Direct Imaging Discovery of a `Super-Jupiter' Around the late B-Type Star Kappa And

Introduction

Stellar mass is emerging as one of the most important parameters in determining the properties of planetary systems, along with stellar metallicity. Radial velocity surveys have indicated that the frequency of giant planets increases with the mass of the stellar host (Johnson et al., 2010), and many of the roughly dozen exoplanets that have been directly imaged so far have had A-type stellar hosts (e.g., Marois et al., 2008; Lagrange et al., 2009), despite such large stars being in the small minority of surveyed targets. These results have motivated targeted imaging surveys for planets around massive stars (e.g., Janson et al., 2011b). The increase in planet frequency with host star mass can be readily explained theoretically, through the consideration that more massive stars are likely to have more massive disks (Mordasini et al., 2012). On the other hand, massive stars also feature an increased intensity of high-energy radiation, which may significantly shorten the disk lifetime due to photoevaporation, and thus decrease the time window in which giant planets are allowed to form. This raises the question whether there is a maximum stellar mass above which giant planets are unable to form.

In this Letter, we report the discovery of a $\sim$12.8 $M_{\rm Jup}$ companion to the $\sim$2.5 $M_{\rm\odot}$ star $\kappa$ And, the most massive star to host a directly detected companion below or near the planetary mass limit. In the following, we describe the acquisition, reduction and analysis of the data used for detection, confirmation, and basic characterization of the companion, $\kappa$ And b.

Observations and Data Reduction

Observations of the $\kappa$ And system extended over a period of seven months (January - July 2012) and were carried out on Subaru Telescope. $JHK$ images were collected with AO188 (Hayano et al., 2010) coupled with HiCIAO (Hodapp et al., 2008). $L^{\prime}$ measurements were carried out with AO188 coupled with the Infrared Camera and Spectrograph (IRCS; Tokunaga et al., 1998). Figure 1 displays the multi-wavelength images of the newly discovered companion. Table 1 provides a summary of the experimental measurements, as well as relevant values from the literature. Figure 2 shows observed astrometric positions of $\kappa$ And b as compared with expected motion of an unrelated background star. The sub-sections below describe the observations in greater detail.

We first detected $\kappa$ And b using AO188 coupled with HiCIAO on Subaru Telescope on January 1, 2012, as part of the SEEDS survey (Tamura, 2009). The observations used a 20′′ $\times$ 20′′ field of view, 9.5 mas pixels, and an opaque 0$\farcs$6-diameter coronagraphic mask, which helped keep the saturation radius $<$ $0\farcs 5$. The images were taken in the near infrared ($H$-band, 1.6 $\mu$m), where young substellar objects are expected to be bright with thermal radiation (Baraffe et al., 2003). Pupil tracking was used to enable angular differential imaging (ADI; Marois et al., 2006).

To optimize the ADI technique, we first reduced the data using a locally optimized combination of images algorithm (LOCI; Lafrenière et al., 2007). HiCIAO observations of M5, combined with distortion-corrected images obtained with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, enabled accurate pixel scale calibration to within 0.2%; the ACS astrometric calibration was based on van der Marel et al. (2007). Figure 1 (left and middle) presents a JHK false-color image and corresponding signal-to-noise (S/N) map after the ADI/LOCI data reduction.

Given the relatively high S/N ratios and the known difficulties in quantifying the impact of LOCI on planet photometry and astrometry, we also performed a classical ADI reduction (Marois et al., 2006) with mean-based point-spread function (PSF) estimation and frame co-adding. Unsharp masking on the spatial scale of 35 pixels ($\approx$ 7 FWHM) was applied to the final image to flatten the residual background. The planet signal was recovered with S/N ratios comparable (within 10%) to the LOCI reduction for all the July data sets. For the somewhat lower quality January data, the measured S/N reduced from about $23\,\sigma$ to $7\,\sigma$.

To achieve unbiased photometry and astrometry, we extracted the combined $\kappa$ And PSF (S/N $>$ 1000) from the neutral density images, and placed it on an empty image frame at the location of $\kappa$ And b. Applying the same unsharp masking and ADI reduction to this data as we did for the science data, we simulated the parallactic angle evolution, as recorded in the science frames. The resulting processed PSF acted as the photometric and astrometric reference for $\kappa$ And b. The only non-linear step in this process was the median-based unsharp masking, but the large spatial scale ($\approx$ 7 FWHM) ensured that subtraction effects were minimal.

We calibrated the astrometry by cross-correlating the $\kappa$ And b signal with the processed calibration PSF. We estimated the uncertainty in the $\kappa$ And b center to be $\mathit{FWHM}/(S/N)$, following Cameron et al. (2008). The uncertainties in the final relative astrometry were dominated by our ability to determine the host star center, which was carried out through Moffat fitting of each individual exposure. We conservatively estimated the uncertainty of the Moffat fit at 0.75 pixels (7 mas). For confirmation, we applied Moffat fitting and peak fitting to unsaturated data of $\kappa$ And and found that the methods agreed at the 0.5 $\sigma$ level. The photometric uncertainties were calculated as a combination of (1) representative noise in an annulus, centered on the host star, with a radius equal to the companion, (2) photometric variability in the neutral density calibration images, which yielded effective accuracies of 7–11% for the combined datasets, and (3) uncertainties in the JHK magnitudes of $\kappa$ And.

