arXiv:1906.04723v1 [astro-ph.SR] 11 Jun 2019 es aalxs n h lse sabout is cluster the ( and old parallaxes, 2 lease from members iay diinloclainb icmiaymat- 2002 ( circumbinary invoked by be the must to ter attributed Additional solely be with binary. duration combination cannot in variability, eclipse, long time the of the and days) of magnitudes) tens to (days (several depth large the as period ( same the binary ( has is variability passage Star B short-term apastron 2010 Star The each of after entirety at while the visible currently, occulted occulted; fully became was A B Star photometric 2009 term short (e.g., and variability long dramatic and unique at is which cluster 2264 NGC 2006 etiiy( centricity cpcbnr AB ihna-qa-ascomponents near-equal-mass (0.715 with (A+B) binary scopic ebte l 2002 al. et Herbst h ceti M iayadepanteavailable the explain and binary PMS ( eccentric dust the of ring nary eido 83 as( days 48.37 of period a H1Di r-ansqec PS spectro- (PMS) sequence pre-main a is 15D KH yee sn L using 2019 Typeset 12, June version Draft eryeg-n apdadpeesn circumbi- precessing and warped edge-on, nearly A ). ; alr2009 Naylor M rnwe l 2018 al. et Aronow 3 yU htn rmtenab asv trH 78 a truncat has Keywords: 47887 HD star massive r nearby value. dust observed the the the e from to that photons possibility extent UV the radial measu discuss by in we also similar We profiles, very variability. these photometric ring modeling system’s gaseous By a 15D. basically KH au, of 5 surface disk hot the blt.Tevraiiyi xlie yacrubnr utrn u h rin the double-peaked but the of ring interpretation dust new circumbinary a a present We by explained detected. is variability The ability. ⊙ ers&Hrs 1998 Herbst & Kearns eateto lntr cecs nvriyo rzn,1 Arizona, of University Sciences, Planetary of Department H1Di elkonsetocpcbnr eas fisuiu and unique its of because binary spectroscopic well-known a is 15D KH o trAad0.74 and A Star for e ≈ 1 eateto srnm,Uiest fAioa 3 ot C North 933 Arizona, of University Astronomy, of Department 0 A euie l 2018 al. et Venuti T . 1. ) eimjrai of axis semi-major a 6), E aeoe l 2012 al. et Capelo ; i Fang, Min ; X obepae Ipol:alkl intr ftegsosrn arou ring gaseous the of signature likely a profile: OI Double-peaked niiul(H1D tr:pemi sequence pre-main stars: — 15D) (KH individual i ta.2016 al. et Lim pncutr n soitos niiul(G 24 protoplane — 2264) (NGC individual associations: and clusters open ono ta.2004 al. et Johnson INTRODUCTION ∼ twocolumn aitne l 2001 al. et Hamilton 4 o5a)i huh ooccult to thought is au) 5 to 1 ieCleeAtooyDprmn,SihClee Northam College, Smith Department, Astronomy College Five 2 ono ta.2004 al. et Johnson atsi te oa ytm em AANxsfrExoplanet for Nexus NASA Team, Systems Solar Other in .I samme fthe of member a is It ). ,2 1, M lraPascucci, Ilaria ; ∼ tl nAASTeX62 in style ⊙ aitne l 2001 al. et Hamilton 5 c ae nthe on based pc, 750 .K 5 exhibits 15D KH ). o trB,hg ec- high B), Star for and rnwe l 2018 al. et Aronow .Fo 06to 2006 From ). .Hwvr the However, ). Gaia ∼ ; ; ebte al. et Herbst .5a,and au, 0.25 ine al. et Winn ∼ aaRe- Data – Myr 2–6 ,2 3, iyugSrn Kim, Serena Jinyoung ABSTRACT ) ; 2 atUiest olvr,Tco,A 52,USA 85721, AZ Tucson, Boulevard, University East 629 in h ytmsosn nrrdecs msinout emission excess 8 infrared no to shows system The tion. ile pe iiso h oa gsds)circumbi- (gas+dust) total the on have wavelengths limits observations millimeter millimeter upper The yielded at dust ALMA. the detected or SMA addition, been with In not has edge. dust ring inner large relatively 2004 ( curves light discus- the (see fitting the in by Sect. excluded in contaminated thus sion range grey-color and The line wavelength sky line). the the (blue marks panel region the in filled shown also is profile 1. Figure eeec rm bando 01Dcme 1 h rfieis H profile the The with 21. December compared 2001 on obtained frame reference r of 2 ure .Hwvr h icmiayds ldddetec- eluded disk circumbinary the However, ). µm er vne usn Z871 USA 85721, AZ Tucson, Avenue, herry eige al. et Deming ( [O rlnnhme l 2016 al. et Arulanantham [O 2 ,2 1, I ). I ] hag&Mra-ly2004 Murray-Clay & Chiang tn A003 USA 01063, MA pton, ] λ λ dteotreg fteds to disk the of edge outer the ed n ua Edwards Suzan and tra hteaoaindriven photoevaporation xternal 30polsa rgntn from originating as profiles 6300 eeitn ai between radii emitting re 30ln rfie(ryln)i h system’s