Search for (sub)stellar Companions of Exoplanet Hosts by Exploring the Second ESA-Gaia Data Release

1 Introduction

Since the detection of the first planet orbiting a star other than the Sun, several thousands of these exoplanets have been discovered by various detection techniques. While the majority of stars are members of multiple star systems (Duchêne and Kraus, 2013), most of the exoplanet host stars are single stars. Nevertheless several multiple star systems hosting exoplanets, could already be revealed by previous multiplicity studies using seeing limited or high contrast AO imaging observations (see e.g. Mugrauer et al., 2014; Mugrauer and Ginski, 2015). In order to explore the effects of the presence of stellar companions on the formation process and orbital evolution of exoplanets, a survey was initiated at the Astrophysical Institute and University Observatory Jena (described in detail by Mugrauer, 2019) to identify and characterize companions of exoplanet host stars, detected in the second data release of the European Space Agency (ESA) Gaia mission (Gaia DR2 from hereon, Gaia Collaboration et al., 2018). Furthermore, in Mugrauer and Michel (2020) a comparable investigation was carried out among potential exoplanet host stars, identified by the TESS mission (Ricker et al., 2015). The study, whose results are presented here, is the third work in the context with Mugrauer (2019). The following section gives a detailed description of this study, and the detected companions and their derived properties are presented in the third section of this paper.

2 Gaia DR2 Search for (sub)stellar Companions of Exoplanet Hosts

The Gaia DR2 is based on data taken by the Gaia spacecraft in the first 22 months of its mission and contains 1.7 billion detected sources up to a limiting magnitude of G = 21 mag. For 1.3 billion sources a five parameter astrometric solution could be derived, i.e. beside their equatorial coordinates (α, δ), also the parallax π and proper motion (${\mu }_{\alpha }\cos \left(\delta \right)$, ${\mu }_{\delta }$) of these sources were determined. Furthermore, for about 88 million detected objects estimates of their G-band extinction and effective temperature are listed in the Gaia DR2, determined by the Priam algorithm, which is part of the astrophysical parameters inference system (Apsis, see Bailer-Jones et al., 2013) in the Gaia data processing.

Using Gaia DR2 data Mugrauer (2019) already explored the multiplicity of all exoplanet host stars, whose exoplanets were detected either by photometric transit observations, radial-velocity (RV), or astrometric measurements, and were listed in the Extrasolar Planets Encyclopedia¹ (EPE from hereon, Schneider et al., 2011) by mid of October 2018. The study, presented in this paper, complements this survey by investigating the multiplicity of the exoplanet hosts (stars but also brown dwarfs), whose planets were indirectly detected either via RV measurements or transit observations in the range of time between mid of October 2018 until end of September 2020, as well as all exoplanet hosts, known so far, with planets, which were directly detected by imaging observations. At the end of September 2020 the EPE lists about 4,350 exoplanets, and about 400 of them were detected around the hosts studied in this work.

(Sub)stellar Companions are expected to be located at the same distance to the Sun as the exoplanet hosts and form common proper motion pairs with them, in particular wide companions with projected separations of hundreds and thousands of au, i.e. the typical targets of this study. Hence, in order to clearly detect such companions and to prove the equidistance of these objects and the exoplanet hosts, in this study we have taken into account only Gaia DR2 sources with an accurate five parameter astrometric solution, i.e. which exhibit precise measurements of their parallax ($\pi /\sigma \left(\pi \right)>3$) and proper motion ($\mu /\sigma \left(\mu \right)>3$). Thereby, sources with negative parallaxes are neglected. As in the Gaia DR2 a parallax uncertainty of 0.7 mas is reached for faint sources down to $G=20$ mag, the survey is furthermore constrained to exoplanet hosts, which are located within a distance of 500 pc around the Sun (i.e. $\pi >2$ mas), to assure $\pi /\sigma \left(\pi \right)>3$ even for the faintest companions, detectable in this survey. This distance constraint is slightly relaxed to $\pi +3\sigma \left(\pi \right)>\approx 2$ mas, i.e. taking into account also the parallax uncertainty of the hosts. By the end of September 2020, in total 289 exoplanet hosts are listed in the EPE, which fulfill this distance constraint, and hence are selected as targets for this study. The properties of all targets are summarized in Table 1 and their histograms are illustrated in Figure 1. On average, the targets are solar like stars most frequently found within 150 pc around the Sun, which exhibit proper motions in the range between about 2 and 10,400 mas/yr, and G-band magnitudes from about 3.7 to 20.8 mag. In particular, the sub-sample of direct imaging exoplanet hosts emerges as a peak in the age distribution at young ages, as all these targets are typically younger than 0.1 Gyr, in contrast to hosts of RV and transiting exoplanets, which are older than 1 Gyr in general.

