Detection of Quiescent Molecular Hydrogen Gas in the Circumstellar ...

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ABSTRACT. We have detected emission from the quiescent, molecular hydrogen gas in the disk around the X-ray bright, classical T Tauri star TW Hya in the 1»0 ...
THE ASTROPHYSICAL JOURNAL, 541 : 767È771, 2000 October 1 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

DETECTION OF QUIESCENT MOLECULAR HYDROGEN GAS IN THE CIRCUMSTELLAR DISK OF A CLASSICAL T TAURI STAR DAVID A. WEINTRAUB,1 JOEL H. KASTNER,2 AND JEFFREY S. BARY1 Received 2000 January 24 ; accepted 2000 May 11

ABSTRACT We have detected emission from the quiescent, molecular hydrogen gas in the disk around the X-ray bright, classical T Tauri star TW Hya in the 1È0 S(1) line at 2.1218 km through high-resolution spectroscopy using CSHELL on NASAÏs Infrared Telescope Facility. The brightness of the observed emission line is consistent with that predicted from models for X-ray excitation of the 1È0 S(1) line of H . This 2 result demonstrates that X-ray ionization is a plausible mechanism for excitation of the H in the 2 gaseous disks of T Tauri stars. In addition, from these high-resolution spectra we have measured the radial velocities of three T Tauri stars in the TW Hya association, CD [33¡7795, HD 98800, and TW Hya. We Ðnd that the radial velocities of these three stars are very similar to each other and to those of at least three other stars presumed to be members of the TW Hya association, Hen 600A, Hen 600B, and CD [29¡8887. This result, combined with the similar proper motions of all six of these stars, lends support to the hypothesis that all of these stars share a common origin in a now-dispersed molecular cloud. Subject headings : infrared : stars È open clusters and associations : individual (TW Hydrae, HD 98800, CD [33¡7795) È stars : preÈmain-sequence È X-rays : stars 1.

INTRODUCTION

Molecular hydrogen represents nearly 90% of the mass of the circumstellar disks around T Tauri stars ; yet, because H has no net dipole moment, the H is extraordinarily 2 2 difficult to detect directly. Thus, conclusions about the presence or absence of disks and about the masses, contents, gas-to-dust ratios, and evolutionary ages of these disks must be made from observations of their trace constituents, e.g., via thermal infrared observations of dust grains or radio-molecular line observations of species such as CO. Molecular hydrogen gas has been observed toward only two T Tauri stars. The detection of H toward T Tauri 2 Beckwith 1997), (Herbst et al. 1996 ; Herbst, Robberto, & however, was of shock-excited gas associated with outÑows, not of quiescent gas in the disk. Only in the case of GG Tau, in which warm H (110^10 K) was detected by the Infrared Space Observatory2 at 28 and 17 km (Thi et al. 1999), has H been observed directly in a circumstellar disk. Thi et al.2 suggest that much of this emission may arise because of the heating of the gas at D100 AU in a Ñared disk by both photospheric emission and ultraviolet emission from the star-disk boundary ; shock heating may account for up to 30% of the line emission. Recently models have been developed (Gredel & Dalgarno 1995 ; Maloney, Hollenback, & Tielens 1996 ; Tine et al. 1997) in which the X-ray luminosity of a preÈmainsequence star may be sufficient to produce fractional ionization but insufficient to fully atomize or ionize the H in the disk. In these environments the absorption of X-rays2 by the gas will generate populations of high-velocity atoms, ions, photoelectrons, and secondary electrons. Collisions of the atoms, ions, and fast electrons with the H molecules will 2 1 Department of Physics and Astronomy, Vanderbilt University, P.O. Box 1807 Station B, Nashville, TN 37235 ;david.weintraub=vanderbilt.edu ; bary=eggneb.phy.vanderbilt.edu. 2 Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623 ; jhkpci=cis.rit.edu.

