The Solar Neighborhood XXX: Fomalhaut C
Eric E. Mamajek, Jennifer L. Bartlett, Andreas Seifahrt, Todd J. Henry, Sergio B. Dieterich, John C. Lurie, Matthew A. Kenworthy, Wei-Chun Jao, Adric R. Riedel, John P. Subasavage, Jennifer G. Winters, Charlie T. Finch, Philip A. Ianna, Jacob Bean
aa r X i v : . [ a s t r o - ph . S R ] O c t The Solar Neighborhood XXX. Fomalhaut C
Eric E. Mamajek , , Jennifer L. Bartlett , Andreas Seifahrt , Todd J. Henry , Sergio B.Dieterich , John C. Lurie , Matthew A. Kenworthy , Wei-Chun Jao , Adric R. Riedel , ,John P. Subasavage , Jennifer G. Winters , Charlie T. Finch , Philip A. Ianna , JacobBean [email protected] Received ; acceptedAccepted to Astronomical Journal Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627,USA Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile US Naval Observatory, 3450 Massachusetts Ave., NW, Washington, DC, 20392, USA Department of Astronomy and Astrophysics, University of Chicago, IL, 60637, USA Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302-4106, USA Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands Department of Physics & Astronomy, Hunter College, 695 Park Avenue, New York, NY10065, USA Department of Astrophysics, American Museum of Natural History, Central Park Westat 79th Street, New York, NY 10034, USA US Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002-1149, USA Department of Astronomy, University of Virginia, PO Box 400325, Charlottesville, VA22904-4325, USA 2 –
ABSTRACT
LP 876-10 is a nearby active M4 dwarf in Aquarius at a distance of 7.6 pc.The star is a new addition to the 10-pc census, with a parallax measured viathe Research Consortium on Nearby Stars (RECONS) astrometric survey on theSmall & Moderate Aperture Research Telescope System’s (SMARTS) 0.9-m tele-scope. We demonstrate that the astrometry, radial velocity, and photometricdata for LP 876-10 are consistent with the star being a third, bound, stellar com-ponent to the Fomalhaut multiple system, despite the star lying nearly 6 ◦ awayfrom Fomalhaut A in the sky. The 3D separation of LP 876-10 from Fomalhautis only 0.77 ± ± ∼ ∼ − . Neither our echelle spectroscopy nor astrometry are ableto confirm the close companion to LP 876-10 reported in the Washington Dou-ble Star Catalog (WSI 138). We argue that the Castor Moving Group to whichthe Fomalhaut system purportedly belongs, is likely to be a dynamical stream,and hence membership to the group does not provide useful age constraints forgroup members. LP 876-10 (Fomalhaut C) has now risen from obscurity to be-come a rare example of a field M dwarf with well-constrained age (440 ±
40 Myr)and metallicity. Besides harboring a debris disk system and candidate planet,Fomalhaut now has two of the widest known stellar companions.
Subject headings: binaries: visual — Stars: activity — Stars: fundamentalparameters — Stars: individual (LP 876-10, Fomalhaut, TW PsA) — Stars: rotation
1. Introduction
Fomalhaut is an important nearby A3 V star, containing a large resolved dusty debrisdisk (Gillett 1986; Kalas et al. 2005) and a candidate extrasolar planet (Kalas et al. 2008,2013; Quillen 2006). Fomalhaut has previously had at least two stars suggested to becompanions. See (1898) reported a 14th magnitude stellar companion to Fomalhaut at30” separation, however this star was later deemed a background star by Burnham (1978) . Luyten (1938) reported discovery of a K-type common proper motion companion to See (1898) reported a single observation of a 14th magnitude companion at θ = 36 ◦ .2,separation 29”.98, at epoch 1896.706. Dubbed “ λ it appearsto be merely a faint field star, having no real connection with Fomalhaut .” Based on thevan Leeuwen (2007) Hipparcos astrometry for Fomalhaut A, we estimate that Fomalhaut Ahas moved 35” since See’s observation, and was at ICRS position 22:57:36.44 -29:37:03.0 atepoch 1896.706. See’s reported position angle and offset corresponds to ∆ α = +17”.7, ∆ δ = +24”.2, hence if this object were stationary, we would predict its ICRS position to benear 22:57:37.8 -29:36:39. No catalogued object appears near this position. Examination ofFig. 3 of Marengo et al. (2009), an IRAC 4.5 µ m full-array, roll-subtracted image taken withSpitzer Space Telescope, shows no obvious point source either at the position See reported,nor where See’s star would appear if it were comoving with Fomalhaut. Given that 1) Seeonly reported a single observation, 2) no subsequent literature characterized the object, and3) we were unable to find the star in the Spitzer imagery and other modern catalogs, weconclude that See’s reported companion to Fomalhaut was likely spurious. 4 –Fomalhaut: TW PsA (HR 8721). The physicality of the Fomalhaut-TW PsA binary systemwas investigated by Barrado y Navascues et al. (1997) and Mamajek (2012), and bothstudies concluded that the pair comprise a physical binary. Mamajek (2012) estimatedthat Fomalhaut and TW PsA have a true separation of only 0.28 pc and share velocitieswithin 0.1 ± − , consistent with constituting a bound system. Mamajek (2012)estimated the age for the Fomalhaut binary system to be 440 ±
40 Myr based upon multipleage indicators, with the isochronal age of Fomalhaut A and the gyrochronology age ofFomalhaut B providing the most weight.During the preparation of the Mamajek (2012) article, another neighboring star wasidentified that appeared to share motion with Fomalhaut and TW PsA: LP 876-10 (NLTT54872, WT 2282, 2MASS J22480446-2422075, PM I22480-2422). LP 876-10 is a highproper motion star first catalogued as such by Luyten & Hughes (1980), situated 5 ◦ .67 NW(20407 . ′′
6; PA = 337 ◦ .91) of Fomalhaut. At the time of writing Mamajek (2012) there wasinsufficient evidence to test whether LP 876-10 was truly associated with the Fomalhautbinary, with the main evidence being the coincidental proper motion and photometricdistance. In this contribution, we combine newly determined accurate astrometric andradial velocity measurements for LP 876-10 to demonstrate that it too, like TW PsA,appears to be a distant companion of Fomalhaut, and should be considered “Fomalhaut C”.
