Depletion of molecular gas by an accretion outburst in a protoplanetary disk
A. Banzatti, K. M. Pontoppidan, S. Bruderer, J. Muzerolle, M. R. Meyer
AAccepted by the Astrophysical Journal Letters
Draft version October 14, 2018
Preprint typeset using L A TEX style emulateapj v. 8/13/10
DEPLETION OF MOLECULAR GAS BY AN ACCRETION OUTBURST IN A PROTOPLANETARY DISK
A. Banzatti , K. M. Pontoppidan , S. Bruderer , J. Muzerolle , M. R. Meyer Draft version October 14, 2018
ABSTRACTWe investigate new and archival 3–5 µ m high resolution ( ∼ − ) spectroscopy of molecular gasin the inner disk of the young solar-mass star EX Lupi, taken during and after the strong accretionoutburst of 2008. The data were obtained using the CRIRES spectrometer at the ESO Very LargeTelescope in 2008 and 2014. In 2008, emission lines from CO, H O, and OH were detected with broadprofiles tracing gas near and within the corotation radius (0.02–0.3 AU). In 2014, the spectra displaymarked differences. The CO lines, while still detected, are much weaker, and the H O and OH lineshave disappeared altogether. At 3 µ m a veiled stellar photospheric spectrum is observed. Our analysisfinds that the molecular gas mass in the inner disk has decreased by an order of magnitude since theoutburst, matching a similar decrease in the accretion rate onto the star. We discuss these findingsin the context of a rapid depletion of material accumulated beyond the disk corotation radius duringquiescent periods, as proposed by models of episodic accretion in EXor type young stars. Subject headings: circumstellar matter — protoplanetary disks — stars: individual (EX Lupi) — stars:pre-main sequence — stars: variables: T Tauri, Herbig Ae/Be INTRODUCTION
During star formation, the rate of accretion of circum-stellar disk material onto the pre-main sequence star isnot constant. Sharp order-of-magnitude enhancementsin the mass accretion rate called accretion outbursts havelong been known to occur in young stars (Herbig 1977).Historically, two classes of eruptive young stars havebeen observationally identified. FUor (FUOri-type) sys-tems are characterized by bursts in accretion rate upto ∼ − M (cid:12) yr − and duration of 100s years, whileEXLupi-type systems (called “EXors” by Herbig 1989)show episodic outbursts that are a factor of ≈
100 weakerboth in accretion rate and in duration. These accretionoutbursts are thought to be linked to the evolution ofprotoplanetary disks. In fact, the inner region of the diskdirectly feeds the central star, and both its energy budgetand its chemistry are modified by high-energy radiationsuch as that generated by accretion shocks (e.g. Hart-mann & Kenyon 1996; Audard et al. 2014). While FUorshave longer been known, EXors have recently attractedincreasing attention due to their shorter accretion cycletimescale that provides exceptional opportunities for ob-serving recurring accretion events and ongoing physicaland chemical processing of disk material ( ´Abrah´am et al.2009; Banzatti et al. 2012, and Figure 1).A theoretical picture of episodic accretion in EXors isemerging in which mass accumulates outside the star-disk corotation radius until the surface density increasesenough to overcome the centrifugal barrier, perturb-ing the magnetospheric radius inward of corotation andbringing the inner disk into a high-accretion phase untilthe built-up material is depleted (Spruit & Taam 1993; [email protected] Space Telescope Science Institute, Baltimore, MD 21218,USA Max-Planck-Institut f¨ur Extraterrestrische Physik, Giessen-bachstr. 1, D-85748 Garching bei M¨unchen, Germany ETH Z¨urich, Institut f¨ur Astronomie, Wolfgang-Pauli-Strasse 27, CH-8093 Z¨urich, Switzerland
