Zodiacal exoplanets in time (ZEIT) XII: a directly imaged planetary-mass companion to a young Taurus M dwarf star

Abstract

We report the discovery of a resolved (0″.9) substellar companion to a member of the 1-5 Myr Taurus star-forming region. The host star (2M0437) is a single mid-M type (Teff≈3100 K) dwarf with a position, space motion, and color-magnitude that support Taurus membership, and possible affiliation with a ∼2.5 Myr-old sub-group. A comparison with stellar models suggests a 2-5 Myr age and a mass of 0.15-0.18M⊙. Although K2 detected quasi-periodic dimming from close-in circumstellar dust, the star lacks detectable excess infrared emission from a circumstellar disk and its Hα emission is not commensurate with accretion. Astrometry based on three years of AO imaging shows that the companion (2M0437b) is co-moving, while photometry of two other sources at larger separation indicates they are likely heavily-reddened background stars. A comparison of the luminosity of 2M0437b with models suggests a mass of 3-5 MJUP , well below the deuterium burning limit, and an effective temperature of 1400-1500 K, characteristic of a late L spectral type. The H-K color is redder than the typical L dwarf, but comparable to other directly detected young planets, e.g. those around HR 8799. The discovery of a super-Jupiter around a very young, very low mass star challenges models of planet formation by either core accretion (which requires time) or disk instability (which requires mass). We also detected a second, co-moving, widely-separated (75″) object which appears to be a heavily-extincted star. This is certainly a fellow member of this Taurus sub-group and statistically likely to be a bound companion.

A Census of the Taurus Star-forming Region and Neighboring Associations with Gaia*

“…In direct imaging, Gaidos et al (2022) resolved a planetarymass companion at a separation of 0 9 from 2MASS J04372171+2651014 (hereafter 2M0437), which was identified as a Taurus member by Esplin & Luhman (2017) and is classified in this work as a member of the Taurus group associated with HD 28354 (6 Myr; Section 5.3). To search for additional components of this system, Gaidos et al (2022) checked Gaia DR3 for sources within an angular distance of 1000″ that are comoving with the primary.…”

“…In direct imaging, Gaidos et al (2022) resolved a planetarymass companion at a separation of 0 9 from 2MASS J04372171+2651014 (hereafter 2M0437), which was identified as a Taurus member by Esplin & Luhman (2017) and is classified in this work as a member of the Taurus group associated with HD 28354 (6 Myr; Section 5.3). To search for additional components of this system, Gaidos et al (2022) checked Gaia DR3 for sources within an angular distance of 1000″ that are comoving with the primary. They found one possible companion, 2MASS J04372631+2651438 (hereafter 2M043726), which has a separation of 75″, a similar proper motion to 2M0437 (differing by ∼2 mas yr −1 ), and an uncertain parallax (8.3 ± 2.8 mas) that is consistent with that of 2M0437 (7.81 ± 0.03 mas).…”

“…Thus, 2M043726 is unlikely to be a companion to 2M0437 or a member of its Taurus group based on those distance estimates. For Gaia DR3 sources within 3°from 2M0437 that are not known Taurus members, the average number of stars with proper motions similar to that of 2M0437 (Δμ  3 mas yr −1 ) and unconstrained values of parallax corresponds to ∼1.7 for an area equal to the field searched by Gaidos et al (2022), so it is not surprising that their search would find a field star that appeared to be comoving with 2M0437. Meanwhile, the proposal that 2M043726 has an edge-on disk can be tested with mid-IR photometry from the Spitzer Space Telescope.…”

“…They found one possible companion, 2MASS J04372631+2651438 (hereafter 2M043726), which has a separation of 75″, a similar proper motion to 2M0437 (differing by ∼2 mas yr −1 ), and an uncertain parallax (8.3 ± 2.8 mas) that is consistent with that of 2M0437 (7.81 ± 0.03 mas). Based on those data, Gaidos et al (2022) concluded that 2M043726 is a Taurus member and is likely to be a companion to 2M0437 (as opposed to an unrelated member). They measured a spectral type of K5-M3 for 2M043726 using near-IR spectroscopy, which is earlier than expected for a Taurus member given its faint photometry (i.e., it is fainter than expected for a Taurus member near that spectral type).…”

“…This is reflected in the fact that 2M043726 appears well below the sequence of Taurus members when placed in a diagram of M G RP versus G − G RP using the distance of 2M0437. To explain the anomalously faint photometry of 2M043726, Gaidos et al (2022) proposed that it is occulted by an edge-on disk. They concluded that the available mid-IR images are inadequate to verify that a disk is present.…”