TABLE 1

TABLE 1. The properties of all targets of this study. The corresponding histograms are shown in Figure 1.

FIGURE 1

FIGURE 1. The histograms of the individual properties of all targets of this study.

The companion search radius, applied in this project around the selected targets, is limited to a maximal projected separation of 10,000 au, which guarantees that the majority of wide companions of the exoplanet hosts are detectable in this study, as described by Mugrauer (2019). This upper separation limit results in an angular search radius around the targets of $r\left[arc\sec \right]=10\pi \left[\text{mas}\right]$. Within this radius around the targets the companionship of all sources, listed in the Gaia DR2 with an accurate five parameter astrometric solution was investigated. For the verification of the equidistance of all detected sources with the associated exoplanet hosts, the difference $\text{Δ}\pi$ between their parallaxes was calculated, taking into account also the excess noise of their astrometric solutions. Common proper motion of the detected sources and the targets was checked with the precise Gaia DR2 proper motions of the exoplanet hosts ${\mu }_{\text{PH}}$ and the sources ${\mu }_{\text{Comp}}$. In addition, we have also derived for all sources the differential proper motion: ${\mu }_{\text{rel}}=\left|{\mu }_{\text{PH}}-{\mu }_{\text{comp}}\right|$, which yields the common proper motion index ($\text{cpm}-\text{index}=\left|{\mu }_{\text{PH}}+{\mu }_{\text{comp}}\right|/{\mu }_{\text{rel}}$), which characterizes the degree of common proper motion of the detected sources and the exoplanet hosts.

Following the companion identification procedure ($sig$-$\Delta \pi \le 3$ and $\text{cpm}-\text{index}\ge 3$), as defined by Mugrauer (2019) the majority of all sources ($>$99.88%), detected within the applied search radius around the targets, can clearly be excluded as companions, as they are either not located at the same distances as the exoplanet hosts and/or do not share a common proper motion with them. In contrast, for 61 detected objects their companionship with the targets could clearly be proven with their precise Gaia DR2 astrometry. For all these companions we have determined their relative astrometry to the exoplanet hosts (angular separation ρ, and position angle $\text{PA}$), as well as their projected separation $sep$, derived with their angular separation and the parallax of the targets.

The absolute G-band magnitude of all companions was derived from their apparent G-band photometry, the parallax of the associated exoplanet hosts, as well as their Apsis-Priam G-band extinction estimate, all listed in the Gaia DR2. If there was no extinction estimate given for a companion, the extinction estimate of the exoplanet host was used instead or if not available, its extinction estimate, listed in the StarHorse catalog (Anders et al., 2019). In the case that no G-band extinction is available at all it was derived from V-band extinction measurements of the exoplanet hosts, listed either in the VizieR data base² (Ochsenbein et al., 2000) or in the literature, adopting $A_G/A_V=0.77$, as described by Mugrauer (2019).