the in line) (grey profile line 6300 n nerdfo oeigthe modeling from inferred ing ytmScience System 2 rmtcpooercvari- photometric dramatic - ()dgtlyetatdfo Fig- from extracted digitally S(1) 1-0 ( tefwsnvrdirectly never was itself g 2004 dK 15D KH nd rdln) h etfi model fit best The line). (red ) aydss—stars: — disks tary 4 ,i iewt the with line in ), ∼ ; 0.5– ine al. et Winn 2 nary disk mass of 1.7 MJUP (Aronow et al. 2018), just to the systemic velocity and present the spectrum with a factor of ∼ 2 lower than the mean protoplanetary the highest S/N, the one obtained on 2001 December disk mass in young (1-3 Myr) star-forming regions (e.g., 21, in Fig. 1. Individual Hα and [O I] λ6300 profiles are Ansdell et al. 2016, Pascucci et al. 2016). Interestingly, shown in Figs. 2 and 5. the SMA CO J = 3 − 2 line reveals a bipolar colli- The [O I] λ6300 emission shows the same double- mated outflow, the northern lobe of which is spatially peaked profile at all observing epochs. Note that the coincident with the H2 jet associated with KH 15D sharp blue-shifted spikes in some profiles are due to sky (Tokunaga et al. 2004; Aronow et al. 2018). Finally, the contamination because: (1) they are truly spikes, i.e. system presents broad Hα line profiles, indicative of ac- their width is much narrower than the spectral resolu- tive accretion (Hamilton et al. 2012), perhaps funneled tion (∼7kms−1); (2) sky lines peak near these spikes from gas surrounding both stars (see the case of the and, as they get stronger, so do the spikes; (3) for four spectroscopic binary DQ Tau, Muzerolle et al. 2019). spectra taken in 2001 covering also the [O I] λ5577 line, The presence of a jet and ongoing accretion point to the we detect the same strong and sharp spikes we see in presence of a gaseous disk. the [O I] λ6300 profiles. The background Hα line, pro- Here, we interpret the double-peaked [O I] λ6300 pro- duced mostly by the HII region (Cone Nebula) around files from KH 15D as originating from the surface of KH 15D, is also present but the ratio of its intensity to this gaseous disk (§2). Modeling of the line profiles re- the source emission is much smaller than the sky lines veals that the emitting gas is radially confined (§3). We at the [O I] λ6300 emission. This explains why we do not discuss how external photoevaporation could have trun- see such spikes in the reduced Hα profiles (see Fig. 2). cated the gaseous disk to the outer edge inferred from The [O I] λ6300 profiles for KH15D differ from those our [O I] λ6300 modeling (§4). found in most accreting T Tauri stars (TTS). In TTS with higher accretion luminosity the [O I] λ6300 pro- 2. DOUBLE PEAKED [O I] λ6300 PROFILES files typically show both a high velocity component The spectroscopic data presented in this work were (usually but not exclusively blueshifted) attributed to obtained from the VLT/UVES ESO archive for 3 a jet and a low velocity component (LVC) which is nights in 2001 and 6 nights in 2004 with a spectral also often blueshifted (Hartigan et al. 1995). The low resolution of ∼42,000 and have been already pub- velocity component is found in a broader range of lished in Hamilton et al. (2003); Mundt et al. (2010); TTS, including those with low accretion luminosity Hamilton et al. (2012). We extracted the raw data and transition disks, and itself can consist of both a from the ESO archive1 and reduced them with the ESO broad and a narrow component, which have been at- UVES pipelines under the ESO Recipe Flexible Exe- tributed to slow disk winds from the inner or outer disk, cution Workbench environment (Freudling et al. 2013). respectively (Hartigan et al. 1995; Simon et al. 2016; We obtain 21 spectra that cover both [O I] λ6300 and Fang et al. 2018; Banzatti et al. 2019). In some cases, Hα lines. The 2001 December 14 and 21 and the 2004 the low velocity component is centered on the stellar ve- December 16–18 (UT) spectra were taken during Star A locity, and may be bound material on the surface of the eclipse phase, the spectrum acquired on 2001 December disk, see e.g. Fig. 24 in Simon et al. (2016). The low 29 was out of eclipse, and the spectra obtained on 2004 velocity components are always single peaked and have December 13–15 were approaching Star A eclipse phase line widths that correlate with disk inclination, indica- (Mundt et al. 2010; Hamilton et al. 2012). tive of broadening by