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excite the H . While most of the excited molecules ultima2 tely will dissociate, a fraction of the H will descend back to 2 the ground state through infrared radiative transitions. This sequence of eventsÈX-ray photon absorption followed by atomization, ionization, photoelectron production, collisional excitation, and radiative de-excitationÈ ultimately depends on the ionization rate, which, in turn, is directly proportional to the X-ray luminosity. In most circumstances, the 1È0 S(1) line at 2.12183 km (4712.91 cm~1) is predicted to be one of the strongest rotation-vibrational lines produced during the radiative cascade (Tine et al. 1997). Relatively old (D107 yr) T Tauri stars generate as much as 0.1% of their bolometric luminosity as X-rays (Kastner et al. 1997 ; Webb et al. 1999). One of the closest, most X-ray bright T Tauri stars in the sky is TW Hya. At a distance of only 56 pc, as determined by Hipparcos, TW Hya has an X-ray luminosity (L ) of 1030 ergs s~1 (Kastner et al. 1999). TW Hya also has axcircumstellar disk containing both gas and dust. Weintraub, Sandell, & Duncan (1989) established the presence of a dust disk around TW Hya, and Zuckerman, Forvveille, & Kastner (1995) and Kastner et al. (1997), via radio observations of selected molecular tracers (CO, HCN, CN, and HCO`), demonstrated the presence of a modest amount of molecular gas (D11 Earth masses) in the TW Hya disk. The gas-poor and dust-rich disk around TW Hya is compact and is viewed nearly pole-on (Weintraub, Sandell, & Duncan 1989 ; Kastner et al. 1997 ; Krist et al. 1999 ; Weinberger et al. 1999). Given its proximity, large L , x and circumstellar molecular disk, TW Hya represents an ideal star for testing the hypothesis of X-ray excitation and near-infrared emission from H molecules around T Tauri 2 stars. In this paper we present high-resolution, near-infrared spectroscopic observations of TW Hya and two other X-ray bright T Tauri stars in the TW Hya association (TWA) (Kastner et al. 1997) and report the detection of H emission toward TW Hya. These results constitute direct2 measurements of emission from quiescent H in a circumstellar disk 2

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around an X-ray bright, preÈmain-sequence star and thereby demonstrate that X-ray ionization is a plausible mechanism for stimulating emission from excited H . 2 2.

OBSERVATIONS

We obtained high-resolution, near-infrared spectroscopic observations of TW Hya, HD 98800, and CD [33¡7795 on 1999 February 23 UT, using CSHELL (Greene et al. 1993) at the NASA Infrared Telescope Facility (IRTF) 3 m telescope on Mauna Kea. CSHELL uses a 256 ] 256 InSb detector array with 0A. 2 (2.7 km s~1) pixels. Our observations were made using a 1A. 0 slit (R \ 21,500) under poor seeing conditions in which the full width at half-maximum (FWHM) for point sources was 1A. 5. Spectra were centered at 2.1231 km, providing spectral coverage from about 2.1204 to 2.1257 km. Atmospheric OH emission lines at 2.12325 and 2.12497 km as well as internal calibrations of an argon lamp provided the absolute wavelength calibration. The mean FWHM of the two OH emission lines is 6.1 ^ 0.4 km s~1, as compared to the slit width of 13.5 km s~1 (5 pixels). Thus, our e†ective velocity resolution with CSHELL during these observations was about 6 km s~1. The observations of the A0 V star HD 92845 were obtained for comparison of telluric spectral features. The similar spectral types of the three target stars, K5 (HD 98800), K7 (TW Hya), and M1.5 (CD [33¡7795) (Webb et al. 1999), permit us to determine whether the selected spectral features, unrelated to the possible H emission feature, are common for this spectral-type range. 2 The observations were obtained while autoguiding on the stars themselves. The spectra of the stars were obtained by placing the star 7A. 5 from the one end of the slit and then nodding the telescope 15A along the 30A slit. A total of 60 minutes of on-source time were spent on each of our three target stars, and 20 minutes were spent on HD 92845. After correcting for dark current and bias, the spectral images were Ñat-Ðelded using Ñats generated by lamps internal to CSHELL. In the spectral region covered by these observations 10 telluric absorption lines are clearly identiÐable in the spectra published by Hinkle, Wallace, & Livingstron (1995), including a strong absorption line at 2.124405 km, a weaker line at 2.12472 km, and eight other approximately evenly spaced weak lines stretching from 2.1210 to 2.1240 km. In order to remove these known telluric absorption features, the spectra of the T Tauri stars were divided by the smoothed spectrum of HD 92845, where the smoothing enables us to remove most of the telluric absorp-

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tion features without introducing additional noise into the data. 3.