The masses and effective temperatures of all detected companions were determined from their derived absolute G-band magnitudes using the evolutionary models of (sub)stellar objects from Baraffe et al. (2015), as well as the ages of the exoplanet hosts, as listed in the EPE. Thereby, we adopt the same age for the planet hosts and their companions. We determined the masses and effective temperatures of the companions via interpolation of the model grid with the age closest to that of the exoplanet hosts. For verification of the obtained results the properties of the companions derived from their G-band magnitudes were compared with those, determined from the near-infrared photometry, taken from the 2MASS Point Source catalog (Skrutskie et al., 2006), if available. For the near-infrared extinction we have used the relations: $A_{Ks}/A_V=0.12$, $A_H/A_V=0.17$, and $A_J/A_V=0.26$, as described in Mugrauer (2019). A graphical comparison of the masses obtained from the G-band and the 2MASS photometry are shown in Figure 2. The identity is illustrated as gray dashed line in this figure. For all companions the derived masses agree well with each other, with deviations that remain below the $3\sigma$ level (the same holds also for the temperature estimates not shown here). Objects, whose masses were determined by extrapolation from the used model grids as such as those with bad quality (quality flags all but A) or contaminated 2MASS photometry were excluded in this comparison.

FIGURE 2

FIGURE 2. Comparison of the mass of the detected companions, derived from their G-band and infrared 2MASS photometry.

Eventually for all companions, which were detected in this study, we have estimated their escape velocity ${\mu }_{\text{esc}}\left[\text{mas}{\text{yr}}^{-1}\right]=2\pi \sqrt{2M{\pi }_{\text{PH}}^3/\rho }$ with their angular separation ρ and the parallax of the associated exoplanet hosts both in the unit of milli-arcsec (mas), as well as the total mass M of the system (in the unit $M_{\odot }$), i.e. the sum of the mass of the companions, derived as described above, and the mass of the associated exoplanet hosts, taken from the EPE. This estimation can be considered as an upper limit of the escape velocity as the projected separation is smaller than the physical separation of the objects.

3 Detected Companions of Exoplanet Hosts

The Gaia astro- and photometry of all exoplanet hosts and their companions, detected in this study, are listed in Table 2. The derived properties of the companions are summarized in Table 3–5. In all tables the exoplanet host systems or the companions are sorted by their right ascension. The used identifier of the targets corresponds either to the one used in the EPE or is a slightly abbreviated version of it. In contrast to the planet definition used by the EPE, in which substellar objects below 60 $M_{\text{Jup}}$ are defined as exoplanets, we follow here the planet definition based on the deuterium burning limit (as described e.g. by Basri, 2000), i.e. all substellar objects below 13 $M_{\text{Jup}}$ are classified as exoplanets, while more massive objects below the substellar/stellar mass limit (at about 0.072 M_⊙ for solar metallicity) as brown dwarfs, respectively. Thereby the given masses of the exoplanets, detected by radial velocity measurements, correspond to minimum-masses ($M\sin \left(i\right)$) due to the unknown orbital inclination, while masses of direct imaging planets are usually derived from their spectrophotometry with evolutionary models.

TABLE 2

TABLE 2. Gaia astro- and photometry of all exoplanet hosts and their companions, detected in this study.

TABLE 3

TABLE 3. The relative astrometry and WDS status of all detected companions.

TABLE 4

TABLE 4. The equatorial coordinates and derived physical properties of all detected companions.

TABLE 5

TABLE 5. List of all detected companions, whose differential proper motion ${\mu }_{\text{rel}}$ exceeds their estimated escape velocity ${\mu }_{\text{esc}}$.

In Table 2 for each exoplanet host and its detected co-moving companion(s) their Gaia DR2 parallax π, proper motion in right ascension and declination (${\mu }_{\alpha }\cos \left(\delta \right)$ and ${\mu }_{\delta }$), astrometric excess noise (epsi) with its significance (sig-epsi), apparent G-band magnitude, as well as the used Apsis-Priam G-band extinction estimate $A_G$ are listed. In the case that the G-band extinction was taken from the StarHorse catalog this is indicated with the SHC flag, or with the $\mathrm{✠}$ flag if the G-band extinction was derived from V-band extinction measurements, either listed in the VizieR database or from the literature. In this table the exoplanet hosts are indicated with *, and known spectroscopic binary stars among them with (SB).

