The Disk Around CoKu Tauri/4: Circumbinary, Not Transitional

“This routine was implemented using the ALLSTAR routine in the IRAF package DAOPHOT (Stetson 1987), and based on previous experience with NIRC2 imaging, we chose to model the PSF using a Gaussian core with Lorentzian wings.”

“To estimate the reddening, we used the following data: intrinsic colors reported in Bessell & Brett (1988) and Bessell (1990) interpolating between giants (log(g) ∼ 1) and dwarfs (log(g) ∼ 5) at an assumed log(g) of 3.7, JHK photometry from 2MASS, V-band photometry averaged between Cohen & Kuhi…”

“In this geometry, Artymowicz & Lubow (1994) calculate that the 3:1 orbital resonance at RD ∼ 2.08a is opened for all but improbably small eccentricities (e & 0.03).”

“5 spectral classification from Cohen & Kuhi (1979) with an assumed 1 sub-class uncertainty.”

“In addition to this method, we used a recursive-phase algorithm (e.g Lohmann et al. 1983) to reconstruct Fourier Phase from bispectrum phase, and then fitted a binary model to the reconstructed phase of the image.”

“This inner hole could be caused by natural processes of grain growth and/or photo-evaporation (Alexander et al. 2006), or truncation of the disk due to dynamical interactions with companions (Lin & Papaloizou 1979).”

“This inner hole could be caused by natural processes of grain growth and/or photo-evaporation (Alexander et al. 2006), or truncation of the disk due to dynamical interactions with companions (Lin & Papaloizou 1979).”

“In addition, the presence of stellar companions may contribute to the disk clearing (Bouwman et al. 2006; Ireland & Kraus 2008), although it may not be the leading cause of inner holes (Pott et al. 2010).”

“The analysis for the SAM data was identical to that used in previous papers (e.g., Ireland & Kraus 2008; Kraus et al. 2008, 2011), combined with the new calibration technique described in Kraus & Ireland (2012).”

“High angular resolution observations have shown that at least some of these inner holes are due to stellar binarity (e.g., CoKu Tau 4; Ireland & Kraus 2008), but the physical mechanism at work in the majority of transitional disks has yet to be clearly determined.”

“As spectral energy distributions are used to discover “transitional” disks implicating planet formation, we must be cognizant of the important role of binarity and the associated circumbinary disks that can mimic signs of planet formation (e.g., Ireland & Kraus 2008).”

“Close binary companions can interact with the primary star’s disk and produce transitional SEDs (e.g., CoKu Tau/4; Ireland & Kraus 2008).”

“Given the criteria applied to select our sample and following the standards for disk classification, we emphasize that these objects should be treated as CDs that possibly are in atransitional phase, and no longer treat them as TDs with a single star.”

“2009), and a number of different mechanisms have been proposed for their origin (Najita and Muzerolle 2007; Chiang and Murray-Clay 2007; Ireland and Kraus 2008; Alexander 2008a; Cieza et al. 2008; Alexander and Armitage 2009).”

“It is clear that observations with high resolution, such as those described by Ireland & Kraus (2008), in which both stars can be characterized, are very useful.”

“There has been general agreement with these predictions from ob- 2 servations of directly imaged circumbinary disks, most notably that found around GG Tauri by Dutrey et al. (1994) (see also Beust & Dutrey 2005; Duvert et al. 1998; Ireland & Kraus 2008).”

“Ireland & Kraus (2008) showed that the famous transition disk CoKu Tau 4 is in fact a near-equal mass binary system, which naturally explains the hole that had been inferred from its SED.”

“(1) Herczeg & Hillenbrand 2014; (2) White & Ghez 2001; (3) Kenyon & Hartmann 1995; (4) Ireland & Kraus 2008; (5) Cohen & Kuhi 1979; (6) Furlan et al. 2009a; (7) Steffen et al. 2001; (8) Kenyon et al. 1998; (9) Kraus et al. 2012; (10) Nguyen et al. 2009; (11) Furlan et al. 2011; (12) Furlan et al.…”

“…et al. 1999; Rice et al. 2003; Quillen et al. 2004; Varnière et al. 2006; Zhu et al. 2011), dynamical interactions in multiple systems (e.g., Artymowicz & Lubow 1996; Ireland & Kraus 2008; Fang et al. 2014), and magneto-hydrodynamical phenomena (Chiang & Murray-Clay 2007) have all been proposed.”