RESULTS

The ratioed spectra of the T Tauri stars TW Hya, HD 98800, and CD [33¡7795 are presented in Figure 1. These spectra are presented in the rest frames of the respective stars (i.e., corrected for the motion of the Earth and the Sun and the measured radial velocities of the stars). The spectra show a number of atomic absorption features, which are identiÐed in Figure 1 and listed in Table 1. The lines of Al, Mg, and Fe appear to be present in the spectra of all three stars, while TW Hya and HD 98800 show strong absorption due to Si. All three stars show broad absorption dips stretching from approximately 2.121 to 2.122 km, while TW Hya and CD [33¡7795 show a second broad dip stretching from 2.123 to 2.124 km. Some prominent telluric absorption features, most notably at 2.1210, 2.1225, 2.1230, 2.1235, 2.1240, and 2.1244 km (Hinkle et al. 1995) are incompletely corrected, and a few of these, most notably at 2.1225 km in the TW Hya spectrum, produce spurious, narrow peaks in the ratioed spectra when they fortuitously fall at positions of positive noise in the object spectrum. Notably, no telluric features fall at or near 2.1218 km (Hinkle et al. 1995). We have used the deeper and sharper spectral features to measure the heliocentric radial velocities of the stars in our sample. For TW Hya, we Ðnd v \ 12.2 ^ 0.5 km s~1 (the error weighted average of thehelio individual line measurements presented in Table 1), which is very consistent with the values of 12.2 ^ 0.1 km s~1 (Kastner et al. 1997), 12.5 ^ 2.2 km s~1 (de la Reza et al. 1989), and 14 ^ 3 km s~1 (Gregorio-Hetem et al. 1992). For HD 98800, we Ðnd \ 10.8 ^ 0.3 km s~1 ; this result is well within the v helio of variation seen for HD 98800, which is a double range spectroscopic binary that shows a range of ^3 km s~1 around the systemic velocity of 12.75 ^ 0.10 km s~1 (Torres et al. 1995). The radial velocities of TW Hya and HD 98800 are very similar and are also comparable to the radial velocities of three other stars in the TWA : CD [29¡8887 (9.7 ^ 0.3 km s~1 and 10 ^ 3 km s~1 ; de la Reza et al. 1989 ; Gregorio-Hetem et al. 1992) and the binary Hen 600A (10.4 ^ 1.1 km s~1 and 10 ^ 3 km s~1 ; de la Reza et al. 1989 ; Gregorio-Hetem et al. 1992) and Hen 600B (14.0 ^ 0.4 km s~1 and 8^3 km s~1 ; de la Reza et al. 1989 ; Gregorio-Hetem et al. 1992). When combined with the very similar proper-motion vectors for all Ðve of these stars

TABLE 1 ATOMIC AND MOLECULAR LINES v (km s~1) helio

j (km)

Species a

TW Hya

HD 98800

CD [33¡7795

2.121021 . . . . . . 2.121396 . . . . . . 2.121732 . . . . . . 2.12183 . . . . . . . 2.121952 . . . . . . 2.122210 . . . . . . 2.123141 . . . . . . 2.124426 . . . . . .

Si Al C H 1È0 S(1) 2 Mg Fe Mg Fe

11.9 ^ 0.5 ... ... ... ... ... ... 14.6 ^ 1.5

11.1 ^ 0.4 ... ... ... ... ... ... 10.4 ^ 0.5

... ... ... ... ... ... ... 20 ^ 3

a From Hinkle et al. 1995.

