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).”

“Simon et al. (2000) used resolved CO 2-1 observations of disks around several T Tauri stars to measure their masses directly.”

“Radial velocity monitoring is most applicable to DM Tau, where the cleared inner hole is expected to have only a 3AU radius (Calvet et al. 2005).”

“5 stars (Luhman 1999), the stars are 4MYr old and are of mass ∼0.52 and ∼0.62M⊙.”

“Most young stars have disks that are optically thick at all wavelengths or have no observable disk at all, while a small subset of young stars are in an intermediate state, thought to be in “transition” (Skrutskie et al. 1990).”

“An uncertainty in distance due to the depth of Taurus (e.g. Torres et al. 2007) can not reconcile this discrepancy, as both RD determined from radiative transfer modeling and the minimum RD determined by dynamical considerations both increase linearly with the distance estimate.”

“The observational scatter in the Taurus HR diagram (e.g Luhman 2004) is consistent with CoKu Tau/4 being either single as assumed by previous authors or an equal brightness double.”

“Many of these intermediate objects have optically thick “cold disks” as seen by large excesses at ∼30µm, but much smaller or nonexistent near- and mid- infrared excesses (Brown et al. 2007).”

“An uncertainty in distance due to the depth of Taurus (e.g. Torres et al. 2007) can not reconcile this discrepancy, as both RD determined from radiative transfer modeling and the minimum RD determined by dynamical considerations both increase linearly with the distance estimate.”

“Aperturemasking interferometry (e.g. Tuthill et al. 2000) is a well established technique for achieving the full diffraction limit of a single telescope, recently applied to observations behind adaptive optics systems (e.g. Lloyd et al. 2006; Kraus et al. 2008).”

“The disk has been modeled as having an inner hole of radius ∼10AU (D’Alessio et al. 2005), with suggestions that this hole is due to a giant planet (Quillen et al. 2004).”

“An example of this is CS Cha, which was announced as a transitional disk (Espaillat et al. 2007) shortly after its discovery as a circumbinary disk (Guenther et al. 2007).”

“Although the most exciting interpretation for the development of gaps is arguably truncation due to planetary formation, it is also possible to dynamically truncate disks due to stellar companions (e.g Jensen & Mathieu 1997; Beust & Dutrey 2005; White & Hillenbrand 2005).”

“CoKu Tau/4 was discovered to have a large ∼20-30µm excess and no excess at wavelengths < 8µm through Spitzer Space Telescope Infrared Spectrograph observations (Forrest et al. 2004).”

“An example of this is CS Cha, which was announced as a transitional disk (Espaillat et al. 2007) shortly after its discovery as a circumbinary disk (Guenther et al. 2007).”

“We extracted photometry and astrometry from the direct AO imaging of these sources using the PSF reconstruction technique described in Kraus & Hillenbrand (2007b), which iteratively fits a template PSF to the primary and then subtracts the secondary to fit an im- proved estimate of the primary.”

“Some authors (e.g. Furlan et al. 2007) have considered the term “transitional” to be an observational term that means the spectral signature of a cleared inner disk rather than an evolutionary term that implicitly applies to single stars.”

“Although the most exciting interpretation for the development of gaps is arguably truncation due to planetary formation, it is also possible to dynamically truncate disks due to stellar companions (e.g Jensen & Mathieu 1997; Beust & Dutrey 2005; White & Hillenbrand 2005).”

“Aperturemasking interferometry (e.g. Tuthill et al. 2000) is a well established technique for achieving the full diffraction limit of a single telescope, recently applied to observations behind adaptive optics systems (e.g. Lloyd et al. 2006; Kraus et al. 2008).”

“Beust & Dutrey (2005) predict a gap size of > 2.6a for eccentricities > 0.1 for GG Tau, which has a similar mass ratio to CoKu Tau/4 (and find this not to be large enough for that system).”

“…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 (1979) and Hanson et al. (2004) with an assumed 0.1mag overall uncertainty, and an M1.”

“We extracted photometry and astrometry from the direct AO imaging of these sources using the PSF reconstruction technique described in Kraus & Hillenbrand (2007b), which iteratively fits a template PSF to the primary and then subtracts the secondary to fit an im- proved estimate of the primary.”