2. Analysis
The stellar parameters for Fomalhaut, TW PsA, and LP 876-10 are summarized inTable 1. Finder charts for LP 876-10 are provided in Figure 1. 5 –Table 1. Stellar Parameters (1) (2) (3) (4) (5) (6)Value α PsA TW PsA LP 876-10 Units Ref.... Fomalhaut A Fomalhaut B Fomalhaut C ... α ICRS (J2000) 344.411773 344.099277 342.018632 deg 1,1,2 δ ICRS (J2000) -29.621837 -31.565179 -24.368872 deg 1,1,2Parallax 129.81 ± ± ± ± ± ± µ α ± ± ± − µ δ -164.67 ± ± ± − v R ± ± ± − V ± ± ± V ± ± ± Rot ... 10.3 0.466 day 7,3SpT A3 Va K4 Ve M4 V ... 8,9,10T eff ±
73 4594 ±
80 3132 ±
65 K K 11,12,3 f bol ± − gal gal gal -6.98 -6.88 -6.70 pc 11,11,3∆ com ± ± ± − ± ± ± − ± ± ± − ± ± − ⊙ ) 1.221 ± ± ± ± +0 . − . ± ⊙ θ . ′′ .7 20407 . ′′ .6 arcsec 14,14PA 0 187 ◦ .88 337 ◦ .91 deg 14,14Note. — ∆ com is the approximate 3D separation between the star and thesystem’s center of mass. ∆S is the difference in velocity compared to FomalhautA. Masses are estimated using evolutionary tracks and are not dynamically mea- Left:
Kron-Cousins I-band image of LP 876-10 taken with SMARTS 0.9-m telescopein 2004.
Right:
Johnson V-band image of LP 876-10 taken with SMARTS 0.9-m telescopein 2012. Star is circled in blue and is near 22:48:04.5 -24:22:08 (J2000). Field of view forboth images is 6.8 arcminutes square. North is up and east is left. Fomalhaut A and B arenot in the field of view. 7 – sured. θ is the projected angular separation from Fomalhaut, and PA is the posi-tion angle as measured north through east. References: (1) van Leeuwen (2007)(distance = 1/parallax), (2) Roeser et al. (2010) (PPMXL), (3) this paper, (4)Gontcharov (2006), (5) Nordstr¨om et al. (2004), (6) Mermilliod & Mermilliod(1994), (7) Busko & Torres (1978), (8) Gray & Garrison (1989) (standard), (9)Keenan & McNeil (1989), (10) Scholz et al. (2005), (11) Mamajek (2012), (12)Casagrande et al. (2011), (13) Davis et al. (2005), (14) this paper, using po-sitions from van Leeuwen (2007) and Roeser et al. (2010). In the Table andthroughout the paper, Galactic velocities U and positions X are defined to-wards the Galactic center, V and Y are in the direction of Galactic rotation,and W and Z is towards the North Galactic pole. Adopted optical and infrared magnitudes from 0.4 µ m (B-band) to 22 µ m (W4-band)are compiled in Table 2. V-band magnitudes of 12.618 ± ± ± photometry program. Pojmanski (1997) presents time series V-band ASASphotometry for this star (349 observations between UT 22 November 2000 and UT 7 October2009 with quality flag A) with mean V = 12.62 and rms scatter of 0.06 mag. The photo-metric errors of the individual measurements are typically ∼ ∼ V T photometric system (Pollacco et al. 2006;Butters et al. 2010; Høg et al. 2000). These photometry are later discussed in Sec. 2.8 forthe purposes of measuring the rotation period, but were not included in our assessmentof the mean Johnson V magnitude. Based on photometry measured independently byReid et al. (2003), Henden et al. (2012), Pojmanski (1997), and the RECONS observa-tions with the SMARTS 0.9-m telescope, we adopt a mean V magnitude of 12.62 ± (1) (2) (3)Band Mag Ref.B 14.31 ± ± KC ± KC ± ± ± s ± ± ± ± ± §
11 – son B from APASS(Henden et al. 2012),and the Johnson V andKron-Cousins R KC I KC photometry from RE-CONS (Jao et al. 2005;Winters et al. 2011),are all photometricallycalibrated to Landolt(1992) standard stars.
