The IPC Line Profiles

The observed line profiles in which at least one component, the redshifted absorption feature, is certainly formed in an accretion flow are those classified as Type IV R. What are the typical velocities at which this feature occurs? How strong are those absorptions? The central velocity of the IPC absorptions and their Equivalent Widths (measured below the continuum level) are shown in Tables 5.5 and 5.6. The tabulated uncertainties in the EW result from the noise in the spectrum only, which is estimated from its point to point variation. There is always an uncertainty in the determination of the level of the continuum which increases the uncertainty in EW. Taken this into account, a more realistic uncertainty for the determined EWs is 1-1.5 km/s. Typical velocities for the absorption features are between 50 km/s and 300 km/s, while the EW can be as low as 2.1 km/s for Haro 6-37's Br Gamma profile and as large as 30.8 km/s for YY Ori's Pa Beta profile.

In Figure 5.23 the EW of the emission component is plotted versus the EW of the absorption component for the IPC (Type IV R) Pa Beta and Br Gamma line profiles. From the available data no correlation between these two quantities is apparent. While a range of emission strength is observed, the strength of the absorption component is fairly constant. This fact is further discussed towards the end of this section.

In the magnetospheric accretion scenario matter falls freely onto the star from the inner radius of the disk along the magnetic field lines. The free-fall velocity obviously depends on the mass () and radius () of the star and on the size of the inner hole.

The velocity of the redshifted absorption feature in the IPC line profiles reflects the free-fall velocity (v_ff) of the infalling matter and can be used as an estimate for the latter. In fact, due to projection effects, it provides a lower limit for . Given such an estimate for and an assumption for the distance from which infall begins, i.e. an assumption for the size of the inner hole, one can, for a given star, determine a lower limit for the / ratio. The results from the work presented here, for which this method of determining / was going to be applied, were combined with published optical line profile data and accepted for publication by Monthly Notices of the Royal Astronomical Society. The accepted paper [Bonnell et al. 1998] is presented in Appendix B and the reader is referred to that appendix for the details of the method, for the results obtained and for a comparison between these and predictions from evolutionary tracks.

In Figure 5.24 the central velocity of the redshifted absorption feature versus the emission strength for the Pa Beta IPC lines, as measured by the EW of the emission component of the line, is plotted. Values for earlier spectral types (K1V to K5V) are represented by open circles and values for later spectral types (K6V to M4V) are represented by filled circles. Stars of earlier spectral types tend to display larger infall velocities and stronger emission.

A somewhat expected result is to find that the emission strength scales with spectral type. On the one hand, stars with earlier spectral types are hotter therefore more energy is available to populate the levels leading to emission. On the other hand, for a given accretion rate, more energy is released if matter falls onto a higher mass star. Since this accretion energy should also play a role in populating the relevant energy levels then one also expects the emission lines to get stronger for higher mass stars, i.e. for earlier spectral types.

It should be noted that these results are based on a small number of stars (there are not many early K stars in the sample) and therefore one should be cautious about them.

Plotting accretion rates from Hartigan et al. (1995) and from Gullbring et al. (1998) against the EW of the absorption feature in the IPC Pa Beta line profiles shows no correlation between the two quantities (see Figure 5.25). When one plots accretion rates versus the EW of the emission component in the Pa Beta IPC lines (see Figure 5.26) there seems to be a trend in the sense that stars with larger accretion rates tend to have larger EW in the emission component of the IPC profiles, the clear exception being GM Aur. This star seems to have a relatively low accretion rate, as computed by Hartigan et al. (1995) and by Gullbring et al. (1998) but still has a large emission component in its Pa Beta line profile. A similar trend is observed if one plots (Figure 5.27) the accretion rate versus the line to continuum ratio at the line peak (as expected). Again, the exception is GM Aur.

In Figure 5.28 is plotted the wind mass loss rate, as determined by Hartigan et al. (1995), versus the equivalent width of the emission component of the IPC line profiles. In this figure, open symbols correspond to lower limits in the mass loss rate. As can be seen, the amount of emission in IPC lines does not seem to correlate with the wind mass loss rate.

A similar analysis cannot be made for the stars in which the Br Gamma line profiles have an IPC characteristic since the number of these stars for which accretion rates are available in the literature is very small.

