Mon. Not. R. Astron. Soc. 000, 1{?? (2002) Printed 13 June 2021 (MN LATEX style file v2.2) The companion candidate near Fomalhaut - a background neutron star? R. Neuh¨auser1∗, M.M. Hohle1;2y, C. Ginski1;3z, J.G. Schmidt1x, V.V. Hambaryan1{, and T.O.B. Schmidt1;4k 1 Astrophysikalisches Institut und Universit¨ats-SternwarteJena, Schillerg¨asschen2-3, 07745 Jena, Germany 2 Graduate School of Quantitative Biosciences Munich, Genecenter of the LMU, Feodor-Lynen-Str. 25, 81377 Munich, Germany 3 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands 4 Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany Accepted 2014 Dec 27. Received 2014 Dec 18; in original form 2014 Jun 26 ABSTRACT The directly detected planetary mass companion candidate close to the young, nearby star Fomalhaut is a subject of intense discussion. While the detection of com- mon proper motion led to the interpretation as Jovian-mass companion, later non- detections in the infrared raised doubts. Recent astrometric measurements indicate a belt crossing or highly eccentric orbit for the object, if a companion, making the planetary interpretation potentially even more problematic. In this study we discuss the possibility of Fomalhaut b being a background object with a high proper motion. By analysing the available photometric and astrometric data of the object, we show that they are fully consistent with a neutron star: Neutron stars are faint, hot (blue), and fast moving. Neutron stars with an effective temperature of the whole surface area being 112,000 K to 126,500 K (with small to negligible extinc- tion) at a distance of roughly 11 pc (best fit) would be consistent with all observables, namely with the photometric detections in the optical, with the upper limits in the infrared and X-rays, as well as with the astrometry (consistent with a distances of 11 pc or more and high proper motion as typical for neutron stars) as well as with non-detection of pulsation (not beamed). We consider the probability of finding an un- related object or even a neutron star nearby and mostly co-aligned in proper motion with Fomalhaut A and come to the conclusion that this is definitely well possible. Key words: stars: individual: Fomalhaut { neutron stars { planets 1 INTRODUCTION cate that it had cleared the gap in this belt. The presence arXiv:1501.07083v1 [astro-ph.SR] 28 Jan 2015 of the belt close to the companion candidate constrained The direct detection of a possibly planetary mass object the upper mass limit of the companion candidate to a few near the star Fomalhaut by Kalas et al. (2008) was widely Jupiter masses (Kalas et al. 2008). regarded as a great success for the direct imaging detection The star Fomalhaut1 (Fomalhaut A) has the following method. The separation between Fomalhaut A and b is some relevant properties (all for the star A): 100 au or 13 arc sec. In addition to this published planetary mass companion candidate (called Fomalhaut b), Fomalhaut • Position J2000.0: α = 22h 57m 39s and δ = A (the central star) is surrounded by a well resolved dust −29◦ 370 2000 (Hipparcos, van Leeuwen 2007). belt, which was most recently studied with Herschel (Acke • The distance as measured by Hipparcos is 7:70 ± 0:03 et al. 2012) and ALMA (Boley et al. 2012). The projected pc (van Leeuwen 2007). position of the tentative companion was interpreted to indi- • Proper motion as also measured by Hipparcos is µα = 328:95 ± 0:50 mas/yr and µδ = −164:67 ± 0:35 mas/yr (van Leeuwen 2007). ∗ [email protected] y [email protected] 1 z [email protected] This star is also called α PsA, i.e. the brightest star in the x [email protected] Southern Fish, the name Fomalhaut comes from the Arabic fam { [email protected] al-h. ¯utal-jan¯ub¯ı meaning mouth of the southern fish, (Kunitzsch k [email protected] & Smart 1986). c 2002 RAS 2 R. Neuh¨auseret al. • The spectral type is A4V as obtained by an optical a planet's photosphere, especially since the flux at 0.6 µm spectrum (Gray et al. 2006); given this spectral type, the is 20 to 40 times brighter than expected for an object with color index is close to zero, e.g. B-V=0.09 mag (e.g. Ducati ∼3 MJ and a few hundred Myr (Fortney et al. 2008, Bur- 2002). rows et al. 2003). On the contrary, Currie et al. (2012) and • The optical brightness is V = 1.16 mag (e.g. Ducati Galicher et al. (2013) argue that an 0.5-1 MJ object would 2002). not have been detected at 4.5 µm (using models by Spiegel & • The age was recently determined to be 440±40 Myr by Burrows 2012 and Baraffe et al. 2003). In addition, Galicher kinematic membership