12 –
The parallax and proper motion of LP 876-10 have been measured during the long-termastrometry program carried out by RECONS at the SMARTS 0.9-m telescope. Jao et al.(2005) describes the astrometry program, however we briefly summarize the program here.A filter is selected from the Johnson-Kron-Cousins
V R RC I RC filterset that provides awell-exposed reference field that, ideally, encircles the target star. Throughout the course ofthe observations, the same pointing (to within a few pixels) and filter are used. Centroidsfor the reference field and parallax star are extracted using SExtractor (Bertin & Arnouts1996) and corrected for differential color refraction using V R RC I RC photometry of thereference and science target stars (see Section 2.1). Relative parallax and proper motionof the target star are solved for using the Gaussfit program . Correction from relative toabsolute parallax is done by estimating the mean distance to the reference field stars, again,using V R RC I RC photometry and the photometric distance relations of Henry et al. (2004).LP 876-10 was included in the RECONS astrometric survey due to its close predictedphotometric distance (7.2 ± ± ± − at position angle (PA)118 ◦ .0 ± ◦ .1 east of north. When the proper and parallactic motions are removed from thestar’s position, the residuals show no hint of curvature or any pattern that would suggestthe existence of an unseen companion (see § d = 7.57 ± ± +2 − kAU), and from TWPsA it lies only 0.987 +0 . − . pc (203 ± Available from the HST Astrometry Team at ftp://clyde.as.utexas.edu/pub/gaussfit/. 13 –and LP 876-10.Blinking images suggests that the neighboring high proper motion star LP 876-11could be a proper motion companion to LP 876-10; LP 876-11 is located 1’.8 away fromLP 876-10 at 42 ◦ east of north. However, we determine a proper motion for LP 876-11of 321.3 ± − at PA 143 ◦ .0 ± ◦ .2 east of north, which is inconsistent with themeasured motion for LP 876-10. Using twelve color-magnitude relations from Henry et al.(2004), we estimate a photometric distance to LP 876-11 of 730 ±
120 pc. We measurea trigonometric parallax of LP 876-11 of 1 ± A spectrum of LP 876-10 was taken with the CRIRES spectrograph on the 8.4-mVLT UT1 (Antu) telescope on UT date 16 June 2009 as part of a near-infrared radialvelocity survey of nearby late-type M dwarfs (Bean et al. 2010). The CRIRES spectrum haswavelength coverage 2.292–2.349 µ m over the effective 4096 x 512 focal plane detector, amosaic of four Aladdin III InSb arrays (Kaeufl et al. 2004). The slit width was 0 ′′ .2, yieldinga resolving power of R ≃ − at 2 pixel sampling). The signalto noise ratio in the continuum of the spectrum was ∼ Photometry for LP 876-11 from 2 nights of observations: V = 17.72, R KC = 16.90, and I KC = 16.04. 14 –rotation velocity of v sin i = 22 ± − ; a heliocentric radial velocity of +6.5 ± − was also measured. Slit viewer images of LP 876-10 appear point-like, and there is no signof duplicity in the CRIRES spectrum. A more detailed spectroscopic analysis of LP 876-10will be presented in a forthcoming paper (Seifahrt et al., in prep.). While neither the astrometry nor the spectroscopy data are consistent with LP 876-10being a binary, it is listed as a double star in the Washington Double Star catalog (WDS;Mason et al. 2001) as WDS 22481-2422 and with discovery identifier “WSI 138” . Asingle observation is reported for epoch 2010, with a reported companion at separation 0”.5at PA = 144 ◦ , with magnitudes 12.80 and 14.80 (presumably V -band, as the combinedmagnitude [12.64] is similar to the adopted V magnitude in Table 2). We are unable toconfirm the existence of the companion reported in the WDS. In the 118 frames takenduring 25 nights, with FWHMs in the range 1”.2 to 2”.8, LP 876-10 appeared to be a pointsource - with no evidence of elongation. With only a single observation, the possibilityremains that the reported WDS companion may be a chance alignment between this highproper motion star and a background star (B. D. Mason 2013, private communication).However, we believe that a background star is unlikely to explain this discrepancy. Basedon the UCAC4 position of LP 876-10 for epoch 2000.0 (Zacharias et al. 2013), the propermotion calculated in this paper, and the separation/PA value listed in WDS, we estimatethat the WSI 138 companion reported in WDS had approximate ICRS position 22:48:04.76 Values are listed from the 03 Mar 2013 update of WDS. WSI = Washington Speckle Interferometer. 15 –-24:22:09.1 (epoch 2010). The only object listed in any Vizier-queryable catalog within2” of this position is the WISE detection of LP 876-10 itself (0.5” away) during 2010.No plausible optical-IR counterpart within 2” of this position exists in the USNO-B1.0,SuperCOSMOS, GSC, and 2MASS catalogs. It seems very unlikely that a bright (V =14.8) background star can explain the faint companion to LP 876-10 reported in theWDS. If the companion were real, and physically associated with LP 876-10, then itsabsolute magnitude (M V = 15.40) would correspond to a 0.11 M ⊙ star on the calibration ofDelfosse et al. (2000). Given the projected separation (0”.5 = 3.8 AU), these values wouldpredict an orbital period of ∼ ∼