Keplerian rotation. We removed telluric absorption and subtracted pho- The [O I] λ6300 profile in KH 15D is distinctive from tospheric features near the [O I] λ6300 and Hα lines other TTS. The profile shows symmetric red and blue 2 −1 using a K7 PMS template (see e.g., Fang et al. 2018 peaks at radial velocities of ±20 kms and looks sim- 3 for details). We adopted a heliocentric radial veloc- ilar over multiple epochs . While such low velocities −1 ity vhelio=18.676 km s as the the systemic velocity (Mundt et al. 2010; Hamilton et al. 2012). This value is 2 We have mirrored and the subtracted the original spectrum consistent with the error-weighted mean in Fig. 1 and find no asymmetry greater than three times the rms (19.4±0.1 km s−1) of other NGC 2264 cluster members next to the line. 3 Note that the claimed factor of ∼ 2 decrease in the [O I] λ6300 (Kounkel et al. 2016). We shifted the [O I] λ6300 lines flux on 2001 December 20 (Mundt et al. 2010; Hamilton et al. 2012) is likely spurious. The ESO archive does not contain any

1 spectrum acquired on that date. Instead, most likely, the correct Based on observations collected at the European Southern spectrum is the one obtained on December 21st, the line profile Observatory under ESO programmes 267.C-5736(A) and 074.C- is the same as in Mundt et al. (2010). Because KH 15D bright- 0604(A). 3

Figure 2. Left: Hα line profiles when star A is fully occulted by the disk. Right: Hα line profiles when star A is partially occulted by the disk or out of the eclipse. Profiles are in the system’s reference frame. The dash-dotted lines mark the two −1 peaks (±20 km s ) from the [O I] λ6300 profiles. Note that the Hα profiles shown here and [O I] λ6300 profiles in Fig. 5 are extracted from the same spectra, hence correspond to the same observing times. are characteristic of TTS low velocity components, no observing seasons (from 2001 to 2006) over which star A other low velocity components to date have shown a was fully visible as well as partially occulted (in ingress double peaked structure. A jet close to the plane of the and egress events) and fully occulted. Some of these sky can have low radial velocities, as can be inferred for spectra are the same from which we have extracted the some TTS where line ratios for several forbidden lines [O I] λ6300 profiles and are presented in Fig. 2 in order reveal properties characteristic of shocked, rather than to emphasize the difference in the line profiles of Hα thermally excited, gas for a few sources that would be and [O I] λ6300. classified as LVC based on their kinematic properties The out of eclipse and ingress spectra (left panel) are (Fang et al. 2018). This is the interpretation proposed characterized by an inverse P-Cygni-type profile with by Mundt et al. (2010), that the [O I] λ6300 profile from a strong redshifted absorption whose changes in cen- KH 15D arises in a pair of identical approaching and troid velocity can be explained by an accretion stream receding jets in the plane of the sky. However, in this onto A (Hamilton et al. 2012). The eclipse and egress interpretation we would expect to see variable profiles at spectra (right panel) present a more symmetric profile different epochs as noticed in other TTS (Simon et al. with often more pronounced broad extended wings (up 2016). Therefore, we put forward an alternate possi- to several hundreds km s−1), which are characteristic bility, informed by recent work on TTS forbidden lines, of actively accreting stars. Additional spectra in this that the [O I] λ6300 traces the surface of a disk and is phase can be seen in Fig. 9 of Hamilton et al. (2012). broadened by Keplerian rotation. Its double peaked Mundt et al. (2010) interpreted the eclipse profiles as structure, unlike other LVC in TTS, would result from tracing the same bipolar jet as [O I] λ6300 since Hα also the fact that the disk is more like a ring, rather than shows red and blue peaks, although the blue/red inten- extended beyond ∼5 au, as required to fill in the cen- sity is quite variable and the velocities somewhat higher tral dip in most TTS (Hartigan et al. 1995; Simon et al. than [O I] λ6300. Alternatively, we might still be see- 2016). ing accretion onto A (and perhaps A+B) through re- flected light from the back of the wall of the circumbi- 3. Hα PROFILES nary disk. This could explain the similarity of some An extensive