“Ireland & Kraus (2008); 2).”

“One is thus tempted to call HD 142527 a circumbinary disk, even if the term is more often used to refer to equal mass binaries, as in GG Tau A and CoKu Tau 4 (e.g. Ireland & Kraus 2008; Beck et al. 2012).”

“…disk objects represent interesting cases in which to initially assess the multiplicity rate, since the inner holes of misidentified transition disks may be due to the presence of a companion that gravitationally truncates the inner radii of a circumbinary disk (e.g., Ireland & Kraus 2008).”

“…et al., 2019; Scally & Clarke, 2001), magnetohydrodynamic (MHD) winds (Armitage et al., 2013; Bai, 2016; Shadmehri & Ghoreyshi, 2019; Suzuki et al., 2016), planet formation (Zhu et al., 2011), grain growth (Dullemond & Dominik, 2005), and close-binary effects (Ireland & Kraus, 2008).”

“Stellar companions have been found to establish large inner holes and to terminate stellar accretion in some transition objects (CoKu Tau/4; Ireland & Kraus 2008; D’Alessio et al. 2005) and not in others (CS Cha; Guenther et al. 2007; Espaillat et al. 2007b).”

“Other sources classified as transitional disks have turned out to be circumbinary disks (Guenther et al. 2007; Ireland & Kraus 2008), where the outer disk may be truncated by tidal interactions with stellar-mass companions (e.g., Lin & Papaloizou 1979).”

“…mass companions can open cavities in disks (e.g., Artymowicz & Lubow 1994; Pichardo et al. 2008), and some observed disk gaps, such as those in CoKu Tau 4 (Ireland & Kraus 2008), HD 98800 (Furlan et al. 2007), and Hen 3-600 (Uchida et al. 2004), have been associated with stellar mass binaries.”

“However, it was afterward shown that CoKu Tauri/4 was in fact a close visual binary (VB) system with a separation of ∼8 au (Ireland & Kraus 2008), and that the secondary star likely carved out the gap.”

“The NIRC2 data were reduced using the pipeline described previously in Ireland & Kraus (2008) and Kraus et al. (2013), providing calibrated closure phases.”

“However, alternative explanations exist, such as the presence of a close binary companion (e.g. CoKu Tau 4, Ireland & Kraus 2008) or grain growth (Dullemond & Dominik 2005).”

“References for inclination and disk morphology: [a] Long et al. (2019), [b] Ireland & Kraus (2008), [c] Ballering & Eisner (2019), [d] Bacciotti et al. (2018), [e] Piétu et al. (2014), [f] Simon et al. (2017), [g] Furlan et al. (2011), [h] Akeson et al. (2019), [i] Long et al. (2018), [j] Kudo et…”

“…and without adaptive optics, resulting in the identification of complex disk structures (e.g., Tuthill et al. 1999, 2001; Sallum et al. 2019) and companions (e.g., Ireland & Kraus 2008; Sallum et al. 2021), including the infrared companion candidates under study here (e.g., Kraus & Ireland 2012).”

“Also, while a few transition discs such as CoKu Tau/4 were found do be close stellar binaries (Ireland & Kraus, 2008), it soon became clear that most transition discs are not due to binarity (Pott et al. 2010).”

“Companions with separations and contrasts similar to those in Model 2 have been found in NRM observations of transition and circumbinary disks (e.g. Ireland & Kraus 2008; Kraus & Ireland 2012; Sallum et al. 2015a).”

“Although observations have shown evidence for the existence of CB disks in young binary systems (Monnier et al. 2008; Ireland & Kraus 2008), magnetic cataclysmic variables (Howell et al. 2006; Brinkworth et al. 2007; Dubus et al. 2007; Hoard et al. 2007), and black hole LMXBs (Muno & Mauerhan 2006;…”

“Observational evidence of stellar or substellar companions has been obtained in some cases (i.e., CoKu Tau4 or T Cha, see Ireland & Kraus 2008; Huélamo et al. 2011, respectively).”