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DETECTION OF MOLECULAR HYDROGEN GAS AROUND TW HYA

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FIG. 1.ÈRatioed spectra of the T Tauri stars TW Hya, HD 98800, and CD [33¡7795 after removal of the telluric absorption features (carried out by dividing by the spectrum of the calibrator star, HD 92845). Many photospheric absorption features are seen in these spectra. Only TW Hya shows clear evidence of emission in the 1È0 S(1) line of H . The spectra are presented in the rest frames of the three stars. 2

(Webb et al. 1999), there is compelling evidence that they all share a common space velocity. For CD [33¡7795, we measure a radial velocity of 20 ^ 3 km s~1 using only the 2.12446 km Fe line ; however,

this line, which is weaker for this star than for the other two stars in our sample, may be strongly a†ected by the incomplete correction of the atmospheric absorption at 2.1244 km, which would bias our velocity calculation redward. We

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WEINTRAUB, KASTNER, & BARY

conclude that this line velocity is consistent with the range of velocities of the other TWA stars and compatible with the conclusion that CD [33¡7795 shares a common space velocity with the other TWA members. A single emission feature is present in the spectrum of TW Hya at j ^ 2.1218 km (Fig. 1). This emission line is asymmetric and unresolved (measured FWHM \ 6 km s~1) and has an equivalent width of 0.02 AŽ . This feature, which falls at the wavelength of the 1È0 S(1) line of H in the 2 rest frame of TW Hya, is not present in the spectra of HD 98800 or CD [33¡7795 (or HD 92845). Although the spectra presented in Figure 1 are corrected for telluric absorption through the division of the source spectra by the spectrum of HD 92845, these spectra still contain photospheric spectral features, some of which we have been able to identify (Table 1). In order to correct for photospheric lines in the spectrum of TW Hya, we have divided the TW Hya spectrum by that of HD 98800, showing the full spectral coverage in the middle panel of Figure 2 and a close-up of the H line, 2 the plotted versus velocity rather than wavelength, in bottom panel. This division removes most photospheric absorption lines common to both systems (which have very similar stellar spectral types) and emphasizes that the H emission line is found only in the spectrum of TW Hya and2 is the only emission feature seen in this spectrum. Other features that remain in this spectrum are likely indicative of small di†erences between photospheric spectra of K5 (HD 98800) and K7 (TW Hya) stars.

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In the top panel of Figure 2 we present a noise spectrum that is obtained by ratioing two fully calibrated spectra of HD 92845. This noise spectrum shows the variability of the atmosphere and reveals the imperfections in our ability to remove the telluric absorption features. Notably, variations from unity are typically 1% and occasionally 2%. There are no noise features that compare with the emission feature seen in the TW Hya spectrum at 2.1218 km, which rises 4% above the continuum and has a width of at least Ðve pixels above a normalized intensity of 1.02. Thus, this comparison emphasizes the strength of the H emission line in the TW 2 Hya spectrum and makes clear that the strong dips below unity in this spectrum are real photospheric features. Careful examination of Figure 1 reveals that the H emis2 sion line in the TW Hya spectrum falls at a position where the spectra of both HD 98800 and CD [33¡7795 are depressed relative to their respective continuum levels. Given the slightly earlier (HD 98800) and later (CD [33¡7795) spectral types of these two stars compared to TW Hya, it is likely that the H emission line in the TW 2 Hya spectrum sits on top of a spectral region that otherwise would fall below the continuum. From this analysis, we conclude that we have detected the 1È0 S(1) line of molecular hydrogen in the spectrum of TW Hya and henceforth refer to this spectral feature as an H emission line. If the H emission line is excited2by the X-ray ionization 2 we can predict the expected Ñux in the 1È0 S(1) mechanism, line based on the models and data in Maloney et al. (1996) and Tine et al. (1997). Taking L D 1030 ergs s~1 for TW x

FIG. 2.ÈT op : Noise spectrum, obtained by ratioing two spectra of the calibrator star HD 92845, illustrating that most imperfections in correcting for telluric lines fall within the range of ^1%. Middle : The ratioed spectrum of TW Hya divided by the spectrum of HD 98800 in the rest frame of TW Hya. The division removes the signatures of most of the absorption features shared by these two stars of similar spectral types. The remaining feature is the H emission line at 2.1218 km. Bottom : The close-up covering 0.0015 km centered on H line at 2.1218 km, with the horizontal axis translated into velocity 2in the rest 2 frame of TW Hya.