D’Angelo & Spruit 2010, 2011, 2012). Determining theamount and extent of gas in the inner disk before, dur-ing, and after an outburst can therefore provide uniqueconstraints to episodic accretion models and help under-stand the basic physics governing the evolution of innerregions in accreting protoplanetary disks.In this paper, we compare new and archival near-infrared (NIR) spectroscopy of molecular gas (CO, H O,and OH) during and after the 2008 outburst of EX Lupi.EX Lupi is a 1–3 Myr old T Tauri star with M ∗ = 0 . M (cid:12) , R ∗ = 1 . R (cid:12) , and spectral type M0 (Gras-Vel´azquez &Ray 2005), at a distance of 155 pc in the Lupus complex(Lombardi et al. 2008). It is the prototypical EXor vari-able (Herbig 1989; Herbig et al. 2001), where the recordof episodic accretion flares dates back to 1901 (McLaugh-lin 1946). In 2008 the strongest outburst on record oc-curred with a 5 mag increase in the V band (Figure 1).On that occasion, EX Lupi was observed over a broadwavelength range from the X-ray and UV (Grosso et al.2010), to the optical (Aspin et al. 2010; Sicilia-Aguilaret al. 2012) and the infrared (Goto et al. 2011; K´osp´al etal. 2011) using a plethora of telescopes. With this work,we build upon the heritage of NIR observations taken in2008. Using new NIR spectroscopy obtained in 2014, wefind that the inner disk is depleted by one order of mag-nitude in molecular gas mass, unveiling a forest of stellarphotosphere lines at 3 µ m. CRIRES SPECTROSCOPY OF EXLUPI AT 3 AND 5 µ M We present velocity-resolved infrared spectroscopy ofEX Lupi using the CRyogenic Infrared Echelle Spectrom-eter (CRIRES, Kaeufl et al. 2004) on the Very LargeTelescope of the European Southern Observatory. Newdata were taken in April 2014, covering the spectral re-gions 2.91–3.07 µ m and 4.66–4.89 µ m (program 093.C-0432). Similar spectral regions had been observed duringthe accretion outburst in 2008, in April (2.91–2.99 µ m)and in August (4.66–4.90 µ m), as part of the ESO LargeProgram 179.C-0151 (Pontoppidan et al. 2011a). In thiswork, we compare spectral regions common to the two a r X i v : . [ a s t r o - ph . E P ] D ec SP I TZE R - I R S V LT - CR I R E S NOT AVAILABLE / NO DATA H O C H HCN H O OH H OH O photospheric standard (veiled) H O H O, OH HCN, C H (no data) H O, OHCO, H O, OH (broad) (no data) weak CO (broad) - M s o l / y r Accumulation Release Accumulation AU AU AU CO (narrow) CO (narrow) v =1-0 v =2-1 v =1-0 PRE-OUTBURST OUTBURST
CO CO
POST-OUTBURST
NOT AVAILABLE (warm phase) OH - M s o l / y r - M s o l / y r EX Lupi transitional disk - TWHya (veiled)
Fig. 1.—
Overview of infrared molecular line emission changes monitored in EX Lupi before, during, and after the 2008 outburst.
Top
Middle : portions of EX Lupi spectra from
Spitzer -IRS (Banzatti et al. 2012) and VLT-CRIRES (this work).
Bottom : cartoon illustrating the inner disk in EX Lupi during the three phases of accretion, highlighting the molecular gas location. epochs: 2.91–2.96 µ m (henceforth referred to as the 3 µ msetting) and 4.66–4.89 µ m (henceforth referred to as the5 µ m setting). The two epochs were taken using identi-cal instrumental parameters. The slit width was ∼ . ∼ . − , and theon-source integration time was 20 min per spectral set-ting. Early-type telluric standard stars were observed atsimilar airmasses to EX Lupi (typically within 0.05, andalways less than 0.1 difference): HR5812 at 3 µ m, BS6175and HR5984 at 5 µ m. The data were reduced applyingprocedures developed for the ESO Large Program 179.C- 0151, as described in Pontoppidan et al. (2011b). Thespectra were photometrically calibrated using the telluricstandards as spectrophotometric references , achieving aprecision of (cid:46)
25% of the absolute fluxes (including fluxuncertainties from the standard stars and variable atmo-spheric and slit transmission at the time of observations).We find that the continuum in EX Lupi decreased from1 . ± . . ± .
04 Jy in April 2014 Absolute fluxes for the standards were interpolated using pho-tometry available through the VizieR catalogue, CDS, Strasbourg,France. at 3 µ m, and from 4 . ± . . ± .