“UY Aur (Duvert et al. 1998) and CoKu Tau 4 (Ireland & Kraus 2008) are additional examples of primordial circumbinary disks but they appear to be rare exceptions rather than the rule.”

“…to explain observations of transition disks, including extensive particle growth (Tanaka et al. 2005; Dullemond & Dominik 2005), photoevaporative massloss in winds (Clarke et al. 2001; Alexander et al. 2006a), and tidal interactions with companions (e.g., Rice et al. 2003a; Ireland & Kraus 2008).”

“Some outliers may be expected from this relationship for circumbinary disks, such as the disk around CoKu Tau 4 (D’Alessio et al. 2005; Ireland & Kraus 2008).”

“This approach has been quite fruitful, resulting in the discovery of companions within these gaps spanning the stellar (CoKu Tau 4: Ireland & Kraus 2008; HD 142527: Biller et al. 2012, Close et al. 2014, Rodigas et al. 2014, Lacour et al. 2015), brown dwarf (HD 169142: Biller et al. 2014, Reggiani…”

“Taurus members that are known to harbor transitional disks include CoKu Tau/4, GM Aur, and DM Tau (Rice et al. 2003; Forrest et al. 2004; Quillen et al. 2004; Calvet et al. 2005; D’Alessio et al. 2005), although the inner hole in the disk of CoKu Tau/4 was probably produced by a stellar companion rather than processes…”

“The example of CoKu Tau/4 demonstrates that “transitional disks” can host binary companions (Ireland & Kraus 2008), and other binary systems, like HK Tau and HV Tau, are known to have wider components which are obscured in this way (Stapelfeldt et al. 1998, 2003).”

“They include transitional and hot debris disks, as well as a small fraction of edge-on disks (e.g. Huélamo et al. 2010) and circumbinary disks (e.g. Ireland & Kraus 2008).”

“One example is the cavity in the inner disk of CoKu Tau/4 which is most likely due to clearing by its stellar companion (Ireland & Kraus 2008) and whose SED has been reproduced by a circumbinary disk model (Nagel et al. 2010).”

“As we showed for the case of CoKu Tau/4 (Ireland & Kraus 2008), the large inner gap caused by a short-period binary system could masquerade as a signpost for planet formation, indicating that binary vetting is required for the so-called “transitional disk” systems.”

“Also, some close binary systems (such as CoKu Tau/4; Ireland & Kraus 2008) clearly still host disks and therefore might plausibly produce planets.”

“…photo evaporation, grain growth, and planet formation shape the structure of circumstellar disks (Clarke et al. 2001; Dullemond & Dominik 2005; Ireland & Kraus 2008), and in particular, the process of planet formation creates gaps, cavities, and asymmetries that can be directly observed (Wolf…”

“A stellar companion orbiting at &1AU, but too close to the primary to be resolved with direct imaging, would clear out a large hole completely (Artymowicz & Lubow 1994), such as is the case for CoKu Tau/4 (Ireland & Kraus 2008).”

“CoKu Tau/4 is not a true disk in transition since dust and gas appears to have been cleared out by a stellar companion and not as a result of disk evolution (Ireland & Kraus 2008).”

“The prosaic explanation for wide holes in transitional disks—tidal truncation of the disk by a stellar companion, as observed in CoKu Tau/4 (Ireland & Kraus 2008)—is also not likely responsible for the unusual and complex disks that concern us in this article.”

“Analysis of our NIRC2 masking data was performed using a pipeline similar to previous papers (e.g., Ireland et al. 2008; Ireland & Kraus 2008) and is described in detail in Section 2.2 of Dupuy et al. (2009c).”

“A notable example is the recent result by Ireland & Kraus (2008) demonstrating that the inner hole in the transition disk around CoKu Tau/4 is caused by a previously unobserved companion.”

“…surveys using high-resoluti n imaging have discovered significant numbers of close binaries even in regions that had previously been well studied (Ireland & Kraus 2008; Kraus et al. 2008), suggesting that the fraction of stars that exist as binaries with separations of∼ 3–30AU is not well…”

“Similar reclassifications were made for CoKu Tau/4 (Ireland & Kraus 2008) (inspiring the first part of our title) and CS Cha (Espaillat et al. 2007; Guenther et al. 2007).”