One would expect a correlation between emission line strength and accretion rate if emission originates in infalling matter in the magnetospheric accretion model (see work by Muzerolle et al. 1998). Such a correlation seems to be present for the emission component of the lines with IPC profiles. Therefore, for this type of line profiles, not only redshifted absorptions below the continuum are observed at velocities that agree with the predicted velocities from the model, but also the strength of the emission component increases with increasing mass accretion rate. It should be noted here that while redshifted absorptions that never dip below the continuum can be interpreted in terms of lack of emission, there is no doubt that absorption below the continuum must indicate real absorbing material. The whole structure of the IPC lines seems therefore to arise in infalling material and is consistent with the magnetospheric accretion scenario.

Comparing Figures 5.26 and 5.24, one sees that, as suspected, the link between the emission strength and the mass accretion rate is tighter than the link between emission strength and spectral type. That can explain why on Figure 5.24 one sees stars of earlier spectral types with relatively small emission strength and a later type star with an emission strength comparable to that of earlier type stars.

The lack of correlation between wind mass loss rate and EW of the emission component in IPC lines points to a direction in which outflowing material is not responsible for the emission component in IPC Pa Beta line profiles.

That the EW of the absorption component of the IPC lines does not correlate with the mass accretion rate is not unexpected. The redshifted absorption feature basically results from seeing the infalling gas against the hot region where the accretion shock occurs. The geometry of the system and the contrast between the temperature of the infalling material and that of the shock region are therefore pivotal in determining the strength of the absorption. Any clear correlation between mass accretion rate and EW of the absorption component can be therefore erased by geometrical and/or temperature contrast effects.

In view of the above, the whole structure of the IPC lines seems to arise in infalling material and it is consistent with the magnetospheric accretion scenario.

From the 14 stars that display IPC line profiles at either Pa Beta or Br Gamma (and observed at both wavelengths) only two stars (HP Tau and RW Aur) have IPC structure for the two lines. FP Tau and GI Tau have IPC structure at Pa Beta while at Br Gamma they show no emission or absorption. For the remaining stars (of the 14) , when one line profile is classified as Type IV R the other is classified as Type I. The Type I profiles of these stars tend to be moderately to strongly asymmetric with more emission in the blue wing. In the cases of DF Tau (at Pa Beta) and of GM Aur (at Br Gamma) the lines display strong asymmetry and are reminiscent of IPC line profiles despite no clear redshifted absorption features being present. CW Tau is somewhat special in the sample of stars studied since it displays a clear wind signature at Pa Beta (blueshifted absorption) and a clear signature of infall at Br Gamma (IPC profile). The different information conveyed by the two lines (IPC/non-IPC structure) can be intrinsic to the origin of the line (having different optical depths and requiring different excitation conditions one would expect them to probe slightly different regions) or can result from the non-simultaneity of the Pa Beta and Br Gamma line profiles (they were obtained one day apart). As will be discussed in Section 5.4.7, some Pa Beta and Br Gamma line profiles are seen to be variable and even change the line shape from Type I to IPC. Therefore, one cannot discard the hypothesis that, at least for some stars, whenever Pa Beta has IPC structure so does Br Gamma and vice-versa. Monitoring the line profiles of the Pa Beta and Br Gamma lines in T Tauri stars would help significantly disentangling the two effects.

A significant percentage of the Br Gamma IPC lines tend to peak more towards the blue than the Pa Beta IPC lines (Section 5.3.7 above). The blueward shift is of about 60 km/s. It is worthwhile noting that amongst the Br Gamma Type I line profiles also a significant number of them have the velocity of the line peak shifted to the blue relative to v_peak in the Pa Beta lines (see bottom panel of Figure 5.11). In the light of the magnetospheric accretion line profile modeling carried out by Hartmann, Calvet, Muzerolle and co-workers, this is an unexpected result. An explanation for this shift would be that, due to their different optical depths (Pa Beta should be optically thinner than Br Gamma), Pa Beta and Br Gamma sample regions where gas moves at different speeds, resulting in line profiles peaking at different velocities. The magnetospheric accretion line profile modeling carried out by Hartmann, Calvet, Muzerolle and co-workers does not show any relative shift in the peak velocity between lines of very different optical depths, such as H Alpha, H Beta, H Gamma, and Br Gamma. The origin for such a shift is not at all clear.

Given the discussion in the previous paragraphs, an infall scenario for the origin of the emission in IPC line profiles seems the most likely. However, current accretion models do not provide completely satisfactory answers when more quantitative comparisons are done.