03 Jy at 5 µ m.Together with a decrease in the continuum flux level,the NIR veiling decreased from 2008 to 2014. TheCRIRES spectra of EX Lupi obtained in 2014 show amoderately veiled stellar photosphere that was not pre-viously detectable in outburst (Figure 1). FollowingMuzerolle et al. (2003), we estimate the veiling ( r λ = EW ref /EW −
1, where EW is the equivalent width ofphotospheric lines) by comparison to a reference spec-trum of a main sequence standard star of similar spectraltype. The standard star spectrum (HIP49986, M2V) wasobtained using CRIRES and the same instrumental setupas for EX Lupi. We find a 3 µ m veiling of r = 3 . ± . µ m, no photospheric main sequence standards wereobserved, so we use TW Hya, a transitional disk (Calvetet al. 2002) that is known to have low veiling ( r = 1 . r = 4 . ± . µ m for EX Lupi. We then deter-mine the excess flux F exc = F λ × r λ / (1+ r λ ), and estimatethe temperature of the veiling dust by assuming black-body emission. While an additional veiling measurementat 2 µ m is needed to improve the fit, we find the 3–5 µ mexcess flux to be best represented by a temperature of ∼ EVOLUTION OF THE MOLECULAR LINE EMISSION
At NIR wavelengths, protoplanetary disks are knownto show line emission from CO at 5 µ m (e.g. Najita et al.2003), and H O, OH, HCN, C H at 3 µ m (Salyk et al.2008; Mandell et al. 2012). Since the CO spectra pro-vide broad coverage of upper level energies and opticaldepths they provide the best constraints on the physi-cal gas properties (Section 3.1). We apply the best-fitgas parameters from CO to the H O and OH spectra inSection 3.2.
CO emission
Two velocity components of CO rovibrational lines aredetected in emission at 5 µ m, both in 2008 and in 2014(Figure 2). A narrow component is detected only in v = 1 − v = 4 in2008 (see also Goto et al. 2011). We characterize theline profile of the different velocity components by stack-ing CO lines between P3 and P18 to increase the signal-to-noise ratio. The number of lines we included in theaverage depends on the spectral coverage obtained, in-terrupted by detector chip gaps and telluric absorption,and on the level of contamination from higher vibrationallevels. In 2008, neighboring lines from higher vibrationallevels build up signal on top of the continuum at bothsides of the stacked v = 1 − v = 2 − v = 2 − v = 1 − v = 1 − Fig. 2.—
Averaged CO line profiles from the CRIRES spectra ofEX Lupi. The individual lines used in the average are listed at thetop of each plot. Gaussian fits to the different velocity componentsare shown as dashed colored lines overlaid to the data. who attributed it to a disk wind developed during theoutburst.We measure a full width at half maximum of ∼ − for the broad component, and of ∼ − for the narrow component in 2008 and2014 respectively. The disk inclination in EX Lupi isstill uncertain, but a value of 45 ◦ was proposed by pre-vious modeling of NIR CO emission (Goto et al. 2011;K´osp´al et al. 2011). Assuming this inclination and gas inKeplerian rotation, and using the line velocity measuredat 10% of the line peak, the CO line widths translate intoemitting inner disk radii R in of 0.02 AU for the broadcomponent, and 0.2–0.3 AU for the narrow component.To measure line fluxes of the two velocity componentsseparately, we fit for the amplitudes of the different Gaus-sian functions in individual CO v = 1 − T , the column density N , and the emittingarea A . The model uses the HITEMP and HITRANmolecular databases (Rothman et al. 2010, 2013) andassumes thermal equilibrium, i.e. a Boltzmann distri-bution for the level populations (an explicit descriptionwas included in Banzatti et al. 2012). We run grids ofmodels to explore a large parameter space that includesthe range of values found for NIR CO emission in pre-vious studies. We include all measurable CO v = 1 − −
1, and 3 − ≤ . v = 2 − v = 1 − τ = 0.05–6), so it can be used to estimate thetotal emitting CO gas mass as N × m co × A , where m co is the mass of the CO molecule. Over the same area, theH gas mass M H is estimated by assuming a fractionalabundance of CO/H = 10 − , as found appropriate forthe inner regions of a disk (France et al. 2014). In Fig-ure 3 we show M H in units of the total mass accretedduring outburst M acc = 6 × − M (cid:12) , estimated usingan average accretion rate of 10 − M (cid:12) /yr (Sicilia-Aguilaret al. 2012) over seven months of the outburst. We findthat M H /M acc ∼ , and that M H de-creased by roughly one order of magnitude from August2008 to 2014 in both the broad and narrow components. H O and OH emission
Emission lines from H O and OH are detected only inthe outburst spectrum, at 3 µ m (Figure 1). They haveline widths comparable to the broad component of the∆ v = 1 CO lines, suggesting that they trace moleculargas extending inward to similar disk radii (0.02 AU). TheH O spectrum is comparable to that seen in a few otherT Tauri systems so far (Salyk et al. 2008; Mandell et al. The 2008 CO spectrum was taken late during the outburst,when some fraction of the gas mass had already been accreted.
