The Astrophysical Journal, 668:1174 Y1181, 2007 October 20 A # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. 1 A LOW-MASS H2 COMPONENT TO THE AU MICROSCOPII CIRCUMSTELLAR DISK Kevin France,2 Aki Roberge,3 Roxana E. Lupu,4 Seth Redfield,5,6 and Paul D. Feldman4 Received 2007 March 25; accepted 2007 July 3 ABSTRACT We present a determination of the molecular gas mass in the AU Microscopii circumstellar disk. Direct detection of a gas component to the AU Mic disk has proven elusive, with upper limits derived from ultraviolet absorption line and submillimeter CO emission studies. Fluorescent emission lines of H2,pumpedbytheOvi k1032 resonance line through the CYX (1Y1) Q(3) k1031.87 8 transition, are detected by the Far Ultraviolet Spectroscopic Explorer. These lines are used to derive the H2 column density associated with the AU Mic system. The derived column density is in the 17 15 À2 range N(H2) ¼1:9 ; 10 to 2:8 ; 10 cm , roughly 2 orders of magnitude lower than the upper limit inferred from absorption line studies. This range of column densities reflects the range of H2 excitation temperature consistent with the observations, T(H2) ¼ 800Y2000 K, derived from the presence of emission lines excited by O vi in the absence of those excited by Ly . Within the observational uncertainties, the data are consistent with the H2 gas residing in P 1 ; À4 ; the disk. The inferred N(H2) range corresponds to H2-to-dust ratios of 30 :1 and a total M(H2) ¼ 4:0 10 to 5:8 À6 10 MÈ. We use these results to predict the intensity of the associated rovibrational emission lines of H2 at infrared wavelengths covered by ground-based instruments, HST NICMOS, and the Spitzer IRS. Subject headinggs: circumstellar matter — planetary systems: protoplanetary disks — stars: individual (AU Microscopii, HD 197481) — ultraviolet: stars Online material: color figure 1. INTRODUCTION low us to examine possible differences between disks of the same age around stars of very different mass. Circumstellar (CS) disks around young main-sequence stars ap- Liu et al. (2004) measured the 850 m dust emission from the pear to be in transition between massive, gas-rich protoplanetary AU Mic disk and inferred a total mass of 0.011 M at T 40 K. disks and low-mass, gas-poor planetary systems. Surveys for CS È dust ¼ No submillimeter CO emission was detected, implying a low CS disks in young stellar clusters suggest that gas-rich protoplane- gas mass; however, this upper limit was not very stringent. A much tary disks dissipate on timescales of 1Y10 Myr (Bally et al. 1998; lower upper limit on the column density of molecular gas was de- Haisch et al. 2001). For solar-type stars, the timescale for gas dis- termined using far-UV H absorption spectroscopy (Roberge et al. sipation is roughly equal to the theoretical time required for gas 2 giant planet formation by the standard core-accretion method (e.g., 2005). This study indicated that the H2-to-dust ratio in the disk is less than about 6 :1, dramatically depleted from the canonical Hubickyj et al. 2005). For low-mass M stars, on the other hand, interstellar ratio of 100 :1. Roberge et al. (2005) mentioned that the time required to form giant planets by core accretion is much there was an indication of weak H absorption at a column den- longer than it is around solar-type stars, and may be longer than 2 sity at least an order of magnitude below their upper limit. They the typical gas disk lifetimes (Laughlin et al. 2004). Consequently, also noted the presence of very weak fluorescent H emission lines observations of the gas component in the disks of low- and 2 in the far-UV spectra, which had been seen in previous work intermediate-mass main-sequence stars undergoing the transition (Redfield et al. 2002). from gas-rich protoplanetary disk to gas-poor debris disk are im- An M star has insufficient continuum flux to excite (‘‘pump’’) portant for constraining giant planet formation scenarios. far-UV fluorescent emission. In general, only stars of type B3 AU Microscopii is a nearby (d 10 pc) M1 star surrounded k by an edge-on (inclination 1 Y3) dust disk (Kalas et al. 2004; and earlier have the necessary spectral energy distribution ( 1110 8) to produce detectable levels of continuum pumped UV Krist et al. 2005). The star is a very active flare star, and the ma- fluorescence (France 2005). However, pre-main-sequence and jority of AU Mic studies prior to imaging of its disk focused on the stellar activity (Redfield et al. 2002 and references therein). low-mass dwarf stars show