Table 3 lists for each detected companion its angular separation (ρ) and position angle ($\text{PA}$) to the associated exoplanet host, which were determined with the Gaia DR2 astrometry of the objects for the (Gaia reference) epoch 2015.5. The relative astrometry of the companions exhibits an uncertainty on average of 0.3 mas in angular separation, and 0.002° in position angle, respectively. In the following columns of Table 3 we list the parallax difference ($\text{Δ}\pi$) with its significance (in brackets calculated by taking into account also the Gaia astrometric excess noise³) between the exoplanet hosts and their detected companions, their differential proper motion ${\mu }_{\text{rel}}$ with its significance, and the cpm-index of all systems. The precise Gaia DR2 astrometry proves the equidistance (sig-$\Delta \pi <2.3\sigma$, average value of $0.5\sigma$) and common proper motion ($\text{cpm}-\text{index}>6$, $\text{average}\text{cpm}-\text{index}=118$) of the exoplanet hosts and their companions. If these companions are not listed yet as companion (-candidates) in the Washington Double Star Catalog (WDS from hereon, Mason et al., 2001) this is indicated with the ★ flag in last column of Table 3. In the case that the companion is not listed in the WDS but was reported in literature before, additional information is given in the notes of this table.

In Table 4 beside the equatorial coordinates (α, δ both for epoch 2015.5) of all detected companions, their derived absolute G-band magnitude $M_G$, projected separation $sep$ to the associated exoplanet host (relative uncertainty about 1%, on average), mass, and effective temperature $T_{\text{eff}}$ are summarized. The flags listed in the last column of this table are defined as follows:

PRI: An Apsis-Priam temperature estimate is available for the detected companion, which could be compared with the effective temperature of the companion, derived from its absolute G-band photometry using the Baraffe et al. (2015) models.

2MA: The companion is listed in the 2MASS Point Source catalog.

BPRP: The $G_{\text{BP}}-G_{\text{RP}}$ color of the exoplanet host and of the detected companion is listed in the Gaia DR2, hence a color comparison was feasible.

EXT: Because of its brightness the companion exceeds the magnitude range of the Baraffe et al. (2015) evolutionary models. Therefore, the properties of the companion were estimated via extrapolation from the two brightest sources of the used model isochrone.

WD: The detected companion is a white dwarf.

BD: The detected companion is a brown dwarf.

Finally, in Table 5 we summarize all those detected companions, whose differential proper motion ${\mu }_{\text{rel}}$ significantly exceeds their expected escape velocity ${\mu }_{\text{rel}}$. Companions, which are already known to be members of hierarchical triple star systems, are indicated with the flag *** in the last column of this table.

Among all 289 targets, whose multiplicity was investigated in the study, whose results are presented in this paper, 41 binary and five hierarchical triple star systems with exoplanets were identified. This yields a multiplicity rate of the targets of 16 ± 2%, very well consistent with the multiplicity rate of exoplanet host stars of 15 ± 1%, reported before by Mugrauer (2019). This is as expected, as the sensitivities of the two surveys should agree well with each other, as the brightness and mass of their targets match, and the distance of the targets from this survey is on average about 40% smaller than that of the targets from Mugrauer (2019), resulting in a reduction in the distance modulus of only about 1 mag. In total, 61 companions (48 stars and 13 brown dwarfs) could be detected in the Gaia DR2 around the targets. The detected substellar companions are all listed as exoplanets in the EPE. The cumulative distribution functions of the derived properties (projected separation, mass and effective temperature) of theses companions, are illustrated in Figures 3–5. The separation-mass diagram of the companions is shown in Figure 6. As described above, the accurate Gaia DR2 astrometry proves the equidistance and common proper motion of all detected companions with the associated exoplanet hosts, and for the majority of these companions their differential proper motion to the exoplanet hosts is slower than their estimated escape velocity, facts that are expected for gravitationally bound systems. In contrast, the differential proper motion of the companions, which are listed in Table 5, exceeds their estimated escape velocity, possibly indicating a higher degree of multiplicity⁴. Indeed, one of these companions (51 Eri BC) is already known to be a close binary itself. The remaining two companions and their primaries are promising targets for follow-up observations to check their multiplicity status e.g. with high contrast AO imaging observations.