TABLE 1
Line Sample Parameter 2008 2014 R in (AU) 0 .
02 0 . T ex (K) 2400 +200 − +100 − CO broad N mol (10 cm − ) 6 . +2 . − . . +0 . − . A (AU ) 0 . +0 . − . . +0 . − . M H (10 − M (cid:12) ) 5 . +1 . − . . +0 . − . R in (AU) 0 .
30 0 . T ex (K) 800 +100 − +100 − CO narrow N mol (10 cm − ) 1 . +0 . − . . +0 . − . A (AU ) 2 . +1 . − . . +0 . − . M H (10 − M (cid:12) ) 10 . +4 . − . . +0 . − . Note . — R in is derived from line widths, using a diskinclination of 45 ◦ (Section 3.1); other parameters comefrom slab model fits. Errors show the 1 σ confidence oneach parameter estimate. O and OH emission, and to model the 3 µ m spec-trum we need column density ratios of CO/H O ∼ O ∼
1, in the range found by Mandell et al.(2012) in other disks. The broad CO component and theH O and OH gas therefore may have undergone the samedepletion and cooling from 2008 to 2014, explaining thedisappearance of line emission at 3 µ m. DISCUSSION
From the evolution in NIR CO emission observed bycomparison of CRIRES spectra of EX Lupi, we estimatedthat M H in the inner disk decreased by one order ofmagnitude from August 2008 to April 2014. EX Lupiunderwent a major accretion event in 2008, when theaccretion rate onto the star increased by two orders ofmagnitude to 10 − –10 − M (cid:12) yr − (Figure 1, and Aspinet al. 2010). However, the system spends most of its timein a quiescent state with a much lower accretion rate(Herbig et al. 2001; Sipos et al. 2009). Sicilia-Aguilaret al. (2012) estimated from monitoring of the H α ≈ − M (cid:12) yr − . Ifthe 2014 data show the typical conditions of the quiescentsystem, what we measure is a higher gas mass in the innerdisk during outburst (2008) by comparison to quiescence.This behavior is consistent with the current view ofepisodic accretion in young stars (see e.g. review byAudard et al. 2014). In particular, one set of mod-els have been shown to be relevant for the timescalesand strengths of EXor outbursts (Spruit & Taam 1993;D’Angelo & Spruit 2010, 2011, 2012). In these models,when the inner disk of a T Tauri star is truncated by thestellar magnetosphere outside (but close to) the corota-tion radius R c , the accretion rate onto the star is inhib-ited and gas accumulates beyond R c . When the surfacedensity becomes large enough to overcome the centrifu- Fig. 3.—
Top:
Two-dimensional confidence regions for the estimated model parameters (1, 2, and 3 σ ). Best-fit models are marked witha star. Bottom:
Population diagrams (Goldsmith & Langer 1999) showing best-fit models overlaid to the data. Crosses show the measuredline fluxes and their errors. Dots connected by a line show the unblended CO transitions from the models, while squares show the modelfluxes blended with higher vibrational transitions as observed. Blending with v ≥ gal barrier, the magnetospheric radius R m is perturbedinward of R c and accretion onto the star can proceedvigorously until the gas reservoir is emptied, eventuallycausing R m to migrate outward of R c again into a low-accretion phase. D’Angelo & Spruit (2012) showed thatthis accumulation-release behavior can lead to excursionsin the accretion rate of about two orders of magnitudeand outburst duration of months, consistent with obser-vations of EX Lupi (see cartoon in Figure 1). Using thedefinition in Bouvier et al. (2007), the radius R m wherethe dipolar field in the stellar magnetosphere truncatesthe disk in EX Lupi should be located at 0.2-0.3 AU in2014 and at 0.03-0.06 in 2008 (adopting a magnetic fieldstrength between 1 and 3 kG as appropriate for T Tauristars, Johns-Krull 2007). The corotation radius, wherethe disk rotation equals the stellar rotation, is instead at R c ∼ .