On July 28, 2012, we followed the JHK observations with $L^{\prime}$-band observations (3.8 $\mu$m; 50 exposures of 30 s) using AO188 coupled with the Infrared Camera and Spectrograph on Subaru Telescope. We employed a 10$\farcs$5 $\times$ 10$\farcs$5 field of view, 20.6 mas pixel scale and no coronagraph. The host star saturated out to $\sim$ $0\farcs 1$. The dithered observations, carried out in ADI mode, were divided into two identical sequences bracketing observations of the star HR 8799, which provided the photometric calibration (Marois et al., 2008). Observations of a third star, S810-A, were collected before the science observations as a secondary calibration check (Leggett et al., 2003).

We sky-subtracted each image using a median combination of frames taken at the other dither positions. To help maximize the high-contrast sensitivity, we processed the data using an “adaptive” LOCI process (A-LOCI; Currie et al. 2012). We also employed a moving pixel mask, where the LOCI algorithm is prevented from using, in PSF construction, pixels lying within the subtraction zone (see Lafrenière et al. 2007 for details). Figure 1 (right) shows the final image.

To quantify the $\kappa$ And b throughput, we used fake point sources added to the image and processed with the same algorithm settings. As an additional check on our flux calibration, we determined the relative brightness between the HR 8799 bcd planets (all detected at S/N $>$ 7–10) using identical procedures, and confirmed its agreement with published values (Currie et al., 2011). The independent calibrations all yielded self-consistent results, ensuring confidence in the 22 $\sigma$ detection of $\kappa$ And b in $L^{\prime}$. As a final check, we re-processed the $L^{\prime}$-band data using a more classical ADI method, similar to that described for the $JHK$ data set, and achieved consistent results. While the July $L^{\prime}$ astrometry was consistent with the July JHK results, we refrained from including it in our proper motion analysis, due to our possession of poorer-quality astrometric calibration.

Host Star Properties

$\kappa$ And is a B9 IV star (Wu et al., 2011) located at a distance of 52.0 pc (Perryman et al., 1997). Fitzpatrick & Massa (2005) report a temperature of 11,400 $\pm$ 100 K with a sub-solar metallicity of $[$Fe/H$]$ = $-$0.36 $\pm$ 0.09, while independent measurements by Wu et al. (2011) report values of 10,700 $\pm$ 300 K and $-$0.32 $\pm$ 0.15. Given the star’s spectral classification, the measured low metallicity is likely due to the details of the star’s accretion and atmospheric physics, as opposed to a true, initial, low metallicity (Gray & Corbally, 2002). We estimate a mass of 2.4–2.5 $M_{\rm\odot}$ using the published temperature and evolutionary tracks from Ekström et al. (2012). Table 1 summarizes the host star properties.

Zuckerman et al. (2011) proposed $\kappa$ And to be a member of the $\sim$30 Myr old Columba association. To further investigate $\kappa$ And’s likely membership in Columba we: (1) independently calculated its Galactic kinematics from astrometry available in the literature (Perryman et al., 1997; Zuckerman et al., 2011) and compared these to the young local associations reported in Torres et al. (2008), and (2) calculated its membership probability in these associations using the Bayesian methods of Malo et al. (2012). Our analyses showed that the star’s kinematics imply a $>$95% probability of the star being part of the Columba association.

As an additional check, we compared the $\kappa$ And$B-V$ color and absolute $V$ magnitude (Perryman et al., 1997; van Leeuwen, 2009) with members of clusters and associations with ages ranging from $\sim$15–700 Myr. These include Lower Centaurus Crux, $\alpha$ Per, Pleiades, Coma Ber, Hyades, Praesepe, and young local associations (Torres et al., 2008; van Leeuwen, 2009). The color–magnitude analysis showed that $\kappa$ And is consistent with other early-type stars having ages $\sim$20–120 Myr. The results of our analyses are consistent with the conclusions reported in Zuckerman et al. (2011); $\kappa$ And’s age range and kinematics suggest it is a member in the Columba association. We therefore adopt a system age of 30${}^{+20}_{-10}$ Myr (following Marois et al. 2010) for all subsequent analyses.

Results

Located 52.0 pc from the Sun, $\kappa$ And exhibits proper motion of 83.5 mas/yr (Perryman et al., 1997), enabling an effective test to distinguish bound companions from unrelated background stars. The $\kappa$ And proper and parallactic motion translate to 76 mas ($\sim$8 HiCIAO pixels) of net movement over the 6 month period between epochs. As shown in Figure 2, the companion exhibits common proper motion with the host star, and deviates from expected background star motion by $7\,\sigma$. In addition to this $7\,\sigma$ deviation in the magnitude of motion, the observed direction of motion and scatter in astrometry are completely inconsistent with that of a background star.

