Abstract
We examine the relationship between metallicity and
J
−
K
color for 64 benchmark late-M and L dwarfs, all of which are wide companions to higher-mass stars, and six of which are new ...discoveries. We assess the correlation between the Δ(
J
−
K
) color anomaly (the difference in an object’s
J
−
K
color from the median color for field objects of the same spectral type) and the metallicity of the host star to investigate how metallicity affects ultracool photospheres. Using Spearman’s rank correlation test and Student’s t-test, the late-M dwarf (L dwarf) sample’s Δ(
J
−
K
) and metallicity show a positive correlation at the 95% (90%) confidence level. A linear fit to color anomaly as a function of metallicity finds a slope of 0.17 ± 0.07 for the late-M dwarfs and a slope of
0.20
−
0.08
+
0.07
for the L dwarfs. We also computed the Δ(
J
−
K
) versus metallicity relationship predicted by multi-metallicity model spectra generated using Drift-Phoenix. The modeled late-M dwarfs show a slope of 0.202 ± 0.03, which is close to our observational results, but the modeled L dwarfs show a slope of 0.493 ± 0.02, steeper than our observational results. Both our empirical results and the models indicate that more metal-rich objects should appear redder photometrically. We speculate that higher metallicity drives more condensate formation in these atmospheres, thus making these ultracool dwarfs appear redder.
Abstract
AF Lep A+b is a remarkable planetary system hosting a gas-giant planet that has the lowest dynamical mass among directly imaged exoplanets. We present an in-depth analysis of the atmospheric ...composition of the star and planet to probe the planet’s formation pathway. Based on new high-resolution spectroscopy of AF Lep A, we measure a uniform set of stellar parameters and elemental abundances (e.g., Fe/H = −0.27 ± 0.31 dex). The planet’s dynamical mass (
2.8
−
0.5
+
0.6
M
Jup
) and orbit are also refined using published radial velocities, relative astrometry, and absolute astrometry. We use
petitRADTRANS
to perform chemically consistent atmospheric retrievals for AF Lep b. The radiative–convective equilibrium temperature profiles are incorporated as parameterized priors on the planet’s thermal structure, leading to a robust characterization for cloudy self-luminous atmospheres. This novel approach is enabled by constraining the temperature–pressure profiles via the temperature gradient
(
d
ln
T
/
d
ln
P
)
, a departure from previous studies that solely modeled the temperature. Through multiple retrievals performed on different portions of the 0.9–4.2
μ
m spectrophotometry, along with different priors on the planet’s mass and radius, we infer that AF Lep b likely possesses a metal-enriched atmosphere (Fe/H > 1.0 dex). AF Lep b’s potential metal enrichment may be due to planetesimal accretion, giant impacts, and/or core erosion. The first process coincides with the debris disk in the system, which could be dynamically excited by AF Lep b and lead to planetesimal bombardment. Our analysis also determines
T
eff
≈ 800 K,
log
(
g
)
≈
3.7
dex, and the presence of silicate clouds and disequilibrium chemistry in the atmosphere. Straddling the L/T transition, AF Lep b is thus far the coldest exoplanet with suggested evidence of silicate clouds.
Abstract
Measurements of the C/O ratio in brown dwarfs are lacking, in part due to past models adopting solar C/O only. We have expanded the
ATMO 2020
atmosphere model grid to include nonsolar ...metallicities and C/O ratios in the T dwarf regime. We change the C/O ratio by altering either the carbon or oxygen elemental abundances, and we find that nonsolar abundances of these elements can be distinguished based on the shapes of the
H
and
K
bands. We compare these new models with medium-resolution (
R
≈ 1700), near-infrared (0.8–2.4
μ
m) Gemini Near-Infrared Spectrograph (GNIRS) spectra of three benchmark late-T dwarfs, GJ 570D, HD 3651B, and Ross 458C. We find solar C/O ratios and best-fitting parameters (
T
eff
,
log
(
g
)
,
Z
) broadly consistent with other analyses in the literature based on low-resolution (
R
∼ 100) data. The model-data discrepancies in the near-infrared spectra are consistent across all three objects. These discrepancies are alleviated when fitting the
Y
,
J
,
H
, and
K
bands individually, but the resulting best-fit parameters are inconsistent and disagree with the results from the full spectrum. By examining the model atmosphere properties we find this is due to the interplay of gravity and metallicity on H
2
–H
2
collisionally induced absorption. We therefore conclude that there are no significant issues with the molecular opacity tables used in the models at this spectral resolution. Instead, deficiencies are more likely to lie in the model assumptions regarding the thermal structures. Finally, we find a discrepancy between the GNIRS, SpeX, and other near-infrared spectra in the literature of Ross 458C, indicating potential spectroscopic variability.
