General Discussion

The veiling observed at optical wavelengths is explained by the presence of continuum radiation which, in the Boundary Layer accretion model is thought to arise in the boundary layer itself [Basri & Bertout 1989], and in the Magnetospheric Accretion model is thought to result from the accretion shock [Guenther & Hessman 1993a]. The optical veiling spectral energy distribution is consistent with temperatures of about 9000 K [Hartigan et al. 1991], implying that the shock veiling at J is one tenth of that at V [Hartigan et al.1995].

In Figure 4.8 is plotted the veiling at V obtained by Gullbring et al. (1998) and by Hartigan, Edwards & Ghandour (1995) versus the J and K veiling obtained here. Some degree of correlation seems to be present in the sense that the higher the veiling at V the higher the near infrared veiling. It is clear from the plots in the top panels that r_J = 0.1 r_V is not verified, since the data points lie above the line that corresponds to that relationship between r_J and r_V (dotted line in Figure 4.8). The J veiling is in fact much higher than expected if it was only due to the accretion shock.

Accretion disks in T Tauri stars contribute significantly to the spectral energy distribution in the near infrared region of the electromagnetic spectrum of these stars. One might expect that the veiling in the near infrared wavelengths has a contribution from the accretion shock and another from the disk. Such contribution might be responsible for the higher than expected veiling found at the J and K bands.

Meyer, Calvet & Hillenbrand (1998) model accretion disks and use the results to predict the contribution of the disk to the veiling in the J, H, K and L bands. They then compare the results with those expected for the shock veiling at these wavelengths, by assuming that the veiling at J is one tenth the veiling at V. The results are shown in their Figure 6 and those for J and K are reproduced in Figure 4.9, of this work. In that figure, the light solid lines represent the model predictions for the near infrared veiling for a disk with: no inner holes (lower line), inner holes with small sizes,i.e. (intermediate line) and large sized inner holes, i.e. (upper line). The dotted line is the result obtained with the assumption that the veiling in the near infrared is due exclusively to the accretion shock. The conclusion of these authors, based on the latter assumption, is that in order to explain the near infrared veiling, inner disk holes with small to large hole sizes () are needed.

The heavy solid lines in Figure 4.9 correspond to the measurements of the veiling at J and K reported in this work (distributions shown in Figure 4.6). Comparing these measurements with the model predictions indicate that smaller inner hole sizes are needed to explain the observations, rather than larger hole sizes as expected if the veiling was solely due to the accretion shock.

The heavy dashed lines correspond to the distributions for the J and K veiling obtained by adding the solid and the dashed lines in Figure 4.7. This corresponds to the conservative assumption that the veiling in the stars for which only lower limits were obtained is the actual lower limit. As expected, the heavy dashed lines reinforce the fact that disks with smaller hole sizes are needed in order to explain the observations. The stars for which only lower limits were obtained may well have r_J and r_K higher than the lower limit itself. In that case, the veiling distributions will be enhanced even further for high values of the veiling, implying that the dashed line reaches unity at even larger values for the veiling. The situation is such that even disks without inner holes can be incapable of explaining the high veiling observed at J and K.

Using the method of Meyer, Calvet & Hillenbrand (1998) which assumes that the veiling at J is solely due to the accretion shock, ie.\ , the J and K veiling expected for the 30 stars in the Strom et al. (1989) sample that are also studied in this work were computed (using Strom et al.'s data). The histograms corresponding to these calculations are shown as dotted lines in Figure 4.10. Also in Figure 4.10 the distributions for the observed J and K veiling (same as solid lines in Figure 4.6) are re-plotted. By comparing the latter with the former, it is clear that the observed J and K veiling tend to be larger than expected if the only source of extra continuum emission is the accretion shock. It should be noted here that given the way in which the histograms were computed (please refer again to Appendix A), the relatively large uncertainties in the observed r_J's and r_K's imply a spread in the histograms that might be able to explain the tail seen in the solid lines of Figure 4.10. In that case the tail, rather than representing genuinely high veiling, results from high uncertainties. To check whether this is the case, one attributes to each value of r_J and r_K obtained from Strom et al.'s data with the assumption that r_J = 0.1 r_V an uncertainty of and respectively. The histograms of these quantities were then re-computed with such uncertainties in each data point and they are plotted as dashed lines in Figure 4.10. It is clear that the tail in the latter are weaker than those in the solid histograms, especially at J, indicating that indeed the observations show larger values of veiling at J and at K than those expected from the accretion shock only. This, in turn, indicates that the disk should contribute significantly to the near infrared veiling.