“Gravitational interaction with an orbiting companion (e.g. Rice et al. 2003; Ireland & Kraus 2008), photoevaporation (Alexander & Armitage 2007), and particle growth (Dullemond & Dominik 2005) have been proposed as clearing processes and innumerable attempts are made to predict the rapidly…”

“…GM Aur (Rice et al. 2003; Bergin et al. 2004; Calvet et al. 2006), DM Tau (Bergin et al. 2004; Calvet et al. 2006) and CoKu Tau/4 (Forrest et al. 2004; Quillen et al. 2004; D’Alessio et al. 2005), though the latter has subsequently been found to harbor a stellar companion (Ireland & Kraus 2008).”

“For most binary systems compact enough to be detected spectroscopically, the companion star is expected to have evacuated the inner disk as in Coku Tau/4 (Ireland & Kraus 2008).”

“The end structure, and spectrum, might resemble those of CoKu Tau/4, for which the outer disk is apparently not very massive, though in this case the disk appears to be truncated by the binary star it contains (Ireland & Kraus 2008) instead of a giant planet.”

“…(Alexander et al. 2006; Cieza 2008; Pascucci & Sterzik 2009; Owen et al. 2010), dynamical interaction with close stellar or substellar companions (Ireland & Kraus 2008), and planet formation via dust locking (Safronov 1969; Lyttleton 1972; Goldreich & Ward 1973; Youdin & Shu 2002; Johansen et…”

“More recently, CoKu Tau/4 has been shown to be a close binary star system with an 8 AU projected separation, rendering its disk a circumbinary one (Ireland & Kraus 2008).”

“Similarly, for CoKu Tau/4, a 53 mas (∼8 AU) binary, the SED-derived transitional radius is consistent with dynamical clearing from the binary pair (Ireland & Kraus 2008).”

“If stellar companions carve out inner holes, accretion may either continue or cease depending on the separation and mass ratio of the two stars (Artymowicz & Lubow 1996; Ireland & Kraus 2008).”

“In the case of CoKu Tau 4, the observations revealed that the cavity is due to a stellar mass companion orbiting at about 8 AU from the central star (Ireland & Kraus 2008).”

“′′05 (Ireland & Kraus 2008), but its astrometric fit is not especially poor, so its astrometry is probably reliable.”

“Planet formation theories predict disk structures such as clumps, spiral arms, gaps and warps (Artymowicz & Lubow 1994; Quillen et al. 2004; Varnière et al. 2006; Jang-Condell & Boss 2007; Ireland & Kraus 2008; Jang-Condell 2009; Zhu et al. 2011).”

“For example, a separation of 10 AU at a distance of 130 pc corresponds to 77 mas, resolvable perhaps with aperture-masking interferometry on large telescopes (e.g. Ireland & Kraus 2008).”

“Two circumbinary disks, CoKu Tau/4 (7.8 AU; Ireland & Kraus 2008) and ROXs42c (23 AU; Kraus et al. 2011), also do not show Pfβ emission, although it should be noted that this does not appear to be universally true, as close binary DP Tau (projected separation 15.5 AU Kraus et al. 2011) has detectable Pfβ emission.”

“Systems empirically classified as TO do not form a uniform class of objects, but include binaries (Ireland & Kraus 2008), objects with giant planets (Setiawan et al. 2008), and cases where strong dust coagulation and/or photoevaporation have produced an optically thin disk (Pascucci & Sterzik 2009).”

“Second, there are the protoplanetary CB disks that originated from high-contrast adaptive optics and/or nonredundant masking searches for binary stars and planetary-mass companions in star-forming regions (e.g., Ireland & Kraus 2008; Ruíz-Rodríguez et al. 2016).”

“…presence of a stellar companion, the formation of a planetary system, disk photoevaporation, magneto-rotational instabilities, disk viscosity, and grain growth (Alexander & Armitage 2007; Chiang & Murray-Clay 2007; Ireland & Kraus 2008; Isella et al. 2009; Strom et al. 1989; Tanaka et al. 2005).”

