T Tauri Stars seen through High Resolution NIR Spectroscopy

Daniel F. M. Folha

Centro de Astrofisica da Universidade do Porto, Porto, Portugal

SETTING THE SCENE

UKIRT's CGS4 is one of the few instruments available to the astronomical community capable of excellent performance at high spectral resolution in the near infrared. A field of study that can take great advantage of this almost unique capability is, of course, star formation.

T Tauri stars (TTS) represent the population of low mass young stars on the final phases of formation. Their spectra may potentially include radiation from several components: a stellar photosphere, shocks resulting from material accreting onto the star, infalling and outflowing material, an accretion disk, starspots, strong chromospheric activity and, possibly, a residual circumstellar envelope. All these are affected by extinction. Understanding these systems requires a multiwavelength approach, of which high resolution NIR spectroscopy plays a very important role. Here, I give an account of the results from a project developed in collaboration with Jim Emerson (Queen Mary & Westfield College), aiming at studying the origin of Hydrogen emission lines in TTS, the kinematic information provided by these lines and detect any excess flux emission relative to the stellar photospheric contribution. For this purpose CGS4 was used in its echelle mode to observe a sample of TTS from the Taurus-Auriga complex. 49 stars were observed at J and 36 were observed at K, respectively around Pa-beta and Br-gamma, with R~25000, during three nights in December 1995. Full description of this work can be found in Folha & Emerson (1999,2000).

THE NIR EXCESS FLUX THROUGH VEILING ANALYSIS

The analysis of photospheric lines in a TTS spectrum is the tool used to study excess emission in these stars. Excess emission veils the photospheric lines, decreasing their equivalent width relative to what they would be in an undisturbed photospheric spectrum. The "veiling" at a given wavelength is defined as the ratio of the excess flux to the photospheric flux at that wavelength and it can be determined by comparing the photospheric spectrum of a TTS with that of a template star of the appropriate spectral type.

Since the late nineteen eighties the veiling has been studied in the optical region of the spectrum of TTS (eg. Hartigan et al. 1995) and explained as the result from shocks where matter accretes onto the stellar surface. The contribution of this excess flux to the NIR is less than 0.1 for a typical accreting TTS (eg. Calvet & Gullbring 1998). Emission from disks in TTS is also not expected to produce significant veiling in the NIR, especially at the J-band, since: the inner disk holes that are thought to exist do not contain enough material emitting at these wavelengths (Meyer et al. 1997); accretion rates through these disks are not likely to be high enough to produce the necessary radial temperature profile. On the basis of these results, NIR veiling in TTS should be very small. What do the observations reveal? Figure 1 shows the J- and K-band veiling distributions for the stars for which it was possible to determine the veiling. Clearly, the excess flux that manifests itself via these veiling measurements is relatively high, and so not easily explained by current models for TTS and their environment. These, in turn, are supported by other lines of evidence.

Pa-beta and Br-gamma LINE PROFILES

Traditionally, the strong hydrogen emission lines found in many TTS were interpreted in terms of mass loss (eg. Hartmann et al. 1982). The last decade saw a shift in interpretation towards mass accretion (eg. Calvet & Hartmann 1992), following the magnetospheric accretion (MA) model (eg. Camenzind 1990). However, the debate is far from settled. These results have been based mostly on observations at optical wavelengths. NIR hydrogen lines impose strong constraints on models and contribute to the understanding of the origin of hydrogen emission in TTS.

Using CGS4 we have carried out the first extensive survey of NIR hydrogen line profiles. It shows that Pa-beta and Br-gamma lines are broad (FWHM~200 km/s) centrally peaked and slightly blueshifted. Their wings extend up to an average velocity of about 250 km/s but often reach velocities in excess of 350 km/s. More than 85% of the observed Pa-beta and Br-gamma profiles fall within two main groups: generally symmetric profiles with weak signs of absorption features (eg. Figure 2a) and inverse P-Cygni (IPC) profiles (eg. Figure 2b) with their typical redshifted absorption feature dipping below the continuum. From the profiles that fall in either of these two groups, about 60% of the Pa-beta and 80% of the Br-gamma profiles belong to the former. These numbers are in complete contrast to what is observed at H-alpha, the most studied hydrogen line in TTS. Most H-alpha profiles in TTS (> 50%) display blueshifted absorption features, about 25% have a generally symmetric shape and only about 5% are IPC. IPC profiles reveal the presence of infalling material, the velocity of which can be estimated from the velocity of the redshifted absorption feature. Typical velocities for the redshifted absorption in Pa-beta and Br-gamma lines are of the order of 200 to 300 km/s, consistent with free fall from a few stellar radii, as expected from the MA model. Comparing the observed NIR line profiles with results from radiative transfer models found in the literature (eg. Muzerolle et al. 1998) shows that MA models provide a qualitative insight on how these lines might be produced in the TTS environment; however they fail when quantitative comparisons are done. The data set discussed here demonstrate that current knowledge about the formation of hydrogen lines in TTS is far from providing a detailed explanation for their characteristics and origin. Also, it constitutes a solid database with which model results should be compared if they are to succeed in their goals.