“…suggest that the circumbinary scenario may be the more appropriate explanation in some cases for the diminished central emission in both transition (Ireland & Kraus 2008) and dense, edge-on disks (Guilloteau et al. 2008). core collapse models that incorporate rotation or magnetic fields (Terebey…”

“The low accretion rate of CoKu Tau/4 (<10−10 M⊙ yr−1; Cohen & Kuhi 1979) and hole size (∼14 AU; Nagel et al. 2010) are consistent with clearing due to the stellar mass companion detected within the disk hole (Ireland & Kraus 2008; Artymowicz & Lubow 1994; Lubow et al. 1999).”

“For the transitional disks DoAr 21, HD 135344B, HR 4796A, and T Cha, the VLT/NACO observations resolve part of the inner hole/gap region of the disk and can potentially discard binaries down to a certain mass limit as the main explanation for the inner opacity cavities.”

“…suggest that circumbinary planets may be abundant: (i) circumbinary disks have been detected around pre-main-sequence systems, such as CoKu Tau 4 (Ireland and Kraus 2008), KH 15D (Herbst et al. 2008), AK Sco (Andersen et al. 1989), GW Ori (Mathieu et al. 1991), DQ Tau (Mathieu et al. 1997; Boden…”

“To date, the structure of the transitional disks has been mainly investigated by model fitting of their SED, which usually does not provide an unequivocal solution (D’Alessio et al. 2005; Calvet et al. 2005; Ireland & Kraus 2008; D’Alessio 2009).”

“Several mechanisms can be responsible for this situation but one of them is the presence of a close (sub)stellar companion at a separation of a few au, as demonstrated in the cases of CoKuTau4 and LkCa 15 (Ireland and Kraus 2008; Kraus and Ireland 2012).”

“It has proven useful in the direct detection of close-in stellar (e.g. Biller et al. 2012; Ireland & Kraus 2008) and substellar (e.g. Sallum et al. 2015a; Kraus & Ireland 2012) mass companions.”

“…have been proposed to cause gaps in proto-planetary disks, including extensive grain growth (Dullemond & Dominik 2005), photo-evaporation (Clarke et al. 2001), binarity (Ireland & Kraus 2008), and tidal barrier created by dynamical interaction with low-mass disk objects (e.g. Bryden et al. 1999).”

“Stellar-mass companions (Mathieu et al. 1997; Ireland & Kraus 2008), dust coagulation and evolution (Dullemond & Dominik 2005; Krauss et al. 2007), inside-out evacuation by the magnetorotational instability (Chiang & Murray-Clay 2007) or photoevaporation (Alexander et al. 2006; Alexander & Armitage…”

“ZZTau and CokuTau/4 are 0 06 binaries (Schaefer et al. 2006; Ireland & Kraus 2008).”

“…to reduce the aperture masking data was similar to that used in previous papers containing NIRC2 masking data (Ireland et al. 2008; Kraus et al. 2008; Ireland & Kraus 2008), except that no comparable single star was observed in KS-band for calibration of the closure phases or squared visibilities.”

“…by other mechanisms such as photoevaporation, dust growth and settling towards the disc mid-plane, dynamic clearing by (sub)stellar companions, or a combination of several of these (e.g. Ireland & Kraus 2008; Birnstiel, Andrews & Ercolano 2012; Alexander et al. 2014; Espaillat et al. 2014).”

“…disk objects represent interesting cases in which to initially assess the multiplicity rate, since the inner holes of misidentified transition disks may be due to the presence of a companion that gravitationally truncates the inner radii of a circumbinary disk (e.g., Ireland & Kraus 2008).”

“This technique has been thoroughly demonstrated from the ground via observations of stellar and substellar companions (e.g. Ireland & Kraus 2008; Biller et al. 2012; Hinkley et al. 2015), circumstellar disks (e.g. Tuthill et al. 2001; Sallum et al. 2019, 2023) and more.”

“For the transitional disks DoAr 21, HD 135344B, HR 4796A, and T Cha, the VLT/NACO observations resolve part of the inner hole/gap region of the disk and can potentially discard binaries down to a certain mass limit as the main explanation for the inner opacity cavities.”

