UV spectroscopy of T Tauri stars: analysis of LH 332-21 and RY Tau. Vitor M. Costa 1, David H. Brooks and M. Teresa V. T. Lago 2 Centro de Astrosica da Universidade do Porto, Rua das Estrelas s/n, 4150-Porto, Portugal Alessandro C. Lanzafame Istituto di Astronomia, Universita di Catania, Viale Andrea Doria 6, I-95125, Catania, Italy Abstract. We report on recent results from our study of IUE (International Ultraviolet Explorer) data of a group of T Tauri stars (TTS). Since the electron densities in their atmospheres are expected to be higher than solar it is important to account for the inuence of density dependent processes and metastable levels. We have improved the treatment of these eects in the analysis of TTS for the rst time by using the ADAS software package (Summers, 1994) whose atomic models and data are based on Collisional-Radiative theory. Here we focus on results for two objects: RY Tau and LH 332-21. Using these models and data we derived the emission measure (EM) distribution for each star. We nd mismatches between the theoretical and observed uxes which may be due to errors in the atomic data or the failure of physical assumptions in the method. We also nd that the distribution for LH 332-21 is 1.5 orders of magnitude (in log) larger and so we assess possible explanations for this dierence. Following geometrical arguments, we suggest that the UV emission in this star is formed in a more extended region. 1. Introduction T Tauri stars are characterised by irregular and peculiar spectra consisting of various emission lines superimposed on late K to early M type absorption spectra. These young low mass and extremely active stars have been studied from radio to X-ray bands because of their importance in the understanding of the overall picture of stellar evolution and of the young Sun. The ultraviolet is a rich spectral range for such study. It includes many emission lines at temperatures from a few 10 4 K up to 2 10 5 K allowing the study of plasma in dierent physical conditions. In this context, emission mea- 1 Departamento de Matematica, Instituto Superior de Engenharia do Porto, Portugal 2 Departamento de Matematica Aplicada, Fac. Ci^encias da Universidade do Porto, Portugal 1
sure (EM) and dierential emission measure (DEM) techniques are powerful tools which provide insight into the structure of the atmospheres of these stars and consequently the mechanisms responsible for the strong observed UV emission. If one physical mechanism or type of structure dominates the emission in a sample of stars one would expect similarities in the EM distributions. In a previous work (Brooks et al. 1999) we reported on an analysis of IUE data of the classical T Tauri star BP Tau. The aim of the analysis was to introduce improved atomic models and data, tested in the laboratory and solar context, to the EM and DEM analyses of the atmosphere of this star. Our objective in the present work is to extend the analysis to other TTS in the IUE database. Here we present a comparison of EM distributions for LH 332-21 and RY Tau. 2. IUE Observations We have used low dispersion calibrated spectra of the stars RY Tau and LH 332-21, selected from the IUE Newly Extracted Spectra (INES) archive. Their spectra are fairly typical for TTS including emission lines from SiIII, SiIV, CIV, HeII, SiII, FeII and MgII. For each line of interest we measured the ux and corrected it for the eects of attenuation by interstellar absorption. The results appear in table 1. RY Tau LH 332-21 CII - 6.11.2(-12) SiII 3.90.1(-13) 1.90.2(-11) CIII 3.60.1(-13) - SiIII/OI 4.30.05(-13) 1.30.2(-11) SiIII] 4.30.3(-13) 2.60.07(-11) SiIV 3.20.5(-13) 1.40.2(-11) OIII] 2.50.9(-13) 7.01.5(-12) CIV 3.90.2(-13) 3.70.2(-11) Table 1. The emission lines in the low resolution ultraviolet spectra of the T Tauri stars obtained with the IUE. The ux is corrected for absorption by the interstellar medium. The uncertainties are given and the numbers in brackets are powers of ten. The units are erg cm?2 s?1. 3. Emission Measure Analysis A full explanation of the EM technique together with a comparison of methods was given by Brooks et al. (1999). We use their method, slightly altered as described below. We adopt a collisional-radiative model and use a modied version of the methods of Jordan et al. (1987) and Laming et al. (1995). Briey, 2
for an atomic transition j to i we compute the EM as F j!i = 1 n j n H n ion P 8d 2? j n A j!i A(z)EM (1) j n 2 e n el where F is the ux, d? is the stellar distance, A(z) is the elemental abundance, EM is the emission measure, n e is the electron density, n H is the hydrogen density, n ion is the total population of the ionisation stage ion, n j is the population of the excited state j, A j!i is the spontaneous radiative decay rate, n el is the total population of the element and R G(Te )dt e = G(T p ) R (2) dt e where T e is the electron temperature, G(T e ) is the contribution function and T p is the temperature of the peak of that function. Following Jordan et al. (1987) we compute the locus of EM which gives an upper limit to the EM distribution. This is done by calculating the EM at a set of temperature points around T p and assuming the entire line is formed at those temperatures. We chose an interval of 0.5 in log T e. In order to estimate the EM reliably we need to determine the electron density in each of the stars' atmospheres. The only line ratio diagnostic available in our spectra was that involving two lines of SiIII: the forbidden 1892A and the allowed 1303.5A multiplet. However, we do not consider this ratio to be reliable due to blending of the last component with OI. In the case of BP Tau we made comparison with results from a DEM analysis and found agreement to around a factor of 2. Here we have attempted to derive an electron density from the EM results by bringing the EM loci for the forbidden lines into agreement with the general trend. For both LH 332-21 and RY Tau the line ratio gives a value of 2.5 10 