“In addition, a stellar companion can inwardly truncate the outer disk as is most likely the cause of the inner hole of CoKu Tau/4 (Ireland & Kraus 2008).”

“To explain the large population of diskless stars in Taurus (∼ 30% of the total population), Currie et al. (2009) argue that stars with close binary companions lose their protoplanetary disks far more rapidly than stars without close binary companions (e.g. Ireland and Kraus 2008).”

“Inside its hole of radius ∼10 AU resides a nearly equal mass Kstar binary having a projected separation of 8 AU (Ireland & Kraus 2008).”

“…2012), and so the operating hypothesis is that the observed structures are produced by dynamical interactions with very faint (e.g., Pott et al. 2010; Kraus et al. 2011), presumably planetary-mass, companions (although more massive stellar companions must be ruled out; e.g., Ireland & Kraus 2008).”

“In certain cases, the inner holes may be also produced by close-in companions (e.g., CoKu Tau/4; Ireland & Kraus 2008).”

“Such is the case of the famous CoKu Tau/4 system (Ireland & Kraus, 2008).”

“In addition we remove any ‘transition’ disc which is caused by a known close stellar companion (e.g. CoKu-Tau 4 Ireland & Kraus 2008).”

“…as well as to measure the dynamical mass of the brown dwarf companion to GJ 802b (Ireland et al. 2008), show that CoKu Tau/4 is a binary system rather than a transitional disk (Ireland & Kraus 2008), and place limits on possible companions existing within 10AU of HR8799 (Hinkley et al. 2011).”

“Some transitional disks with unmeasurably low stellar accretion rates may harbor stellar binaries whose torques are strong enough to stave off accretion (e.g., CoKu Tau/4; Ireland & Kraus 2008; Najita et al. 2007).”

“Theory and observations agree that stellar companions can open large gaps in disks, or truncate them to much smaller radii than they would otherwise have (e.g., Artymowicz & Lubow 1994; Ireland & Kraus 2008).”

“…by Megeath et al. (2005); Hernández et al. (2007); Merín et al. (2008); Monnier et al. (2008); Hughes et al. (2008); Sicilia-Aguilar et al. (2008); Ireland & Kraus (2008); Kim et al. (2009); McClure et al. (2010); Najita et al. (2010); Espaillat et al. (2011); Furlan et al. (2011) Article number,…”

“…used 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 for calibration of the closure phases or squared visibilities.”

“OurM ′ band detection demonstrates that it is possible to directly image planets at separations approaching the telescope diffraction limit without sparse aperture masking interferometry (SAM; Ireland and Kraus 2008).”

“Both stars are of spectral type M1 - M2 and are ∼ 3 - 4 Myr old (Ireland & Kraus 2008).”

“Our technique, which we describe in greater detail in Sections 2 and 3, bolsters ongoing ground-based NRM efforts (Ireland & Kraus 2008; Kraus et al. 2008; Bernat et al. 2010) as well as serving as a precursor for the NRM efforts to be used with the James Webb Space Telescope, hereafter “JWST”…”

“This is somewhat different from the case of Coku Tau/4, a weak line TTauri star surrounded by a transition disk (D’Alessio et al. 2005) which has recently been found to be an equal mass binary surrounded by a circumbinary disk (Ireland & Kraus 2008).”

“The disk around CoKu Tauri/4 is of circumbinary rather than the transitional (Ireland & Kraus 2008).”

“…object (e.g. Alexander et al. (2006a,b); Owen et al. (2010, 2011a)), planet formation (e.g. Zhu et al. (2011)), grain growth (e.g.Dullemond & Dominik (2005)), close-binary effects (e.g. Ireland & Kraus (2008)) and external photoevaporation by nearby high-mass stars (e.g. Scally & Clarke 2001).”

“Objects like CoKu Tau4, DM Tau and GM Aur show no or at most small continuum excess emission at wavelengths .”

“More recently, it became evident that, although some TO harbor planets (e.g., TW Hya; Setiawan et al. 2008), others may have their inner part cleared since their formation due to the presence of close-in binaries (e.g., CoKu Tau/4; Ireland & Kraus 2008).”