In Figure 4.11 is plotted the mass accretion rate, as determined by Gullbring et al. (1998) and by Hartigan, Edwards & Ghandour (1995), versus the computed J and K veiling. The J veiling correlates well with the mass accretion rate, in the sense that the higher the accretion rate the higher the J veiling is. A similar correlation might occur with the K veiling as well, however the large uncertainties in its determination make it difficult to make a clear judgment. The M vs. r_J correlation is further enhanced by noting that CW Tau, DG Tau, DR Tau, RW Aur and YY Ori, in which photospheric features are not detected in the J band spectra presented here, therefore implying high veiling at J, have all M larger or equal than , as determined by Hartigan et al.

One expects the near infrared veiling to correlate with the mass accretion rate through a circumstellar disk since the temperature dependence with radius in the disk depends on M [Calvet et al. 1997]. The higher the accretion rate the higher the temperature at a given radii implying that the disk becomes more capable of producing the near infrared veiling.

Calvet, Hartmann & Strom (1997) compute near infrared spectra from circumstellar disks around T Tauri stars. With assumptions that maximize the disk contribution relative to that of the stellar photosphere, i.e. with the disk extending right to the surface of the star and seen face-on ( inclination) the disk contribution at the J band only exceeds that of the photosphere for mass accretion rates of the order of yr^-1. At mass accretion rates typical of T Tauri stars, that is -- yr^-1 the disk contribution at the J band is small relative to the stellar photosphere. The disk emission in the near infrared should be even smaller than this though, since the presence of inner disk holes, as expected for magnetospheric accretion scenarios, remove the hotter regions of the disk which are responsible for the emission at these wavelengths. In fact, the expected veiling at J due to a circumstellar disk with a inner hole of size and for accretion rates typical of those in T Tauri stars is zero, independently of the inclination of the disk (Calvet, private comunication). The expected veiling at K is, in these circumstances, smaller than (Calvet, private comunication).

Given the results presented in the last paragraph it is not clear how such high values for the J and K veiling in T Tauri stars as those obtained in this work can be achieved. Calvet, Hartmann & Strom (1997) are able to obtain values for r_J similar to those reported here from thermal emission of an infalling dusty envelope. Such an envelope is characteristic of Class I sources but it is not present around T Tauri stars. Also, the results from Calvet, Hartmann & Strom for infalling envelopes are far too high when compared with those obtained in this work.

The veiling measurements presented here allow one to estimate the temperature of the region producing this near infrared veiling. Assuming that the excess flux that veils the stellar photosphere at these wavelengths is characterized by a black body of temperature one can use the determined values of r_J and r_K to compute the excess fluxes at J (F_J (excess)) and at K (F_K (excess)). A black body fit through these two points yields . From Equation 4.1 above one sees that in order to compute F_J (excess) and F_K (excess) from r_J and r_K one needs the stellar flux at J and K respectively. Given the limitations of the data set presented here (no flux calibration was done nor simultaneous photometry is available) the stellar flux was taken to be that that results from a black body of temperature corresponding to the spectral type of the star. This is only an approximation, which is justified given the relatively large uncertainties in r_J and r_K.

Taking T_eff = 4000 K corresponding roughly to stars of spectral type K7V/M0V and the average values for r_J and r_K (< r_J> = 0.56 and < r_K> = 1.31) one gets T_veil ~ 3400 K. Taking the individual r_J's and r_K's for each star and the appropriate T_eff one arrives to T_veil ranging from 2100 K to 4800 K. For most stars T_veil is between 3000 and 4000 K. DN Tau and AA Tau yield the lowest values (~ 2100 K and ~ 2600 K respectively) while HP Tau, T Tau, V807 Tau, V773 Tau and DO Tau yield the highest values (respectively ~ 4800 K, ~ 4800 K, ~ 4200 K, ~ 4400 K and ~ 4100 K).

Such temperatures are expected to occur in the innermost regions of accretion disks, ie. , if inner disk holes are not present [Calvet et al. 1997]. Such holes are expected to be present in many T Tauri stars, as will be discussed in the next chapter.

A different possibility for the origin of the near infrared veiling observed here is that it arises in the accretion flow itself. Martin (1996) determines the thermal structure of magnetic accretion flows in T Tauri stars. He concludes that the temperature of the gas in the flow is typically between 3000 K and 6000 K, depending on the region of the flow. These temperatures are consistent with those needed in order to obtain the observed veiling at J and at K (see above). However, the latter temperatures were estimated from considering black body emission (i.e. optically thick emission). Martin (1996) shows that for the 3000 to 4000 K temperature regime Bremsstrahlung is the dominant coolant in the accretion flow, which, at these near infrared wavelengths, is optically thin. If Bremsstrahlung emission is the source of infrared veiling then the excess flux at K should be higher than the excess flux at J since for this wavelength regime free-free emission increases with wavelength. However, the observations show the opposite, i.e. the excess emission at J is higher than that at K. If the accretion flow is to be the source of near infrared veiling continuum then some other cooling process other than free-free emission must also play a role.

Given the unexpectedly high values obtained for the veiling at J and K and the difficulties in explaining it, the reliability of the results presented here are assessed below.