27 deg yr − and a photocentric amplitude of ∼
110 mas.The predicted photocentric amplitude would be about half (50 mas over 8 years) of thefull amplitude (110 mas over ∼ <
10 mas level over ∼ ∼ − (rms) of the 8-year baseline propermotion calculated in this survey, further suggesting that it would be difficult to hide a ∼ − perturbation of the photocentric motion. As the purported WDS companionshould have a period only somewhat longer than the duration of our RECONS astrometricdataset, and with a predicted photocentric amplitude similar in size to the observed par-allax, we conclude that it is unlikely that the companion reported in the WDS catalog is real. 16 – We estimated T eff for LP 876-10 by fitting the photometry in Table 2 to the BT-Settlgrid of synthetic stellar spectra which vary by effective temperature, metallicity, and surfacegravity (Allard et al. 2012). Twenty-two colors consisting of combinations of the bands V , R RC , I RC , J , H , K s , W W
2, and W eff = 3132 K and solarmetallicity. We estimated the uncertainty in T eff due to metallicity and surface gravityby individually varying these parameters by one increment (0.2 dex) and measuring theeffect on the resultant T eff . The uncertainty in the T eff breaks down approximately asfollows: ±
33 K from the dispersion in color-based T eff estimates for the best fit, ±
50 Kdue to metallicity uncertainty, and ±
25 K due to uncertainty in log( g ). Together thisyields an overall T eff uncertainty of ±
65 K. The systematic error due to the validity ofthe BT-Settl models is unknown, however our derived T eff should be comparable to Mdwarf T eff values derived using the same models (indeed Rajpurohit et al. 2013, similarlyderives T eff ≃ eff = 3100 K, [Fe/H] = 0.0, log( g ) = 5.0. Fromconsiderations of the star’s color-magnitude diagram position (Sec. 2.6), we predict thatLP 876-10 has a slightly subsolar metallicity, and lies near the zero-age main sequence for ∼ ⊙ stars (log( g ) ≃ λ -dependent polynomial correction factorthat is applied to the synthetic spectrum to cause small modifications in order to producethe best fit to the photometry (details of the technique are described in Dieterich et al. 2013).By directly integrating the spectral energy distribution made by fitting the photometry 17 –in Table 2 with solar composition BT Settl models, we estimate m bol = 9.994 ± ± × erg s − , log(L/L ⊙ ) = -2.337 ± bol = 10.597 ± V = m bol - V = -2.62 ± eff estimate, we estimate a radius of 0.23 ± ⊙ .Combined with our estimate of the projected rotation velocity v sin i (22 ± − ), thisplaces an upper limit on the rotation period of LP 876-10 of 0.55 ± § Using our new parallax and the photometry in Table 2, we estimate absolutemagnitudes of M V = 13.21 ± K s = 7.81 ± s ) color of 5.40 ± s ) vs. M V relations from Henry et al.(2004) and Johnson & Apps (2009), we predict photometric distances of 7.9 ± ± s ) color versus absolute magnitude M V and use the metallicitycolor-magnitude calibration of Johnson & Apps (2009) to predict a metallicity of [Fe/H]= -0.07 dex (estimated accuracy ± ± ± ≃ -0.1 dex. Using the Delfosse et al. (2000) M V vs. mass calibration for field M dwarfs (i.e. mixedmetallicities and ages), the approximate mass of LP 876-10 is ∼ ⊙ . Interpolatingwithin the Baraffe et al. (1998) tracks, one finds that solar composition stars with massesof greater than 0.163 M ⊙ are not ever predicted to be as faint as M V = 13.21 mag (seeFig. 4). As the tracks are first and foremost tracing luminosity evolution as a functionof mass and age, we also examine the constraints that the luminosity of LP 876-10 canprovide. Through fitting BT-Settl models to the photometry, we estimate the luminosity tobe log(L/L ⊙ ) = -2.337 ± and Dotter et al.(2008) solar composition tracks give essentially identical predictions that no stars withwith masses greater than 0.197 M ⊙ are predicted to have luminosities this low. Using thosetracks, we estimate that it takes a 0.2 M ⊙ star approximately ∼
300 Myr to reach within ∼ ∼ ≃ -0.1 is commensurate with the adopted age for Fomalhaut A and B (440 Myr;Mamajek 2012).As seen in Fig. 4, the Baraffe et al. (1998) isochrones do not accurately reproducethe empirical main sequence from Johnson & Apps (2009) in this color regime, so our 19 –lower bound on the age of LP 876-10 is only approximate. Naively interpolating themass and age of Fomalhaut C from the evolutionary tracks and isochrones would yielda mass of ∼ ⊙ and age of ∼
60 Myr. However, as can be seen in Fig. 4, a 125Myr isochrone (log(age/yr) = 8.1) from the same tracks fails to replicate the intrinsiccolor-magnitude sequence for the ∼