FIGURE 3

FIGURE 3. The cumulative distribution function of the projected separation (sep) of all detected companions to the associated exoplanet hosts.

FIGURE 4

FIGURE 4. The cumulative distribution function of the mass of all companions, detected in this study.

FIGURE 5

FIGURE 5. The cumulative distribution function of the effective temperature of all detected companions.

FIGURE 6

FIGURE 6. The mass of all companions, detected in this study, plotted over their projected separation (sep) to the associated exoplanet hosts. The white dwarf companion HIP 38594 B, is illustrated as open circle.

All detected companions exhibit projected separations to the associated exoplanet hosts in the range between 52 and 9,555 au (average separation of about 2,310 au). The highest companion frequency is found at projected separations between about 240 and 400 au and half of all companions are located at projected separations below about 1,240 au. The closest detected companion is K2-288 A, which is separated from the exoplanet host star K2-288 B by 52 au, and it is the only companion identified in this study within a projected separation of 100 au. The masses of the companions range between 0.016 and 1.66 $M_{\odot }$ (average mass of 0.36 $M_{\odot }$) and companions are found most frequently in the substellar mass regime between 0.016 up to 0.033 $M_{\odot }$, while more massive companions are detected at a lower but constant frequency up to about 0.7 $M_{\odot }$, and only about 10% of all the detected companions exhibit masses larger than 0.7 $M_{\odot }$. The companions exhibit effective temperatures in the range between about 1850 and 6350 K (average temperature of about 3400 K), which corresponds to spectral types of L3 to F6 (M3, on average), according to the $T_{\text{eff}}-SpT$ relation⁵ from (Pecaut and Mamajek, 2013).

In general the effective temperature of the detected companions, determined with their derived absolute G-band magnitude, using the evolutionary Baraffe et al. (2015) models, agree well with their Gaia DR2 Apsis-Priam temperature estimate (if available) with a characteristic deviation of about ±350 K, consistent with the typical uncertainty of the different temperature estimates, which is in the order of about 330 K. Only in the case of HIP 38594 B the temperature estimate, based on the absolute G-band photometry of the companion significantly deviates by more than 2300 K from its Apsis-Priam temperature estimate, which is also about 900 K higher than the one of the associated exoplanet host star HIP 38594 A. Furthermore, the companion appears bluer ($\Delta \left(G_{\text{BP}}-G_{\text{RP}}\right)=-0.669\pm 0.004$ mag) than its primary although it is about 7 mag fainter in the G-band than the exoplanet host star. The intrinsic faintness and high temperature of HIP 38594 B clearly indicates that this companion is a white dwarf. This conclusion is consistent with the results of Subasavage et al. (2008), who have already classified the companion spectroscopically as a white dwarf, and have denote it as WD 0751-252. For this degenerated companion we adopt here a mass of about 0.6 $M_{\odot }$.

In Figure 7 the G-band magnitude difference of all detected companions to the associated exoplanet hosts is plotted vs. their angular separation. For comparison we show as dashed line in this figure the estimate of the Gaia detection limit, reported by Mugrauer (2019) which was further constrained by Mugrauer and Michel (2020). Companions of exoplanet hosts brighter than 12.8 mag are plotted as open circles those of hosts, which are fainter than that magnitude limit, as filled black circles, respectively. A magnitude difference of about 5 mag is reached at an angular separation of about two arcsec, consistent with the estimate of the Gaia detection limit, determined by Mugrauer (2019). Only two companions significantly exceed the limit estimate, namely K2-288 A at an angular separation of about 0.8 arcsec with $\text{Δ}G\sim 1.2$ mag and HIP 77900 B, at 22.3 arcsec with $\Delta G\sim 13.5$ mag. While K2-288 A is a companion of a target fainter than $G=12.8$ mag for which Gaia reaches a higher sensitivity at angular separations slightly below one arcsec (up to 3 mag, as described by Mugrauer and Michel, 2020) the detection of HIP 77900 B indicates that the given limit estimate might be too conservative at angular separations beyond about 20 arcsec.