05 AU or outward at R c (cid:38) .
07 AU (estimatedusing a stellar v sin i = 4 . < − from Siposet al. 2009; K´osp´al et al. 2014, respectively). Althoughall these estimates are uncertain, the conditions for theD’Angelo & Spruit (2012) model may therefore be foundin EX Lupi, as R m is larger than R c in 2014 (when accre-tion onto the star is inhibited) and smaller than R c dur-ing outburst (when the accretion instead proceeds vigor-ously). The broad CO component traces the region atand inward of R c , as found by Salyk et al. (2011) for CO emission in other T Tauri disks; in the D’Angelo &Spruit (2012) scenario, the increase in gas mass by an or-der of magnitude would measure the release of gas when R m approaches R c . The narrow CO component, instead,may probe the gas accumulation/feeding region beyond R c . Indeed, during the outburst M H /M acc approachesunity, while in 2014 M H is just sufficient to feed a qui-escent accretion rate of ∼ − M (cid:12) yr − . R m in 2014is very similar to R in of the narrow CO lines, and mayexplain the reason of two physically separated emissioncomponents as due to the inner disk truncation by thestellar magnetosphere. An R in < R m for the broad com-ponent in 2014 may present an inconsistency with thispicture, unless R m can be put to 0.02 AU by the combi-nation of dipolar and octupolar field components (Gre-gory et al. 2008; Adams & Gregory 2012). To test thevalidity of this interpretation of the CO emission seen inEX Lupi, future monitoring should measure a steady gasmass from the CO broad component (until the next out-burst occurs) and an increasing gas mass in the narrowcomponent as gas accumulates during quiescence. Moni-toring variations in the NIR CO emission observed fromEXors could provide unique tests of episodic accretionmodels.EX Lupi should also be discussed in the context ofsimilarly young (1-5 Myr) disks. In Figure 4 we compare Fig. 4.—
Changes in gas emission in EX Lupi between 2008 and2014 (broad/narrow component in large/small symbols). Valuesfor “classical” (C) and “transitional” (T) are taken from Salyk etal. (2011). M H is estimated from N and A as in Section 3.1. Thecircumbinary disk (B) is taken from Carr et al. (2001). the gas properties as measured from NIR CO emission inEX Lupi to individual values found in other young disksby Salyk et al. (2011), who used methods comparable tothose used in this work. We include in the figure both COvelocity components from EX Lupi, although a multi-component analysis has not been performed yet in otherdisks. The narrow CO emission in EX Lupi shows prop-erties well within those found in other disks, while thebroad emission lies at the lower extreme in R in values.In 2008, the gas in EX Lupi reached masses consistentwith those of classical T Tauri disks. In 2014, instead,EX Lupi lies within the systematically lower values found in “transitional” and circumbinary disks, i.e. disks thatshow dissipation of gas and dust outward of the dustsublimation radius. An inner dust radius larger thansublimation has been proposed for EX Lupi by Sipos etal. (2009) and Juh´asz et al. (2012), at a location con-sistent with R in of the narrow CO lines (0.2–0.3 AU),and a close-in (0.06 AU) low-mass ( m sin i ∼
15 M
Jup )binary companion has been recently claimed by K´osp´alet al. (2014). Conclusive evidence is still awaited, andour findings raise questions on EX Lupi that may ap-ply to episodically accreting T Tauri systems in general.Does a link exist between accretion outbursts and thedissipation status of inner disks? Is EX Lupi a clas-sical T Tauri system that resembles a transitional diskin its inner disk gas properties, or a transitional diskthat looks classical only during accretion outbursts? Asa step toward answering such questions, we have foundthat high-resolution infrared spectroscopy of moleculargas helps to determine the dissipation and evolution ofinner disks, even in regimes where these are insufficientto produce significant signatures in the broad-band spec-tral energy distribution.The authors thank referee Greg Herczeg for commentsand suggestions that helped improving this work. A.B.acknowledges financial support by a NASA Origins of theSolar System Grant No. OSS 11-OSS11-0120, a NASAPlanetary Geology and Geophysics Program under grantNAG 5-10201. M.R.M. acknowledges financial support ofthe Swiss National Science Foundation within the frame-work of the National Centre for Competence in Research“PlanetS”. This work is based on observations madewith ESO telescopes at the Paranal Observatory underprograms 093.C-0432 and 179.C-0151.