125 Myr-old Pleiades (Barrado y Navascu´es et al.2004). For the V-K s color (5.4) of LP 876-10, the combination of Pleiades color-magnitudesequence from Stauffer et al. (2007) and mean Pleiades distance from Soderblom et al.(2005) yield a Pleiades absolute magnitude of M V = 12.24. The Baraffe et al. (1998)isochrones for age 125 Myr (log(age/yr) = 8.1) predict absolute magnitude M V = 13.57for V-K s = 5.4 , i.e. 1.33 mag too faint! As summarized by Bell et al. (2012), “for alloptical colours, no pre-MS models follows the observed Pleiades sequence for temperaturescooler than 4000 K.” Estimating isochronal ages using pre-MS evolutionary tracks is quiteproblematic, with large systematic differences between tracks (see review by Soderblom2010). For all of these reasons, we do not adopt the pre-MS mass and isochronal ageinterpolated from the evolutionary tracks and isochrones in Fig. 4, and instead constrainthe age based on its proximity to the main sequence, and infer the mass based on mainsequence absolute magnitude vs. mass considerations. Given the empirical and theo-retical constraints previously discussed, we adopt a mass of 0.18 ± ⊙ for Fomalhaut C. The Baraffe et al. (1998) tracks use the CIT
J HK photometric system. We convert theBaraffe et al. (1998) CIT photometry to 2MASS following (Carpenter 2001). 20 –
Photometric data from the online SuperWASP archive (Butters et al. 2010) consistingof 14,991 measurements for LP 876-10 were extracted for two observing seasons (2007-2008).To search for a rotation period, we selected SuperWASP photometry from a singlewell-sampled season (2008) with V SuperW ASP magnitudes between 12.46 and 12.70, withmagnitude and photometric error of less than 0.2 mag, and with a good TAMFLUX2 flagextraction. SuperWASP photometry is calibrated to the Tycho-2 V T system (Pollacco et al.2006; Høg et al. 2000). There were 3162 points for subsequent analysis. To remove 1-dayaliasing effects, all points during a single observing night were adjusted so that their averageequalled the average seasonal magnitude of LP 876-10. A Lomb-Scargle periodogram withassociated False Alarm Probabilities (FAP) was calculated following Press et al. (1992), andthe resultant periodogram is plotted in Fig. 5. There is significant power (FAP < − , whichis only slightly larger than the observed v sin i (22 km s − , corresponding to a maximumperiod of 0.55 day). For the star’s mass and radius, we estimate a breakup velocity andperiod (following Townsend et al. 2004) of 386 km s − and P breakup = 0.03 day, respectively.Hence, any of the periods between ∼ ∼ v sin i constraints, respectively. The fastest rotation period among 41 nearby field Mdwarfs in the MEarth survey of Irwin et al. (2011) is 0.28 days. We test the robustnessof the detection by injecting artificial sinusoidal ( P = 0 .
466 day) signals into a Gaussiandistributed photometric data set with the same time cadence as the LP 876-10 data set.These tests indicate that the 0.195, 0.242, and 0.318 day peaks are aliasing effects due tothe irregular time sampling of the light curve. We conclude that the P = 0 .
466 d peak is ∼ ∼ ⊙ star places negligibleconstraint on its age. Mid-M stars with rotation periods faster than 1 day are a nearlyubiquitous feature of stellar samples between ages of ∼ ∼
10 Gyr (see Fig. 12of Irwin et al. 2011). Figure 11 of Irwin et al. (2011) plots the masses of field M dwarfsvs. their rotation periods measured by the MEarth survey. For stars of ∼ ⊙ , arotation period of ∼ > Not only is LP 876-10 fast rotating, but, unsurprisingly, it appears to be a coronallyactive star as well. Voges et al. (1999) ranked LP 876-10 as the most likely opticalcounterpart of the ROSAT All-Sky Survey (RASS) Bright Source Catalog (BSC) X-raysource 1RXS J224803.5-242240. The X-ray counterpart is 35” away from LP 876-10.However, the RASS BSC position error is 15”, and LP 876-10 is the brightest optical sourcewithin 40”, indicating that it is the likely X-ray source (Neuhaeuser et al. 1995). 1RXSJ224803.5-242240 appears to be the brightest RASS X-ray source within a degree of LP876-10. The fact that the position of the brightest RASS X-ray source within a degree of LP Listed under its Guide Star Catalog alias “GSC6964.01226”. 22 –876-10 lies within 40” of the rapidly rotating, nearby M dwarf suggests to us that LP 876-10is almost certainly the optical counterpart of 1RXS J224803.5-242240. The RASS-BSCcatalog (Voges et al. 1999) reports a soft X-ray flux of 0.142 ct s − (28% uncertainty) withHR1 hardness ratio of -0.23 ± × − erg/s/cm − . At d = 7.57 pc, this corresponds to an X-ray luminosity of L X ≃ . erg/s. This implieslog( L X / L bol ) ≃ -3.41, i.e. a very active star close to X-ray saturation. This corroboratesthe very high projected rotational velocity measured spectroscopically ( v sin i = 22 km s − ),which should induce strong magnetic activity. With our best measurements of the proper motion, radial velocity, and parallax, wecalculate the 3D Galactic velocity of LP 876-10 to be (U, V, W) = -5.3 ± ± ± − . Comparing these values to those for Fomalhaut and Fomalhaut B(TW PsA), we find that LP 876-10’s velocity only differs from that of Fomalhaut by1.1 ± − , and that of Fomalhaut B by 1.1 ± − . Using the LSR velocityellipsoid for both dM and dMe dwarfs estimated by Reid et al. (2002, their unweighted solu-tion), and adopting the solar peculiar velocity with respect to the LSR from Sch¨onrich et al.(2010), we naively only expect roughly 1 in ∼ − of Fomalhaut, and roughly 1 in ∼ − . 23 –Henry et al. (2006) report 239 M dwarfs within 10 pc having accurate trigonometricparallaxes. These numbers are updated at recons.org, with a count as of 1 January2012 of 248, which corresponds to a number density of 0.059 pc − . This space densityimplies that within a sphere of radius 1 pc surrounding Fomalhaut, we would expect tofind 0.25 M dwarfs. Hence, we estimate the probability that a random M dwarf couldappear within 1 pc of Fomalhaut, and sharing its velocity within less than 2 km s − , asapproximately 1 in ∼ . (and sharing its velocity within less than 1.1 km s − as roughly1 in ∼ . ). For comparison, one would expect to have to encircle a sphere ∼