“The NIRC2 data were reduced using the pipeline described previously in Ireland & Kraus (2008) and Kraus et al. (2013), providing calibrated closure phases.”

“In the paradigm “central clearing” disk around CoKu Tau/4 the companion is a star (Ireland & Kraus 2008), but in most others it is likely to be a gas-giant planet (Artymowicz & Lubow 1994; Calvet et al. 2005; Kim et al. 2009; Pott et al. 2010).”

“For example, CoKu Tau/4 is a very close binary with a cleared central cavity created by the binary tidal forces, creating an SED that resembles a transition disk (Ireland & Kraus 2008).”

“Given that unresolved companions could provide an explanation for transition-like SEDs (Ireland & Kraus 2008), some knowledge of the binary frequency of the Upper Scorpius latetype stellar population is warranted.”

“…instability may play a role in removing the inner disk (Clarke, Gendrin, & Sotomayor 2001; Quillen et al. 2004; Alexander & Armitage 2007; Chiang & Murray-Clay 2007), but also close binary companions can clear inner regions due to orbital resonances (Artymowicz & Lubow 1994; Ireland & Kraus 2008).”

“The depleted inner disk would likely result from dynamical clearing by the two components of the spectroscopic binary, similar to the CoKu Tau 4 system that was previously recognized as a transition disk source (Ireland & Kraus 2008).”

“For more massive disks where photo-evaporation can not yet play a significant role in disk shaping, the key clearing possible mechanisms are significant grain growth and dynamical interactions with forming protoplanets (Dodson-Robinson and Salyk 2011) or companions (Ireland and Kraus 2008).”

“The Spitzer IRS spectrum of HBC 340, however, is not suggestive of a cleared inner disk analogous to CoKu Tau 4, the pre-main sequence binary in Taurus-Auriga resolved by aperture masking interferometry (Ireland & Kraus 2008; Nagel et al. 2010).”

“The NACO data were reduced using our data reduction pipeline that was used already in various earlier studies (e.g. Ireland & Kraus 2008; Kraus & Ireland 2012; Kraus et al. 2013), providing absolute calibrated visibilities and closure phases.”

“Although sometimes confused with circumbinary discs (Espaillat et al. 2007; Ireland & Kraus 2008), these discs remain an important stage in single star evolution (e.g. Rúız-Rodŕıguez et al. 2016) and may be a signpost for giant planet or multiple giant planet formation (Zhu et al. 2011;…”

“Some appear to have inner cleared regions due to the presence of previously unseen faint companions at small orbital radii (Ireland & Kraus, 2008).”

“Observations using NRM have been successful in probing close-in protoplanetary disk structures (e.g. Sallum et al. 2019) and identifying companions (e.g. Ireland & Kraus 2008).”

“A binary star system can be embedded in a circumbinary disc (e.g., Jensen et al. 2007; Ireland & Kraus 2008; Nagel et al. 2012; Biller et al. 2012; Takakuwa et al. 2013; Gillen et al. 2014).”

“…2) there is observational evidence that the inner hole of a transitional disk has low gas density (Pontoppidan et al. 2008; Brown et al. 2008; Ireland & Kraus 2008); 3) it allows us to isolate the effects of collisional dynamics; 4) in this work dust is the instantaneous collisionally…”

“Ireland & Kraus (2008) show that the prototypical transitional disk, CoKu Tau/4, is in fact an 8 AU separation binary, and its disk is therefore circumbinary and not (necessarily) in the process of dispersal.”

“Aperture Masking Interferometry The technique of aperture masking interferometry (e.g. Tuthill et al. 2000, Ireland & Kraus 2008), achieves contrast ratios of ∼102 - 103 at very small inner working angles, usually within ∼ 13 λ/D − 4λ/D (∼ 20-300 mas for Keck L′-band imaging).”

“…been proposed to cause these gaps, including extensive grain growth (Dullemond & Dominik 2005), photo evaporation (Clarke et al. 2001) and binarity (Ireland & Kraus 2008), however most exciting for exoplanetary science is the potential for such systems to arise from the tidal barrier created by…”

“19 Here we present a progress update on the Search for Protoplanets with Aperture Masking (SPAM) survey, the highest angular resolution NIR direct imaging transition disk survey.”