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DETECTION OF MOLECULAR HYDROGEN GAS AROUND TW HYA

Hya (Kastner et al. 1997) and extrapolating from Figure 6a in Maloney et al., we Ðnd that the intensity of the line emission would be ^10~4 ergs s~1 cm~2 sr~1 for a plausible H disk density of n \ 107 cm~3. Adopting a disk 2 radius appropriate for TW Hya of D0A. 5, the total line Ñux from the disk would be ^2 ] 10~15 ergs s~1 cm~2. The observed H line, which we estimate has a signal-to2 noise ratio of D12.5 in these data, has a total intensity of 1.0 ] 10~15 ergs s~1 cm~2. Given the uncertainties in the model and in our assumptions, the actual line intensity is remarkably close to that predicted for TW Hya. The line peaks at the systemic velocity of TW Hya although the line center, as determined by a Gaussian Ðt to the slightly asymmetric proÐle, appears slightly blueshifted with respect to the heliocentric radial velocity of TW Hya. A small blueshift relative to the star could arise from the line-of-sight component of a region of high-velocity (several tens of km s~1) shocked gas whose dominant velocity component is close to the plane of the sky ; however, there is no evidence for shocked gas or molecular outÑows in the TW Hya system. In addition, there is considerable evidence that the TW Hya system is viewed nearly pole-on (see ° 1). We conclude that the Mg absorption feature at 2.12195 km biases the H emission slightly blueward and the emission in fact arises at2 the stellar radial velocity. The H molecular line emission, unlike the other identi2 Ðed features in the near-infrared spectrum of TW Hya, cannot be associated with the K7 stellar photosphere. On the other hand, the narrow width of the H line is consistent with those of radio emission lines from2 numerous other molecules seen toward TW Hya (Kastner et al. 1997). Such narrow molecular emission lines are consistent with gas in the Keplerian orbits in circumstellar disks viewed pole-on. Thus, we conclude that the H emission from TW Hya originates from its circumstellar2disk. In contrast, the nondetections of H emission from HD 98800 and CD 2 [33¡7795 are consistent with the lack of CO emission from the former star (Zuckerman et al. 1995) and with the lack of a detectable IR excess in the spectral energy distribution of the latter star. 4.

DISCUSSION

The spectrum of TW Hya demonstrates the existence of an emission from the quiescent H in the circumstellar disk around a classical T Tauri star. 2Only two related mechanisms could account for the presence of the near-infrared H emission : X-ray or UV excitation. While the U[B color for2 TW Hya is in the normal range for T Tauri stars (Herbig