36 pc inradius in the local Galactic disk in order to find another M dwarf whose motion randomlyagreed with that of Fomalhaut within less than 2 km s − . Our probability estimates donot take into account the similarity in the spectroscopic metallicity of TW PsA and thephotometric metallicity of LP 876-10, which provides further agreement. We concludethat LP 876-10 appears to be related to Fomalhaut and TW PsA beyond a reasonable doubt. Fomalhaut was listed by Barrado y Navascues (1998) as a potential member of theCastor Moving Group (GMG). The co-motion of LP 876-10 with Fomalhaut may be lesssignificant if Fomalhaut is immersed in a swarm of co-moving stars like the purportedCMG. The origin and nature of moving groups like CMG is an active field of study (e.g.Famaey et al. 2005; Murgas et al. 2013). That the CMG represents a kinematic group ofstars of common age and birthsite is unlikely.Calculating revised space motions for the 14 CMG “members” (“Y” or “Y?” members)from Barrado y Navascues (1998), using revised Hipparcos astrometry (van Leeuwen 2007)and the best available radial velocities (Barbier-Brossat & Figon 2000; Gontcharov 2006), 24 –we find that the CMG stars have median velocity of ( U , V , W ) = -11.1 ± ± ± − , with standard deviations of 6.1, 3.6, and 4.2 km s − . The scatters aremuch larger than the typical velocity errors, and larger than the one-dimensional velocitydispersions of nearby clusters and associations ( < − ; Madsen et al. 2002; Mamajek2010). The velocity for Fomalhaut differs from the CMG median velocity by 5.6 ± − . The list of “final” members in Barrado y Navascues (1998) comprises ∼
27 M ⊙ ofstars spread out over a volume of ∼ , implying that the density of CMG membersin the solar neighborhood is roughly ∼ × the local disk density (0.12 M ⊙ pc − ;van Leeuwen 2007). The stellar systems in the CMG have negligible interaction with oneanother, and so their motions are completely dominated by the local Galactic potential.The velocity differences between Fomalhaut and individual CMG members is illumi-nating, and we discuss the famous CMG members Vega, LP 944-20, and Castor itself,in more detail. Vega is a proposed fellow CMG star of either similar age (455 ±
13 Myr;Yoon et al. 2010) or somewhat older age (700 +150 − Myr; Monnier et al. 2012) than thatof Fomalhaut (440 ±
40 Myr; Mamajek 2012). Could Vega and Fomalhaut be related?Using the revised Hipparcos astrometry for Vega and its mean radial velocity reportedby Parthasarathy & Lambert (1987), we estimate for Vega a velocity of ( U , V , W ) =-15.9 ± ± ± − . Vega’s velocity differs from that of Fomalhautby 10.9 ± − , and only 10 Myr ago their separations differed by ∼ ±
10 pc.Another nearby famous CMG “member” is the nearby candidate brown dwarf LP 944-20
A new RECONS parallax has been measured which places LP 944-20 at a distance of6.4 pc (Dieterich et al., submitted), making it most likely a star near the H-burning limitrather than a brown dwarf. The new distance revises LP 944-20’s space motion to ( U , V , W ) = -14.9, -5.9, -1.5 km s − , which differs from that of Fomalhaut by 13.5 km s − , and does 25 –(Ribas 2003). Adopting the astrometry from Tinney (1996) and a mean radial velocityof +9.0 ± − (based on measurements from Mart´ın et al. 2006), we calculatea velocity for LP 944-20 of ( U , V , W ) = -12.2 ± ± ± − . LP944-20 is currently situated 6.6 pc away from Fomalhaut, and its velocity differs fromthat of Fomalhaut by 10.9 ± − . Only 10 Myr ago, LP 944-20 and Fomalhautwere separated by ∼ ± − in our calculations). These values are consistent with theCastor system having a velocity of ( U , V , W ) = -7.5 ± ± ± − .Fomalhaut is currently ∼
21 pc away from the Castor system, differing in velocity by asignificant margin (4.9 ± − ), and only 10 Myr ago Fomalhaut and Castor were sepa-rated by ∼ ± One predicts that stellar companions in multiple systems can exist with separationsup to their tidal (Jacobi) radius with respect to the Galactic potential. Jiang & Tremaine(2010) parameterize the tidal radius r t as: r t = (cid:26) G ( M + M )4Ω A (cid:27) / (1)where G is the Newtonian gravitational constant, M and M are the masses of thestars, Ω is the Galactic angular circular speed (orbital velocity dividied by Galactocentricradius), and A is the Oort parameter. Adopting modern estimates of the relevant Galacticparameters, and rewriting the expression from Jiang & Tremaine (2010), we estimate thetidal radius to be: r t = 1 .