high-temperature emission lines aris- ing in their chromospheric and coronal regions (Linsky & Wood AU Mic is a member of the Pictoris moving group, indicating 1994; Wood et al. 1997; Redfield et al. 2002, 2003). These stellar an age of t ¼ 12þ8 Myr (Zuckerman et al. 2001). The star AU Mic À4 emission lines can coincide in wavelength with transitions of H , Pic (A5 V) itself has a well-studied edge-on debris disk (Roberge 2 providing the necessary flux to excite detectable fluorescence when et al. 2000; Lecavelier des Etangs et al. 2001). These two stars al- CS material is present. Line pumped H2 fluorescence has been observed from a variety of environments, including T Tauri disks 1 Based in part on observations made with the Far Ultraviolet Spectroscopic (Valenti et al. 2000; Wilkinson et al. 2002; Herczeg et al. 2006), Explorer, operated by the Johns Hopkins University for NASA. sunspots (Jordan et al. 1977), solar system comet comae (Feldman 2 Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8; [email protected]. et al. 2002), and recently from planetary nebulae (Lupu et al. 3 Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight 2006). Center, Greenbelt, MD 20771; [email protected]. Here we present a detailed description and analysis of the fluo- 4 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, rescent H emission lines in Far Ultraviolet Spectroscopic Ex- MD 21218; [email protected], [email protected]. 2 5 Department of Astronomy, University of Texas, Austin, TX 78712; plorer (FUSE) spectra of AU Mic. The majority of line pumped sredfi[email protected]. fluorescence studies have focused on H2 excitation by Ly emis- 6 Hubble Fellow. sion (Shull 1978; Black & van Dishoeck 1987). However, in the 1174 H2 GAS IN AU MIC 1175 Fig. 1.—FUSE LiF 1B data for the CYX (1Y3) Q(3) k1119.08 8 and (1Y 4) Q(3) k1163.81 8 emission lines. The broad feature in the left panel is coronal Fe xix ( Redfield et al. 2002, 2003). For display, measurement uncertainties on the flux calibration are shown as the error bars on the crosses. [See the electronic edition of the Journal for a color version of this figure.] case of AU Mic, we show that the observed H2 emission lines are 1187 8) detector segments. Archival HST Space Telescope Imag- pumped by the O vi k1032 stellar emission line, which is an im- ing Spectrograph (STIS) spectra of the chromospheric C iii k1176 portant excitation source in some situations (Wilkinson et al. 2002; multiplet were used to establish the wavelength calibration of the Redfield et al. 2002; Herczeg et al. 2006). In x 2 of this paper, we FUSE LiF 1B channel (Roberge et al. 2005). Due to a lower back- briefly describe the observations and the characteristics of the ground in the (1Y3) Q(3) k1119.08 8 region, we present the LiF detected fluorescent H2 emission lines. In x 3, we calculate the 1B data here. total fluorescent H2 flux, constrain the H2 temperature, and deter- The H2 emission lines observed in the LiF 1B channel are mine the total column density of absorbing H2. Our estimation shown in Figure 1. The CYX (1Y3) Q(3) k1119.077 8 and CYX of the total mass of H2 gas in the AU Mic system appears in x 4. (1Y 4) Q(3) k1163.807 8 transitions are shown in the left and This section also contains a discussion of the H2 heating and its right panels, respectively. These lines are excited by absorption implications for the gas location. A comparison between the ob- in the CYX (1Y1) Q(3) k1031.87 8 line, which is coincident with served AU Mic and Pic disk properties is given in x 5, and pre- the strong stellar O vi k1032 emission line. A description of the dictions for the near- and mid-IR H2 emission line fluxes of AU electronic excitation of H2 and the molecular notation is given in Mic appear in x 6. Our results are summarized in x 7. Shull & Beckwith (1982). The (1Y3) and (1Y 4) lines are expected to be the brightest ones in the O vi pumped fluorescent cascade 2. FUSE OBSERVATIONS (Abgrall et al. 1993b). A Gaussian least-squares fitting routine AND H2 LINE IDENTIFICATION was used to derive the line strength, width, velocity, and detec- FUSE performs medium-resolution (Áv 15 km sÀ1)spectros- tion levels. The lines were marginally resolved and displayed a copy in the far-UV bandpass (905Y1187 8). The FUSE obser- slight blueshift with respect to the stellar velocity (và ¼4:98 Æ À1 Y vatory is described in Moos et al. (2000), and on-orbit performance 0:02 km s ). The (1 3) line was detected at 2.6 with an integ- ; À15 À1 characteristics are given in Sahnow et al.