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& Bell 1988), TW Hya is one of the brighter T Tauri stars in the near-ultraviolet based on this criterion. However, TW Hya is perhaps the brightest classical T Tauri star in X-ray emission as a fraction of its bolometric luminosity (L ), bol with L /L ^ 10~3 (Kastner et al. 1997). Thus, while we X bol cannot dismiss UV excitation as a mechanism for exciting the H , these results raise the intriguing possibility that TW 2 Hya provides the Ðrst example in which the presence of H 2 has been detected in a circumstellar disk through X-ray excitation (Gredel & Dalgarno 1995 ; Maloney et al. 1996 ; Tine et al. 1997). The direct detection of H line emission in the near2 infrared spectrum of TW Hya, an X-ray bright, classical T Tauri star for which the presence of a gaseous disk was previously established (Kastner et al. 1997), lends support to the notion that the hypothesized X-ray excitation mechanism operates in these environments. Since Zuckerman et al. (1995) have established the relatively gas-poor nature of the TW Hya disk, our results demonstrate that the H emis2 sion at 2.1218 km is a useful diagnostic of the presence of residual circumstellar gas. Additional measurements such as these, coordinated with new theoretical work, could enable the determination of the fractional mass of H that is emitting in the 1È0 S(1) line as 2 a result of X-ray excitation. Then observations of nearinfrared H lines could be used to estimate the total mass 2 the gas-to-solid fractionation of CO and the and perhaps gas-to-dust ratios in such disks. Such observations could provide important constraints on the timescale for the formation of planets around Sun-like, preÈmain-sequence stars. We also have demonstrated that the radial velocities of TW Hya and the multiple systems HD 98800 and CD [33¡7795 can be determined from atomic transitions in high-resolution near-infrared spectra and that the systemic velocities of all three systems are within a few km s~1 of each other. Webb et al. (1999) have already shown that TW Hya, HD 98800, and CD [33¡7795 have similar common proper-motion vectors (which correspond to tangential velocities of 1.8È2.2 km s~1). Thus, the space velocities of these three stars are dominated by their radial velocities, and the overall similarity of their motions adds greater support to the theory that the TW Hya association stars share a common origin. This work is supported by NASA grants NAG5-8295, NAG5-4428, and GO07861.01-96A to Vanderbilt University.

REFERENCES de la Reza, R., Torres, C. A. O., Quast, G. R., Castilho, B. V., & Vieira, Krist, J. E., Stapelfeldt, K. R., Burrows, C. J., Menard, F., & Padgett, D. L. G. L. 1989, ApJ, 343, L61 1999, BAAS, 31, 935 Gredel, R., & Dalgarno, A. 1995, ApJ, 446, 852 Maloney, P. R., Hollenback, D. J., & Tielens, A. G. G. M. 1996, ApJ, 466, Greene, T. P., Tokunaga, A. T., Toomey, D. W., & Carr, J. S. 1993, Proc. 561 SPIE, 1946, 313 Thi, W., van Dishoeck, E. F., Blake, G. A., van Zadelho†, G., & HogGregorio-Hetem, J., Lepine, J. R. D., Quast, G. R., Torres, C. A. O., & de la erheijde, M. R. 1999, ApJ, 521, L63 Reza, R. 1992, AJ, 103, 549 Tine, S., Lepp, S., Gredel, R., & Dalgarno, A. 1997, ApJ, 481, 282 Herbig, G. H., & Bell, K. R. 1988, Lick Obs. Bull., No. 1111 Torres, G., Stefanik, R. P., Latham, D. L., & Mazeh, T. 1995, ApJ, 452, 870 Herbst, T. M., Beckwith, S. V. W., Glindemann, A., Tacconi-Garman, L. E., Webb, R. A., Zuckerman, B., Patience, J., White, R. J., Schwartz, M. J., Kroker, H., & Krabbe, A. 1996, AJ, 111, 2403 McCarthy, C., & Platais, I. 1999, ApJ, 512, L63 Herbst, T. M., Robberto, M., & Beckwith, S. V. W. 1997, AJ, 114, 744 Weinberger, A. J., Schneider, G., Becklin, E. E., Smith, B. A., & Hines, D. C. Hinkle, K., Wallace, L., & Livingstron, W. 1995, Infrared Atlas of the 1999, BAAS, 31, 934 Arcturus Spectrum, 0.9È5.3 km (San Francisco : ASP) Weintraub, D. A., Sandell, G., & Duncan, W. D. 1989, ApJ, 340, L69 Kastner, J. H., Huenemoerder, D. P., Schulz, N. S., & Weintraub, D. A. Zuckerman, B., Forveille, T., & Kastner, J. H. 1995, Nature, 373, 494 1999, ApJ, 525, 837 Kastner, J. H., Zuckerman, B., Weintraub, D. A., & Forveille, T. 1997, Science, 277, 67