35 pc (cid:26) M total M ⊙ (cid:27) / (2)Summing the masses of the Fomalhaut system components (2.83 M ⊙ ), one predicts a tidalradius of ∼ ∼ ∼ − effects of convective blueshift and gravitational redshift, may lend itself to providing a test 27 –as to whether Fomalhaut C is orbiting Fomalhaut AB either retrograde or prograde to theGalactic rotation (V. Makarov, priv. comm.).Fomalhaut A and B are separated by ∆ AB = 57.4 +3 . − . kAU, and Fomalhaut C isseparated by ∆ AC = 158.2 +2 . − . kAU from A, and by ∆ BC = 203.4 +1 . − . kAU from B. Wecalculate the position of the barycenter (center of mass) for the system using the Galactic(X, Y, Z) positions and masses in Table 1: ( X, Y, Z ) com = 3.08, 1.13, -6.93 pc. Convertingthis position to the equatorial ICRS coordinate system yields ( α , δ ) = 344 ◦ .179, -29 ◦ .792,at distance 7.67 pc. We can make a rough estimate of the orbital period of C around theAB pair. C is currently located ∼ a ∼ e ∼ ⊙ ),this translates to an approximate orbital period of ∼
20 Myr, or ∼
5% the system’s age. Thepredicted orbital velocity of LP 876-10 around the Fomalhaut system barycenter would be ∼ − . Given the masses and configuration of the AB pair, the escape velocity of Cis ∼ − .How stable is Fomalhaut C’s orbit with respect to A and B? Obviously, the orbit ofAB and AB-C are not well constrained. We only have fairly accurate estimates of therelevant mass ratios and current separations, while the semi-major axes and eccentricitiesare unknown. The mass of C is very small compared to that for the AB pair ( µ = M C /( M A + M B ) ≃ e = 0; a =57.4 kAU), then the minimum stable semi-major axis for C is predicted to be ∼
135 kAU.Tokovinin (1998) estimates that the mean eccentricity for wide binary pairs is < e > ≃ e ∼ a ∼
34 kAU, and theminimum stable semi-major axis for Fomalhaut C is ∼
140 kAU. There are plausible rangesof orbital parameters for Fomalhaut B and C that would be dynamically stable over manyorbits.Could LP 876-10 be genetically related to Fomalhaut AB but we are “catching it inthe act” of being an unbound escapee of the Fomalhaut system? We argue that this is veryunlikely. LP 876-10 has velocity statistically consistent with that of Fomalhaut A and B(∆ S = 1.1 ± − ). If the star actually had a velocity difference of > − (i.e.above escape velocity), with respect to the Fomalhaut AB barycenter, then it would notspend much time in the vicinity of Fomalhaut or near its tidal radius. The approximatetimescale that LP 876-10 would spend within Fomalhaut’s tidal radius is approximately t ≃ r t /∆ S , where r t ≃ v mustbe larger than the escape velocity (0.2 km s − = 0.2 pc Myr − ). Hence: t Myr ≃ r t ∆ S < . S ∼ v esc ∼ − , LP 876-10 could spend of order ∼
10 Myr within the tidalradius of Fomalhaut. For a velocity difference of ∼ − , LP 876-10 would spend only ∼ S greater than 2.5 29 –km s − are ruled out at 95% confidence, so timescales for LP 876-10 being unbound andwithin the tidal radius of Fomalhaut shorter than ∼ t wouldhave to be of order ∼ ∼ unbound member of the Fomalhaut system in a state of disintegration, then we would haveto be witnesses to an unusual dynamical state predicted to occur over ∼ . This seems rather unlikely, and the simplest explanation for theagreement in velocities at the kilometer-per-second level between LP 876-10 and FomalhautA & B, and its position within the tidal radius of Fomalhaut AB, is that LP 876-10 is athird bound component of the Fomalhaut system.