to the young Castor Moving Group et al. (2013) present upper detection limits at 1.1 µm, which (Barrado y Navascues 1998, Mamajek 2012). are consistent with this upper mass limit. However, there is a general agreement in all aforementioned studies that the The companion was originally discovered in the opti- flux in the optical wavelength range cannot stem completely cal bands of the Hubble Space Telescopes (HST) Advanced (or at all) from a planet's photosphere. Camera for Survey (ACS, Ford et al. 1998). However, several In addition to the discussed over-luminosity in the op- attempts to detect the object in the near and mid infrared tical wavelength range, Kalas et al. (2008) and Janson et (see e.g. Kalas et al. 2008 and Janson et al. 2012) failed al. (2012) report significant (5-8 σ) variability of the flux (see Table 1). This was most troublesome, given that a (few at 0.6 µm, which could not be explained by a thermal emis- hundred Myr) young cooling Jovian-mass object should be sion from a planet's photosphere. Kalas et al. (2008) propose much brighter in the infrared than in the optical. Further- that there might be a 20-40 R accretion disk around the as- more, the latest astrometric measurements by Kalas et al. J sumed planet. The disk would reflect light from the primary (2013) indicate that the object would either cross the dust star, which explains the optical excess flux. Furthermore, belt or that it would be on a highly eccentric orbit which they argue that the variability could then be explained by is not in alignment with the belt at all. These two facts accretion driven H emission. Janson et al. (2012) strongly together have prompted us to seek for an alternative expla- α disagree, stating that at the high system age moons should nation which might explain all the observations and finally have formed in a possible accretion disk, and thus the reflec- resolve some of the apparent contradictions. In the follow- tive surface should be reduced. Also, they think it is unlikely ing we will first briefly discuss the various scenarios that that accretion driven Hα emission can explain the variabil- have been proposed so far and will then present our own ity, because the accretion rate would have to be similar to considerations. young T Tauri stars. Currie et al. (2012) and Galicher et In this paper, we first review the observations of Fo- al. (2013) re-analyzed the same optical data and did not de- malhaut b (Sect. 2) and the interpretations as Jupiter-mass tect any significant variability at 0.6 µm. Thus, they are not planet (Sect. 2.1), as super-Earth (Sect. 2.2), and as dust excluding a dust disk around a Jupiter-mass planet. cloud (Sect. 2.3). Then, we consider the background hy- The most recent study by Kalas et al. (2013) incorpo- pothesis as either a White Dwarf (Sect. 3.1) or neutron star rates new astrometric measurements taken with the HST (Sect. 3.2.); we discuss the astrometry, the X-ray data, and STIS (Space Telescope Imaging Spectrograph, Woodgate et the optical and IR photometry, to constrain the neutron star al. 1998) in 2010 and 2012. They find that the object is properties (to be consistent with all observables). In Sect. most likely on a ring-crossing orbit with a high semimajor 3.2.5, we also discuss the probability to find a background axis and eccentricity. One explanation for that, assuming object or even a neutron star close to a star like Fomalhaut. Fomalhaut b is a planet, would be that it had a close en- We conclude in Sect. 4. counter with a further-in massive planet and was scattered out. However, Kenworthy et al. (2013) performed deep coro- nagraphic imaging and can rule out further-in objects with 2 FOMALHAUT b AS A GRAVITATIONALLY 12-20 MJ at 4-10 au. They state that this effectively rules BOUND OBJECT out scattering scenarios, which makes the orbit elements re- covered by Kalas et al. (2013) somewhat peculiar. 2.1 Fomalhaut b as a Jupiter-mass planet Given that five astrometric data points are available The first interpretation of the available data by Kalas et only for four different epochs separated by a few years, any al. (2008) led to the conclusion that the object may be a orbit fits with periods of hundreds of years (Kalas et al. giant planet. From stability considerations of the dust belt, 2013) suffer from high uncertainties anyway. Given the large Kalas et al. (2008) and Chiang et al. (2009) inferred that the separation between the two objects (star and presumable planet), even if bound, other planet detection techniques like mass of the object should be 6 3 MJ.