2 Physical Properties of κ𝜅\kappa And b

Figure 3 shows that the $\kappa$ And b colors are most consistent with cloudy L dwarfs and overlap with several other benchmark exoplanets and low-mass companions, including HR 8799 bcd, AB Pic b, and 1RXS1609 b. Figure 4 compares $\kappa$ And b colors and absolute magnitudes with DUSTY and COND evolutionary tracks (Baraffe et al., 2003; Chabrier et al., 2000), as well as low-mass companions around HR 8799 and AB Pic. The plots show $\kappa$ And b as well situated between HR 8799 cde and AB Pic b. Its infrared colors are slightly bluer than those of typical field L dwarfs, possibly indicating a low surface gravity (Cruz et al., 2009). However, improved photometry is required to confirm whether this color deviation is real.

While the DUSTY models are likely the more relevant, we estimate a possible alternative mass using the COND evolutionary tracks. In this scenario, we determine a mass of 11.5${}^{+2.4}_{-1.2}$ $M_{\rm Jup}$ and a temperature of 1640${}^{+40}_{-20}$ K. More recent evolutionary models by Spiegel & Burrows (2012) offer alternative “Warm Start” scenarios that consider formation with lower levels of initial entropy. While these models do not consider combinations of mass and temperature similar to that of $\kappa$ And b, they do predict generally higher masses than that of the DUSTY and COND models. In the case of $\kappa$ And b, such models place the most probable mass at a value above the typical deuterium burning limit. While we currently adopt a nominal mass estimate of 12.8${}^{+2.0}_{-1.0}$ $M_{\rm Jup}$ for the analyses in this discovery paper (based on the DUSTY models), we defer a deeper investigation of companion mass for a follow-up paper, where we will focus on a more thorough comparison of multiband photometry with synthetic spectra.

3 Orbital Properties of κ𝜅\kappa And b

We estimate the semimajor axis of $\kappa$ And b from its observed separation. Assuming a uniform eccentricity distribution of 0 $<$ e $<$ 1, and random viewing angles, Dupuy et al. (2010) compute a median correction factor between projected separation and semimajor axis of 1.1${}^{+0.91}_{-0.36}$. Using this relation, we derive a semimajor axis of 61${}^{+50}_{-20}$ AU based on its projected separation of 55.2 AU (1$\farcs$07) in January 2012.

4 Possible Secondary Companions

Discussion

$\kappa$ And is the most massive star to host a directly imaged planet, or brown dwarf near the deuterium burning boundary. The mass ratio between $\kappa$ And b and its host is $\sim$0.5%, similar to the $\sim$0.4% ratios of the $\beta$ Pic and HR 8799 planets (Lagrange et al., 2009; Marois et al., 2008). In comparison, this value is noticeably smaller than those of reported directly imaged planets around 1RXS 1609 (Lafrenière et al., 2008) and 2M 1207 (Chauvin et al., 2004). The projected separation of $\kappa$ And b is also intermediate between the two outer planets in HR 8799. The similarities between $\kappa$ And b, $\beta$ Pic, and HR 8799 could imply a similar formation mechanism, which may be distinct from recently discovered brown dwarf companions of approximately an order of magnitude larger mass ratios (e.g., GJ 758 B; Thalmann et al., 2009) or semimajor axes (e.g., HIP 78530 B; Lafrenière et al., 2011). Strengthening the possibility of a planet-like formation for Kap And b, theoretical models (e.g. Rafikov, 2011) show that, for a minimum mass solar nebula, the region of the primordial disk where core accretion formation of giant planets can occur overlaps with the separation range of $\kappa$ And b. Furthermore, this formation mechanism may be significantly enhanced for a star as massive as $\kappa$ And, assuming it had a correspondingly more massive protoplanetary disk. Further studies will be needed to more stringently constrain the population properties of planets and brown dwarfs on intermediate and wide orbits.

The best-fit mass of $\kappa$ And b lies just below the deuterium burning limit according to conventional evolutionary models, but may be above this limit if initial entropy at formation is lower than such models assume (Spiegel & Burrows, 2012). This leads to an ambiguity in whether the companion can be classified as an “exoplanet” by the present IAU definition. Such a classification scheme can however be misleading, given that $\kappa$ And b may well have formed in the same way as previously imaged planets, regardless of whether its mass falls just below or above this limit. Indeed, radial velocity studies have shown that massive stars tend to have massive planets, sometimes with companions having masses above the deuterium burning limit (e.g. Lovis & Mayor, 2007) and which apparently form a high-mass tail of a lower-mass planetary population (e.g. Hekker et al. 2008). On the other hand, formation history can be difficult to assess in individual cases. In order to avoid these uncertainties, we simply classify $\kappa$ And b as a ‘Super-Jupiter’, which we take to mean a group of objects that includes the previously imaged planets around HR 8799 and $\beta$ Pic as well as the most massive radial velocity planets, and which one might suspect have formed in a similar way to lower-mass exoplanets, but for which this has not necessarily been unambiguously demonstrated. This suggested class includes substellar objects with masses at or moderately above the deuterium burning limit, but excludes objects with orbital separations well beyond a typical disk truncation radius, or systems with mass ratios more indicative of a binary-like formation.

References