analysis of Hα profiles and associ- ingress and mid-eclipse Hα profiles and has been pro- ated emission variability is provided in Hamilton et al. posed by Hamilton et al. (2012) to explain the presence (2012). The analyzed spectra covered five contiguous of the broad wings during these phases. While a detailed analysis of the Hα profiles is beyond ened by 0.77 magnitude in the R band from December 20 to 21 the scope of this work, we draw the reader’s attention to (Windemuth & Herbst 2014) and this magnitude is used to con- the difference between the Hα and [O I] λ6300 profiles. vert the line equivalent width into flux, an incorrect assignment While the Hα profiles, even during eclipse, are mostly of the R band can explain the factor of ∼ 2 underestimation in I λ the [O I] λ6300 flux. asymmetric and variable in flux, the [O ] 6300 profiles 4

Figure 3. Corner plot showing the posterior distributions from the MCMC fit of the [O I] λ6300 line obtained on 2001 December 21 (see Fig. 1). The vertical dashed lines are the 16 and 85 percentiles, respectively. The solid lines indicate the medians of the posterior distributions. are persistently more symmetric through all the phases from modeling the photometric variability of KH 15D (Fig. 5) and show only minor variations in flux (Fig. 1 in (Chiang & Murray-Clay 2004). Mundt et al. 2010). This is different from the [O I] λ6300 To model in more depth the entire line profile, we emission tracing jets which shows strongly variable line adopt a power-law distribution for the O I surface den- α intensities and profiles as the emission arises in time sity: Σ(O I)∝ r , where r is the radial distance from the variable knots of shocked gas (e.g., Hartigan et al. 2011, star. We assume optically thin emission and an edge-on Simon et al. 2016.) Therefore, our preferred explanation (i = 90◦) disk for simplicity, since (1) photometric vari- is that the two lines trace different phenomena, with the ability points to such high disk inclination (Herbst et al. [O I] λ6300, being more stable, probing the surface of the 2002) and (2) the [O I] λ6300 line profile is nearly sym- gaseous circumbinary disk. metric with two peaks (see Fig. 1). The line model- ing includes thermal broadening for a temperature of 4. A CIRCUMBINARY GASEOUS RING 5,000 K (Simon et al. 2016) and convolution with the instrumental width of ∼7 kms−1. We vary α from −4 Here, we explore the possibility that the [O I] λ6300 r emission originates from the surface of a circumbinary to 4, the inner disk radius ( in) from 0.05 to 2.55 au and r r gaseous disk and use the line profile to constrain its outer radius ( out) from in out to 13au to perform a grid search and find the parameters leading to the low- size. In the assumption of Keplerian broadening, the 2 − χ I λ ∼40 kms 1 separation of the two peaks points to a est reduced . We focus the fit on the [O ] 6300 line profile obtained during the night of 2001 December 21 characteristic [O I] emitting radius of ∼3 au, consistent with the mean circumstellar dust ring radius inferred because of its high S/N. We further normalize the ob- 5 served line profile to its peak and fit it with modelled 5. INNER AND OUTER TRUNCATION OF THE line profiles that are similarly normalized to their peaks. DISK From this grid search we find the following best-fit pa- Several theoretical papers have shown that the rameters: α = −2.9, rin=0.57au, and rout=5.2 au. We inner region of a disk surrounding a close binary also implement a Markov Chain Monte Carlo (MCMC) is quickly dissipated (e.g., Lin & Papaloizou 1993; procedure (Sharma 2017) to find the best-fit parameters Artymowicz & Lubow 1994). Recently, Pichardo et al. and evaluate their uncertainties. The posterior distribu- (2008) carried out numerous simulations, using the test tions with the best-fit parameters and their uncertain- particle approah, to derive a simple relation between ties are shown in Fig. 3. These values are consistent the gap radius and binary semi-major axis (a), eccen- with those obtained through the grid search. Note that tricity (e), and primary/companion mass ratios (q): this best fit profile to the 2001 December 21 spectrum 0.32 0.043 Rgap ≈ 1.93a(1 + 1.01e )[q(1 − q)] . For the reproduces rather well all the other profiles (see Fig. 1 KH 15D system, a ∼ 0.25 au, e ∼ 0.6, and q ∼ 0.83– and 5), again highlighting the temporal stability