FIGURE 7

FIGURE 7. The G-band magnitude difference of all detected companions, plotted over their angular separation to the associated exoplanet hosts.

4 Summary and Outlook

The study, presented here, is a continuation of a survey, which was initiated at the Astrophysical Institute and University Observatory Jena, to investigate the multiplicity status of exoplanet hosts and to characterize the properties of their detected (sub)stellar companions, using accurate Gaia astro- and photometry. In this paper the multiplicity of 289 exoplanet hosts was explored and (sub)stellar companions were detected around 60 targets. The companionship of these objects with the exoplanet hosts could be proven with their accurate Gaia DR2 astrometry (equidistance, common proper motion, and differential proper motion smaller than the expected escape velocity). The mass and effective temperature of all companions were determined with their derived absolute G-band photometry and the Baraffe et al. (2015) evolutionary models of (sub)stellar objects. In total, 61 companions (beside 48 stellar companions, among them the white dwarf HIP 38594 B, also 13 brown dwarfs) were detected in this project, and 14 of these objects are neither listed in the WDS as companion (-candidate)s of the targets nor were described in the literature before. A total of 41 binary and five triple star systems with exoplanets, were identified in this study, yielding a multiplicity rate of the targets of about 16%, which is very well consistent with the multiplicity rate of exoplanet host stars, reported by Mugrauer (2019). Following the standard procedure of our survey, all detected companions and their derived properties will be made available online in the VizieR database. The survey, whose latest results are presented here, is an ongoing project as more and more exoplanet hosts are detected by different planet detection methods, whose multiplicity status needs to be investigated. Furthermore, there are sources, listed in the Gaia DR2, within the applied search radius around the targets, which still lack a five parameter astrometric solution. Hence, further companions of the exoplanet hosts, investigated here, should exist, whose companionship can be proven with accurate astrometric measurements, provided by future data releases of the ESA-Gaia mission, e.g. the Gaia EDR3, planed to be published end of 2020.

The results of this survey, which is mainly sensitive for wide companions of exoplanet hosts, combined with those of our currently ongoing large high contrast imaging surveys (sensitive for close companions), carried out with SPHERE/VLT and AstraLux/CAHA (first results are already published e.g. by Ginski et al., 2020) will yield a complete characterization of the multiplicity status of the observed targets. This will eventually allow to draw conclutions on the impact of the stellar multiplicity on the formation process of planets and the evolution of their orbits.

Data Availability Statement

Publicly available datasets were analyzed in this study. This data can be found here: https://vizier.u-strasbg.fr/viz-bin/VizieR-3?-source=I/345/gaia2&-out.max=50&-out.form=HTML%20Table&-out.add=_r&-out.add=_RAJ,_DEJ&-sort=_r&-oc.form=sexa.

Author Contributions

K-UM and MM have worked together on the data analysis and its publication.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank the two anonymous referees for their helpful and constructive comments on the manuscript. We made use of data from: (1) the Simbad and VizieR databases, both operated at CDS in Strasbourg, France. (2) the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. (3) the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

Footnotes

¹Online available at: http://exoplanet.eu/

²Online available at: https://vizier.u-strasbg.fr/

³The astrometric excess noise is conservatively considered here as additional parallax uncertainty of the source.

⁴Additional close companions either of the exoplanet hosts or of the companions force these objects on close orbits with high orbital velocities around a common barycenter that could induce the observed high differential velocities.

⁵Online available at: http://www.pas.rochester.edu/∼emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt

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