11 cm?3, so we have used this value in the computation of level populations, ion fractions etc. Figures 1 and 2 show the EM loci distributions for both stars against temperature. The two stars appear to have very similar distributions although the values for LH 332-21 are about 1.5 orders of magnitude (in log) higher than in RY Tau. The SiIII 1303.5A loci appear to lie above the general trend which may be due to the blending with OI. The CIII loci in RY Tau are also rather high and since this line does not appear in the LH 332-21 data it is not possible to say if a similar eect would be observed. However, the absence of the CIII line in LH 332-21 does imply that the EM for RY Tau would have to be well below the upper limit indicated for the distributions to match. Figures 1 and 2 also show the variation of the EM with density for the forbidden lines (OIII] and SiIII]). The other lines did not show any signicant variation. Results are presented for densities of 1 10 4 (representing the low density limit), 2.5 10 11 and 1 10 12 cm?3. Evidently the EM loci decrease with decreasing electron density in both stars. However, in neither case do they fall into agreement with the general trend. We investigated the eects at high density and found that the EM loci continued to increase. The disagreement between theory and observation then points to problems in either the atomic data or the physical assumptions in the EM technique, e.g. ionisation equilibrium, constant elemental abundances etc. This is discussed further in section 5. 3 line
Figure 1. The EM loci plotted against temperature for LH 332-21. Figure 2. The EM loci plotted against temperature for RY Tau. 4
4. Opacity Since the electron densities in these two stars appear somewhat higher than in the Sun it is evident that optical depth eects could alter the emergent uxes. We have thus investigated the inuence of opacity on some of the lower temperature lines used in the analysis. Other lines may also be aected and we are currently investigating this issue. We have used simple escape factor techniques to estimate the optical depths in the lines. The techniques are described elsewhere (Brooks et al. 1999, Loch 1999, Fischbacher 1999) and are implemented as a computer code as part of the ADAS package. We used the EM results, assuming solar coronal abundances, to calculate column densities for CII and SiII. These values, along with various geometrical options, were entered into the ADAS code which computed the optical depths and population escape factors. For CII in RY Tau the optical depth is 24.7. For SiII it is 3.43. In this case we chose an extended slab geometry to approximate a layer of atmosphere. The population density was chosen to peak at layer centre and decrease parabolically toward the edges. The optical depths were used to calculate layer averaged emergent ux escape factors which were then used to compute an opacity modied EM, G*(T e ). The observed ux was then combined with the G*(T e ) to compute the modied EM. Figure 3 shows the EM loci at a density of 2.5 10 11 cm?3 (solid lines) with the modied loci overplotted as dashed lines. Clearly, although there are substantial dierences in the results, the eect of opacity is not sucient to account for the dierence between the two distributions, at least for the geometry considered. In addition, since the escape factor falls with optical depth the result would be to increase the EM values for LH 332-21 by greater amounts than in RY Tau (due to the higher column densities) thus increasing the already considerable discrepancy. 5. Discussion There are clearly unexpected variations in the EM distributions due to the forbidden lines and the SiIII line and additionally the CIII line in RY Tau. As mentioned, in the case of SiIII this could well be due to blending but the fact that the other lines do not follow the general trend (for any density) may point towards unreliabilities in the uxes, uncertainties in the atomic data or failures in one or more of the physical assumptions made by the method. To investigate the former we computed the EM loci for 1 ORI using the uxes given by Jordan et al. (1987) and found similar results to those presented there i.e. we found no discrepancy for the forbidden lines and were able to bring the lines into agreement at similar densities to their results. This suggests that the problems are not in the atomic data but rather are in the assumptions made in the method. We are investigating this issue further. Another interesting result is that none of the processes investigated accounts for the sizeable dierence in the magnitude of the EM between the two stars. If the electron densities are in fact the same (as suggested by the line ratio), the dierence would be due to the volume of the emission region. Assuming 5
Figure 3. The EM loci distribution for RY Tau with the CII 1335A and SiII 1816 A lines modied for the eects of opacity as described in the text. spherical symmetry, and noting that the radii of the two stars are nearly the same, we nd that the vertical extent of the emission region in LH 332-21 would be 50 times greater than in RY Tau. Since the distributions are very similar we could well have similar processes operating in the two stars but over a more extensive region in LH 332-21. Acknowledgments. VMC would like to acknowledge the award of nancial support to attend the conference from the LOC. This work was partially supported by the Portuguese Fundac~ao para a Ci^encia e Tecnologia through grant PESO/P/PRO/1196/97 and PESO/P/INF/1197/97. References Brooks D.H., Costa V.M., Lago M.T.V.T., Lanzafame A.C., 1999, MNRAS, 307, 895 Fischbacher, G.A., 1999, PhD Thesis, Univ. of Strathclyde - in preparation. Jordan, C., Ayres, T.R., Brown, A., Linsky, J.L., Simon, T., 1987, MNRAS, 225, 903 Laming, J.M., Drake, J.J., Widing, K.G., 1995, ApJ, 443, 416 Loch, S., 1999, PhD Thesis, Univ. of Strathclyde - in preparation. Summers H.P., 1994, JET Internal Report, JET-IR (94)-06 6