of the 0.97 (Winn et al. 2006; Aronow et al. 2018), thus the [O I] λ6300 emission. disk inner radius is expected to be at ∼0.8 au according It is also worth noting that the [O I] λ6300 sur- to these simulations. However, as shown in the more face brightness distribution follows a rather steep detailed physical models of Mu˜noz & Lai (2016) and −2.8 power law: I ≈ r , in comparison to other disks Mu˜noz et al. (2019) , it is actually difficult to define (Hartigan et al. 1995; Simon et al. 2016). As pointed the cavity radius for an eccentric binary, as gas from out in Hartigan et al. (1995), a steep power law index the circumbinary disk flows in generating complex and of −3 is expected if the emission is proportional to the transit streams. Therefore, we consider our estimate of flux from the central star and the disk is flat. Thus, our ∼ 0.5 au from modeling the [O I] λ6300 line pretty close modeling results further support a disk origin. Interest- to the possible cavity radius. ingly, the inner and outer gas disk radii inferred from Chiang & Murray-Clay (2004) argued that the disk of the [O I] emission are very close to those of the dust KH 15D cannot extend beyond ∼5 au to maintain rigid ring derived from light curve modeling (see also next precession and explain the long-term photometric vari- Section). These results together point to a disk with ability of the system (but see Lodato & Facchini 2013 a relatively large inner radius and a very small outer for the possibility of the disk extending to 5–10 au). Re- radius, a circumbinary ring of gas. cently, Arulanantham et al. (2016) confirmed the rigid Meijerink et al. (2012) investigated the effect of high- precession of the ring and estimated a velocity (across energy stellar photons on the disk structure and the the sky) of ∼15m s−1 for the projected edge of the ring. gas chemical composition by varying the X-ray radia- Then, the precession rate of the ascending node of the 32 −1 29 tion from 0 to 10 erg s and the FUV from 10 to ring can be estimated to be 0◦.18 yr−1/¯a, wherea ¯ is 32 −1 10 erg s . Their calculations show that there ex- mean radius of the circumbinary ring in au. Using the ists a radially extended hot (∼5000K) region on top of approximate expression relating the precession rate of 4 6 8 the disk surface with hydrogen densities of ∼ 10 − 10 ascending node of an inclined circumbinary ring to the −3 cm . The [O I] λ6300 line should be tracing this region. mean ring radius from Chiang & Murray-Clay (2004), Mundt et al. (2010) claim that there is a slight blue the masses (0.715 and 0.74 M⊙ Aronow et al. 2018) and asymmetry (14%) in the [O I] λ6300 line profiles. For a (0.25 au, Winn et al. 2004) of the binary, the mean system like the KH 15D with a close binary, the [O I] radius of the ring should be ∼4 au. This value is also emitting regions can be disturbed by the time varying close to the characteristic [O I] λ6300 emitting radius, potential of the binary system (Thun et al. 2017), caus- see Sect. 4. ing some variability and asymmetry in the [O I] λ6300 Preventing the outer boundary from viscous spread- line profile as proposed by Mundt et al. (2010). How- ing may require some sort of outside confinement. ever, as discussed in Sect. 2, the blue part of [O I] λ6300 Chiang & Murray-Clay (2004) proposed a planet ex- line profile is also contaminated by sky lines, hence terior to the ring. Here, we explore external photoe- changes restricted to a narrow range of velocities close vaporation as an alternative. At a projected separation to the sky lines should be interpreted with caution. Ex- of ∼37′′, south-west from KH 15D, there is a mas- cluding these, likely spurious, features we do not identify sive star HD 47887 (Teff =24000±1000K, Fossati et al. any asymmetry in the highest S/N [O I] λ6300 profile. 2014). Based on Gaia Data Release 2, KH 15D and HD 47887 have similar parallaxes (1.2691±0.0751mas 4 from z/r ∼0.2 at r∼0.6 au to z/r ∼0.5 at r∼5 au, where z is vs. 1.3272±0.0751mas). Thus, it is very likely that the disk vertical height and r is the radial distance from the star both of them belong to the NGC 2264 cluster. With 6

Figure 4. The sizes of disks around a 1.5 M⊙ young star decrease with the ages due to the external photoevaporation within the UV strength of 4470 G0, like KH 15D. The different symbols are different starting disk masses and sizes.