CONCLUSION

High resolution NIR spectroscopy revealed unexpected results both in terms of the excess emission and the hydrogen lines profile shapes. Current models for TTS have to be refined/reviewed in order to account for these observations. From an observational point of view, extremely important clues regarding the origin of the excess emission and hydrogen line emission will result from variability studies of both veiling and line profiles. Good sampling over, at least, one rotation period of a typical TTS, i.e. up to a week is necessary. UKIRT together with CGS4 is where it can be done.

REFERENCES
Calvet & Hartmann (1992), ApJ, 386, 239
Calvet & Gullbring (1998), ApJ, 509, 802
Camenzind (1990), Rev. Mod. Astron., 3, 24
Folha & Emerson (1999), A&A, 352, 517
Folha & Emerson (2000), to appear in A&A
Hartigan, Edwards & Ghandour, ApJ, 452, 736
Hartmann, Edwards & Avrett (1982), ApJ, 261, 279
Meyer, Calvet & Hillenbrand (1997), AJ, 114, 288
Muzerolle, Calvet & Hartmann (1998), ApJ, 492, 743

Surveying the Distant Universe through Massive Clusters Lenses.

Ian Smail¹, Graham Smith¹, Jean-Paul Kneib², Oliver Czoske² and Harald Ebeling³

¹Durham University, U.K.
²Observatoire Midi-Pyrenees, France
³University of Hawaii, U.S.A.

The properties of massive clusters of galaxies are expected to predominantly reflect gravitational processes and can thus provide unique insights into the nature and distribution of dark matter. One particularly striking demonstration of the masses of the richest clusters is their ability to act as gravitational lenses and focus light from background sources creating highly-magnified (and in rare cases multiple) images of distant galaxies. Moreover, any distant galaxy seen through the lens is magnified (in a tangential direction) shearing the images of background galaxies into `arclets' - the distortion of these arclets can be used to trace the shape of the mass distribution within the lens. To understand more about the distribution of mass within these massive clusters we are therefore undertaking a lensing survey with Hubble Space Telescope of 12 X-ray luminous clusters in a narrow redshift slice at z~0.2. The gravitationally-lensed features identified in our HST exposures allow us to construct detailed mass maps for the central ~1Mpc regions of these clusters. By incorporating wide-field imaging from CFHT and sensitive X-ray observations from Newton we will trace the dark matter and baryonic profiles of the clusters out to the point where they merge into the surrounding field.

Our HST observations provide information not only about the mass distribution within the lens, but also can be analysed to determine the redshifts for any background galaxy which is significantly magnified (and thus distorted) by the lens. The images of some of these galaxies can be magnified by >10-times thus offer an enormous increase in sensitivity for the detailed study of faint, high-redshift galaxies. But more typically the galaxies in the HST fields are boosted by factors of 2-3. This amplification is dependent upon both the mass distribution in the lens and the redshift of the background galaxy. Thus, if we can construct an accurate mass model for the cluster we can `invert' it to predict redshifts for the faint galaxies seen in the background. The original predictions of the redshifts of galaxies seen through the cluster lens Abell 2218 using this `inversion' technique (Kneib et al. 1996), were tested and confirmed with very-deep spectroscopic observations by Ebbels et al. (1998).

FIGURE 1: A true colour image of the core of the massive cluster Abell 383, created from ground-based B- and K-band frames and the HST R-band exposure. The cD galaxy in the cluster centre clearly dominates this region and our lensing analysis shows that it makes a large contribution to the mass profile in the central 20 kpc of the cluster. The z=1.01 giant arc (B0) is visible to the south of the cD and has an apparently blue nucleus at the right-hand end. Several other multiply-imaged sources are visible by virtue of their blue colours in the halos of bright cluster ellipticals near the bottom edge of the frame (see also Fig. 2). However, arguably the galaxy with the most unusual colours is the Extremely Red Object (ERO, called B14) in the bottom-left of the field. This galaxy has (R-K)=6.0 and is barely detected on our HST frame with R~26. This field is 50x50 arcsec in size.

