectroscopic methodologies used by each author to derive accretion luminosity and the mass accretion rate, as well as to the different evolutionary models adopted to estimate the stellar parameters (as also recently pointed out by Alcalá et al. 2014).

thumbnail Fig. A.1

Comparison between our acc values calculated using the EWHα and those obtained by several authors. Dashed and dotted lines represent the one-to-one relation and the position of the typical mean error in acc of ± 0.5 dex. The legend in the upper left corner explains the meaning of the symbols.

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