a distance from us of ∼750pc, the projected distance UV field strength of 4470 G0 can decrease to 5 au at between them is ∼0.135 pc. the age of the cluster (2–6 Myr, Naylor 2009; Lim et al. Taking this projected distance, the model atmosphere 2016). Thus, photoevaporation can explain the outer from Lanz & Hubeny (2007), and the broad-band pho- truncation of the disk. tometry from Kharchenko (2001), the UV field strength 5 near KH 15D produced by HD 47887 is ∼4470 G0 . Un- 6. CONCLUSION der such UV field strength (4470 G0), we make a toy We provide a new interpretation to the double-peaked model to assess the timescale for dissipating the outer [O I] λ6300 emission line as arising from the hot surface disk of KH 15D as close in as 5 au. In the models, of the gaseous disk surrounding the spectroscopic binary the initial disk sizes are set to be 100, 150, or 200 au, KH 15D. Line profile modeling constrains the emission disk masses are 0.0075 or 0.015 M⊙ (0.5% or 1% of the to a relatively narrow radial extent ∼0.5–5 au, sim- KH 15D system mass), and the inner disk radius is fixed ilar to the extent of the dust ring inferred from mod- to be 0.1 au. The disk material has a surface density eling the source photometric variability. The relatively −1 Σ ∝ r , and the central binary is simplified as one large inner edge is likely set by the binary dynamical single star with a mass of 1.5 M⊙. Furthermore, we as- interaction. We show that the small outer disk radius sume that the bulk of the mass loss due to external pho- could be shaped by external photoevaporation driven toevaporation is driven from the disk outer edge, as in by UV photons from the nearby massive star HD 47887. Haworth et al. (2018), and neglect the disk viscous evo- Newer high signal-to-noise [O I] λ6300 line profiles would lution. We interpolate the disk mass-loss rates within be useful to further test our interpretation and detect the UV field strength of 4470 G0 during the disk dissi- any subtle changes that might arise from disk preces- pation using the FRIED grid of mass-loss rates for ex- sion. ternally irradiated protoplanetary disks (see details in Haworth et al. 2018). The mass-loss rates range from −6 −10 −1 We thank the anonymous referee for comments that ∼10 to ∼10 M⊙yr when disk size decrease from hundreds au to 5 au. The resulting disk sizes from our helped to improve this paper and K. Kratter for in- toy models are shown in Fig. 4 as a function of age. puts on the theory of circumbinary disks. I.P. and Our calculations show that the size of a disk within the S.E. acknowledge support from a Collaborative NSF As- tronomy & Astrophysics Research Grant (ID: 1715022 and ID:1714229). This material is based upon work 5 G0 is the Habing unit of UV radiation corresponding to the supported by the National Aeronautics and Space Ad- × −3 −1 −2 −1 integrated flux (1.6 10 erg cm s ) over the wavelength ministration under Agreement No. NNX15AD94G for range 912A˚ to 2400A˚ used in Haworth et al. (2018). the program ”Earths in Other Solar Systems”. The 7 results reported herein benefitted from collaborations tion network sponsored by NASA’s Science Mission Di- and/or information exchange within NASA Nexus for rectorate. Exoplanet System Science (NExSS) research coordina-

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Figure 5. [O I] λ6300 line profiles (grey and red lines) obtained between 2001 and 2004, compared to our best-fit line profile (blue lines), see Appendix A for details. The red line shows the [O I] λ6300 line profile taken on 2001 December 21 and used for the fitting. For multiple observations during 2001 December 14 and 2004 December 13-16, we only show one spectrum per night since there are no detectable variations in [O I] λ6300 profiles. The zero velocity marked with the dash line is that of the KH 15D rest frame. The grey-color filled region marks wavelength ranges where sky contamination is likely.

APPENDIX

A. COMPARISON OF OBSERVED [O I] λ6300 LINE PROFILES AND THE MODEL LINE PROFILE Figure 5 shows the comparison of the line profiles observed at different epochs (grey lines) with the best-fit model line profile (blue lines). The latter is obtained by fitting the 2001 December 21 profile (outermost right panel). We chose this line profile because (1) it has the highest S/N, and (2) it shows the least contamination from sky lines. The best fit parameters are: α = −2.82, rin=0.54au, and rout=5.0 au, see also Sect. 4.