We are using UFTI on UKIRT to obtain high quality K-band imaging of gravitationally-lensed galaxies seen through the rich clusters in our HST survey. This article deals with the observations of the X-ray luminous cluster Abell 383 (at z=0.19) the analysis of which we have recently completed (Smith et al. 2000). The UFTI image of the cluster core, combined with ground-based and HST optical images is shown in Fig. 1. The median seeing for the stacked 2.4-ks K-band observation was 0.42", compared to 0.17" in the 7.5-ks HST image and 0.88" in the 7.2-ks CFHT B-band exposure.

FIGURE 2: A false colour version of the HST R-band frame, overlaid with an isodensity representation of our lens model (Smith et al. 2000). Contours correspond to projected surface mass densities of 3, 4, 6, 8, 11, 15 x 10⁹ M_o/kpc². Note the well-defined circular symmetry exhibited by the cluster mass, as well as the local perturbations produced by individual cluster ellipticals. The numerical labels indicate the cluster members used in the lens model. The alpha-numeric labels identify the multiply-imaged systems used to constrain the model, and a number of other singly-imaged arclets. The very faint object labelled B14 is the ERO which is so clearly visible in Fig. 1.

The most obvious lensed feature in Figs. 1 & 2 is the giant arc (labelled as B0a+B1a/b/c). This comprises four images of two background galaxies. We have obtained a redshift for B0a which places it at z=1.01 and provides an absolute normalisation for the mass model of the cluster. We find that B1 must lie at a similar redshift to B0, z~1. Several other unusual lensed galaxies are visible in the HST image - of these the brightest are the five-images of a z~3.5 galaxy seen as B2a/b/c/d/e, and the four visible images from a five-image system of a galaxy at a similar redshift seen as B3/a/b/c/d.

However, our UFTI imaging turned up an equally unusual galaxy seen through the core of the cluster - an Extremely Red Object (ERO, Fig. 2) - this galaxy has K=19.7 and R-K=6.0 - meaning it is only just visible in our deep HST R-band exposure. EROs comprise a rare class of galaxies which is believed to consist of a mixture of evolved early-type galaxies at z=1-2 and dusty starbursts at similar (and higher) redshifts. The latter class includes many members of the faint submm population identified by SCUBA. However, by definition EROs are faint at optical wavelengths and hence only a couple have reliable redshifts measurements. This lack of information about their redshifts and hence luminosities and restframe colours has hampered efforts to disentangle the properties of this unusual population.

Using the positions and shapes of the various images of the lensed galaxies seen in Fig. 2 we have constructed a detail model of the mass distribution in the central regions of the cluster (Smith et al. 2000). This is overlayed on the HST frame in Fig. 2 and shows that the cluster's potential well appears relaxed and circular. We can then employ this model to interpret the shapes of other gravitationally lensed arclets seen in this region and predict redshifts for these faint, background galaxies. Using this approach we can constrain the redshift of the ERO, B14. As only a single image of this galaxy is visible it must lie at z<3.9. Moreover, the very faint arclet visible at the position of B14 in our HST image appears to be highly elongated (a/b~4) and this would suggest a redshift of around z~3 and would give the galaxy unmagnified apparent magnitudes of K=21 and R=27 - well-beyond the reach of even 8-m class telescopes. When magnified to K=19.7, however, this galaxy is just within reach of efficient near-infrared spectrographs on 8-m telescopes and so our estimate of the redshift of this ERO could be confirmed in the near future.

References

Ebbels, T.M.D., Ellis, R.S., Kneib, J.-P., Le Borgne, J.-F., Pello, R., Smail, I., Sanahuja, B., 1998, MNRAS, 295, 79
Kneib, J.-P., Ellis, R.S., Smail, I., Couch, W.J., Sharples, R.M., 1996, ApJ, 471, 643
Smith, G.P., Kneib, J.-P., Ebeling, H., Czoske, O., Smail, I., 2000, ApJ, submitted.

Image Credits: The UFTI, HST and CFHT imaging of this cluster was taken for a collaborative study by our group. These observations were obtained with UKIRT, which is operated by the Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom; CFHT, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii and the NASA/ESA Hubble Space Telescope, which is operated by the Space Telescope Science Institute for the Association of Universities for Research in Astronomy Inc., under NASA contract NAS 5-26555.