3. Summary
LP 876-10 is an active (log( L X / L bol ) ≃ -3.4), fast-rotating (P ≃ ∼ − . Mamajek (2012) showed that the isochronal age of Fomalhaut, and variousage diagnostics for TW PsA (rotation, X-ray emission, Li abundance) were consistent withan age of 440 ±
40 Myr for the pair. The appearance of LP 876-10 on the main sequencehints that it is >
300 Myr in age, and its photometric metallicity ([Fe/H] ≃ -0.1) is ingood agreement with spectroscopic metallicity estimates for TW PsA. We argue that thepurported membership of the Fomalhaut system to the Castor Moving Group does notprovide a useful age-constraint on the system.Based on its position, velocity, and color-magnitude data, we argue that LP 876-10 isa third stellar component in Fomalhaut system. The chances of an interloper field M dwarfsharing the velocity of Fomalhaut within 1 km s − and lying within 1 pc of Fomalhaut is 30 – < − , hence LP 876-10 is almost certainly physically related to Fomalhaut A and B. Thechances that we are catching the Fomalhaut system in a state of disintegration, where LP876-10 is currently escaping with velocity greater than its predicted escape velocity (0.2km s − ), is statistically unlikely ( < − ). Hence, we argue that LP 876-10 is most likely a bound low-mass stellar companion to the Fomalhaut system, which has a well-determinedage of 440 ±
40 Myr (Mamajek 2012). This makes the previously barely-studied M dwarfLP 876-10 (“Fomalhaut C”), only recently added to the census of stars within 10 pc via theRECONS astrometry program, one of the few red dwarfs in the solar neighborhood witha strongly constrained ( ∼ ≃ -0.1). Given the difficultyin calibrating the age and metallicity scale for M dwarfs, Fomalhaut C provides a usefulanchor among the mid-M stars, and another rare of example of a low-mass companion withseparation approaching a parsec. The existence of both Fomalhaut C (LP 876-10) and B(TW PsA) should be considered for future dynamical calculations trying to explain theunusual offset ( ∼
13 AU) between Fomalhaut A and its debris disk (Kalas et al. 2005), andthe eccentric orbit for the planet candidate Fomalhaut Ab (Kalas et al. 2013).We thank the referee for a prompt and thoughtful review which significantly improvedthe paper. We thank Brian Mason, Mark Pecaut, Valeri Makarov, Massimo Marengo,John Bangert, Christine Hackman, Demetrios Matsakis, Sean Urban, Alice Quillen, andPaul Kalas for discussions on LP 876-10, Fomalhaut, and comments on the paper. EEMacknowledges support from NSF award AST-1008908. JLB acknowledges support from theUniversity of Virginia, Hampden-Sydney College, and the Levinson Fund of the PeninsulaCommunity Foundation. The RECONS effort is supported primarily by the NationalScience Foundation through grants AST 05-07711 and AST 09-08402. Observations wereinitially made possible by NOAO’s Survey Program and have continued via the SMARTSConsortium. This research has made use of NASA ADS, SIMBAD, Vizier, and data 31 –products from 2MASS, WISE, UCAC, SuperWASP, ASAS, and Hipparcos. This researchwas made possible through the use of the AAVSO Photometric All-Sky Survey (APASS),funded by the Robert Martin Ayers Sciences Fund. This research has made use of theWashington Double Star Catalog maintained at the U.S. Naval Observatory. 32 –Fig. 2.— Positions and proper motion vectors for Fomalhaut A, B (TW PsA), and C (LP876-10). The system barycenter is estimated to be at the position marked with an X, atdistance 7.67 pc (more details are discussed in § (1) (2) (3)Reference µ α µ δ ... mas yr − mas yr − Wroblewski & Costa (1999) 290 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
34 –Fig. 3.— Astrometric residuals in RA ( top ) and Dec ( bottom ) for LP 876-10 in V-band imagesafter subtracting the parallactic motion with ̟ = 132.07 ± µ = 378 ◦ .1 ± − at PA 118 ◦ .0 ± ∼
110 mas level with period ∼
13 yr. Any perturbations due tounseen companions must be at the <
10 mas level over the ∼ V ) for Fomalhaut C (LP 876-10). The empirical mainsequences for [Fe/H] = 0.0 and -0.1 from Johnson & Apps (2009) are plotted as thick solidlines , along with the color-magnitude sequence for the ∼
125 Myr-old Pleiades cluster fromStauffer et al. (2007, adopting d = 133.5 pc from Soderblom et al. 2005) ( thin solid line ). The theoretical evolutionary tracks from Baraffe et al. (1998) are plotted as dotted lines , and theisochrones for the approximate age of the Pleiades (log(t/yr) = 8.1) and Fomalhaut A & B(log(t/yr) = 8.6) are plotted as long dashed lines . The evolutionary tracks do not accuratelypredict the solar composition main sequence nor Pleiades sequence for this color-magnitudecombination. Note that the Pleiades has a well-determined Lithium depletion boundary andmain sequence turn-off age consistent with ∼
125 Myr (Ventura et al. 1998; Stauffer et al.1998; Barrado y Navascu´es et al. 2004), and the main sequence “turn-on” appears to beconsistent with this age as well (Barenfeld et al. 2013). Fomalhaut C appears to lie on theempirical main sequence of Johnson & Apps (2009) with [Fe/H] ≃ -0.07. 36 –Fig. 5.— Lomb-Scargle periodogram for SuperWASP photometry for LP 876-10. The periodat 0.466 d appears to be the real period, and tests indicate that the periods at 0.242 and0.318 d are due to aliasing. 37 – REFERENCES