“Other processes such as dynamical interactions in binary stellar systems are also likely to play a role (Ireland & Kraus, 2008).”

“Binaries (Ireland & Kraus 2008), planet formation (Quillen et al. 2004; Varnière et al. 2006) and photoevaporation (Clarke et al. 2001; Alexander et al. 2006) are some of the suggested mechanisms for this removal of warm dust.”

“Another possibility is tidal truncation in close binaries (Ireland & Kraus 2008).”

“According to the IAU definition, this object would be a brown dwarf, and not a planet, making HD 100546 a misinterpreted binary system with a disk like CoKu Tau / 4 (Ireland & Kraus 2008), rather than a transitional disk.”

“…processes taking place within them (e.g., accretion, photoevaporation, interaction with companions, dust growth and settling, and radial migration; Takeuchi & Lin 2002; D’Alessio et al. 2006; Ireland & Kraus 2008; Alexander et al. 2014) require that we consider several factors for their study.”

“…of WTD+PTD. Sensitive searches have been made for 10 AU-scale binaries in Taurus (Pott et al. (2010), Kraus et al. (2011)), but only one (CoKu Tau/4; Ireland & Kraus (2008)) of the nine transitional disk systems so studied has a stellar companion been detected with binary separation similar to the…”

“In the case of CoKu Tau/4, where the binary separation is about 8 AU, the truncation radius would be of the order of 13-16 AU, depending on the eccentricity of the binary (Ireland & Kraus 2008).”

“To use the closure phase quantity to search for companions, we follow the analysis outlined in Kraus et al. (2008), Ireland & Kraus (2008) and Hinkley et al. (2011), briefly summarized here.”

“Finally, the process of (d) dynamical interaction ofthe disk with a (sub)stellar companion (e.g. Ireland & Kraus 2008) may also remove dust from the inner part of the disk.”

“Similar phase signals have been observed on several other transitional and pre-transitional disks (e.g. CoKu Tau/4: Ireland & Kraus 2008; TCha; Huélamo et al. 2011; LkCa 15: Kraus & Ireland 2012; HD142527: Biller et al. 2012; FLCha: Cieza et al. 2013) and have often been interpreted as signatures of a close companion.”

“As noted above, recent high-resolution aperture-masking by Ireland & Kraus (2008) has revealed that the supposedly transitional disk around CoKu Tau/4 is instead a circumbinary disk around a near-equal flux binary with a (projected) star-star separation of 53 mas.”

“The disk of the star Coku Tau/4 is circumbinary, and its structure is a result of tidal truncation due to the binary orbit (Ireland & Kraus 2008).”

“Several processes can be responsible for the dissipation of the innermost material, such as the gravitational influence of a stellar companion (e.g., CoKu Tau 4, Ireland & Kraus 2008), which can truncate the cir- Article number, page 1 of 13 ar X iv :1 30 2.”

“Analysis of our masking data was performed using a pipeline similar to that used in previous papers containing NIRC2 masking data (e.g., Ireland et al. 2008; Ireland & Kraus 2008) and is described in detail in Section 2.2 of Dupuy et al. (2009c).”

“Analysis of masking data was done using a pipeline similar to that in previous papers containing NIRC2 masking data (e.g., Ireland et al. 2008; Ireland & Kraus 2008) and is described in detail in Section 2.2 of Dupuy et al. (2009b).”

“…could in principle explain CoKu Tau/4’s hole since this object has a weak outer disk (Mdisk ∼ 0.0005 M⊙; Andrews & Williams 2005) and a non-detectable accretion rate, however, its disk clearing is most likely caused by its recently discovered stellar companion (Ireland & Kraus 2008).”

“Several mechanisms can be responsible for dust clearing, such as a binary companion (Jensen & Mathieu 1997) which is the case for CoKu Tau 4 (Ireland & Kraus 2008), or photo-evaporation processes (Alexander et al. 2006).”

“Aperture masking observations on the Keck II telescope by Ireland and Kraus (2008) demonstrated the presence of a near-equal binary companion at an angular distance of 53mas (∼ 8AU).”