UKIRT and CGS4 Define the New Methane-Dwarf Sequence(s)

Sandy Leggett

UKIRT/Joint Astronomy Centre

UKIRT continues to produce definitive data for the rapidly advancing field of brown dwarf studies. In the last Newsletter (March 2000) Lucas & Roche described their discovery of a population of very young brown dwarfs in Orion, possibly with masses as small as 8 Jupiter-masses (MNRAS 314). This work used UFTI with I, J and H filters. In the September 1999 Newsletter I described UKIRT's contribution to the discovery of a population of brown dwarfs cool enough at T_eff ~ 1000 K to show methane absorption in their near-infrared spectra. Such objects are provisionally being called T dwarfs. Prior to 1999 only one methane dwarf was known, Gliese 229B, which was discovered in 1995 (Nakajima et al. Nature 378). In 1999 the Sloan Sky Survey reported their discovery, confirmed with CGS4 spectra, of two 229B-like brown dwarfs (Strauss et al. ApJ 522 and Tsvetanov et al. ApJ 531), followed by announcements from the 2-Micron All Sky Survey (2MASS) of 4 others (Burgasser et al. ApJ 522), and of another detected by the NTT/VLT (Cuby et al. A&A 349). 2MASS have found further examples of 229B-like methane dwarfs this year. All of these T dwarfs have spectral energy distributions that are extremely similar to each other. They all have infrared colours of J-H ~ H-K ~ 0, and their spectra show extremely strong absorption bands due to water and methane, with atomic features due to absorption by the alkalis cesium and potassium. This is in contrast to the warmer L dwarfs whose near-infrared spectra show no indication of absorption by methane and which have extremely red colours of J-K ~ 1.5. For these objects carbon is predominantly in the form of carbon monoxide and CO bands are seen at 2.3-2.4 microns.

FIGURE 1: CGS4 spectra showing the L- to T-dwarf spectral sequences with weakening CO and strengthening CH₄ (based on Leggett et al. 2000, ApJ, 536)

It was suggested by Kirkpatrick et al. (2000 AJ 120) that the lack of objects with near-infrared spectra showing both CO and CH₄ implies that they are rare and that the spectral transition from L to T must occur over a very small temperature range. However in March Tom Geballe (Gemini) and myself, using CGS4 and working with Princeton representatives of the Sloan consortium Jill Knapp and Alex McDaniel, disproved this by finding three examples of the so-called transition objects. These objects link the L dwarfs with T_eff ~ 2000 K to the 229B-like objects with T_eff ~ 1000 K (Leggett et al. 2000 ApJ 536). Their JHK colours are very similar to the much hotter, and very numerous, K and M dwarfs, and hence they are extremely difficult for near-infrared surveys to identify; however in the Sloan survey they are distinguished by extremely red I-Z colours. The three, with the previously known L and T types, form a clear sequence, shown in Figure 1, of strengthening CH₄ absorption at 1.6 microns and 2.2 microns while the CO bandhead at 2.3 microns is still seen but weakens. H₂O absorption at around 1.15, 1.35, 1.85 and 2.40 microns strengthens. These objects will form the hot end of the T dwarf spectral sequence, where T dwarfs are defined by the presence of CH₄ absorption in the H and K bands.

FIGURE 2: CGS4 spectra showing the onset of methane absorption at 3.30 microns in L-dwarfs (based on Noll et al., 2000, astro-ph/0007449).

There was still the question: at which spectral type can methane first be detected? The methane band at 3.3 microns is two orders of magnitude stronger than the bands seen in the near-infrared. Keith Noll (STScI) initiated a program with CGS4 to search for the onset of methane and in May we detected methane in an L5 and an L7 dwarf (and not in an L3). The spectra are shown in Figure 2 and are reported in Noll et al. 2000 (ApJ accepted, astro-ph/0007449). There is telluric CH₄ absorption at 3.312-3.323 microns but the (sub)stellar band is broader; preliminary models can reproduce the shape of the band quite well. Clearly the term ``methane brown dwarf'' must be used with caution. As these CGS4 spectra show, methane is detectable in the photospheres of the later L dwarfs. Use of the label ``T dwarfs'' to mean methane detected at wavelengths shortwards of 2.5 microns is a useful definition for classification, but it should be remembered that the warmer L dwarfs are not methane-free.

The CGS4 spectra shown in the figures reveal vital diagnostic features in the atmospheres of the very cool L and T dwarfs. Certainly sky surveys will continue to add to the known population of free-floating brown dwarfs, and we can look forward to detecting objects with temperatures between the T dwarfs and that of Jupiter, while theorists work hard to develop models that incorporate clouds so that we can better understand the physics of these planet-like atmospheres.