Abstract
We present the latest and most precise characterization of the architecture for the ancient (≈11 Gyr) Kepler-444 system, which is composed of a K0 primary star (Kepler-444 A) hosting five ...transiting planets and a tight M-type spectroscopic binary (Kepler-444 BC) with an A–BC projected separation of 66 au. We have measured the system’s relative astrometry using the adaptive optics imaging from Keck/NIRC2 and Kepler-444 A’s radial velocities from the Hobby-Eberly Telescope and reanalyzed relative radial velocities between BC and A from Keck/HIRES. We also include the Hipparcos-Gaia astrometric acceleration and all published astrometry and radial velocities in an updated orbit analysis of BC’s barycenter. These data greatly extend the time baseline of the monitoring and lead to significant updates to BC’s barycentric orbit compared to previous work, including a larger semimajor axis (
a
=
52.2
−
2.7
+
3.3
au), a smaller eccentricity (
e
= 0.55 ± 0.05), and a more precise inclination (
i
=
85
.°
4
−
0
.°
4
+
0
.°
3
). We have also derived the first dynamical masses of B and C components. Our results suggest that Kepler-444 A’s protoplanetary disk was likely truncated by BC to a radius of ≈8 au, which resolves the previously noticed tension between Kepler-444 A’s disk mass and planet masses. Kepler-444 BC’s barycentric orbit is likely aligned with those of A’s five planets, which might be primordial or a consequence of dynamical evolution. The Kepler-444 system demonstrates that compact multiplanet systems residing in hierarchical stellar triples can form at early epochs of the universe and survive their secular evolution throughout cosmic time.
Abstract
51 Eri is well known for hosting a directly imaged giant planet and for its membership to the
β
Pictoris moving group. Using 2 minute cadence photometry from the Transiting Exoplanet Survey ...Satellite (TESS), we detect multiperiodic variability in 51 Eri that is consistent with pulsations of Gamma Doradus (
γ
Dor) stars. We identify the most significant pulsation modes (with frequencies between ∼0.5 and 3.9 cycles day
−1
and amplitudes ranging between ∼1 and 2 mmag) as dipole and quadrupole gravity modes, as well as Rossby modes, as previously observed in Kepler
γ
Dor stars. Our results demonstrate that previously reported variability attributed to stellar rotation is instead likely due to
γ
Dor pulsations. Using the mean frequency of the
ℓ
= 1 gravity modes, together with empirical trends of the Kepler
γ
Dor population, we estimate a plausible stellar core rotation period of
0.9
−
0.1
+
0.3
days for 51 Eri. We find no significant evidence for transiting companions around 51 Eri in the residual light curve. The detection of
γ
Dor pulsations presented here, together with follow-up observations and modeling, may enable the determination of an asteroseismic age for this benchmark system. Future TESS observations would allow a constraint on the stellar core rotation rate, which in turn traces the surface rotation rate, and thus would help clarify whether or not the stellar equatorial plane and orbit of 51 Eri b are coplanar.
Abstract
Giant planets grow by accreting gas through circumplanetary disks, but little is known about the timescale and mechanisms involved in the planet-assembly process because few accreting ...protoplanets have been discovered. Recent visible and infrared imaging revealed a potential accreting protoplanet within the transition disk around the young intermediate-mass Herbig Ae star, AB Aurigae (AB Aur). Additional imaging in H
α
probed for accretion and found agreement between the line-to-continuum flux ratio of the star and companion, raising the possibility that the emission source could be a compact disk feature seen in scattered starlight. We present new deep Keck/NIRC2 high-contrast imaging of AB Aur to characterize emission in Pa
β
, another accretion tracer less subject to extinction. Our narrow band observations reach a 5
σ
contrast of 9.6 mag at 0.″6, but we do not detect significant emission at the expected location of the companion, nor from other any other source in the system. Our upper limit on Pa
β
emission suggests that if AB Aur b is a protoplanet, it is not heavily accreting or accretion is stochastic and was weak during the observations.
We are conducting a proper-motion survey for young brown dwarfs in the Taurus-Auriga molecular cloud based on the Pan-STARRS1 3π Survey. Our search uses multi-band photometry and astrometry to select ...candidates, and is wider (370 deg2) and deeper (down to 3 MJup) than previous searches. We present here our search methods and spectroscopic follow-up of our high-priority candidates. Since extinction complicates spectral classification, we have developed a new approach using low-resolution (R 100) near-infrared spectra to quantify reddening-free spectral types, extinctions, and gravity classifications for mid-M to late-L ultracool dwarfs ( 100-3 MJup in Taurus). We have discovered 25 low-gravity (vl-g) and the first 11 intermediate-gravity (int-g) substellar (M6-L1) members of Taurus, constituting the largest single increase of Taurus brown dwarfs to date. We have also discovered 1 new Pleiades member and 13 new members of the Perseus OB2 association, including a candidate very wide separation (58 kau) binary. We homogeneously reclassify the spectral types and extinctions of all previously known Taurus brown dwarfs. Altogether our discoveries have thus far increased the substellar census in Taurus by 40% and added three more L-type members ( 5-10 MJup). Most notably, our discoveries reveal an older (>10 Myr) low-mass population in Taurus, in accord with recent studies of the higher-mass stellar members. The mass function appears to differ between the younger and older Taurus populations, possibly due to incompleteness of the older stellar members or different star formation processes.
Abstract
AF Lep b is a rare directly imaged exoplanet with properties consistent with core-accretion evolution models. Using its precise dynamical mass and spectroscopically inferred bolometric ...luminosity alongside the Spiegel & Burrows evolution models, I determined an initial entropy >8.7
k
B
/baryon at 3
σ
and an age of 12 ± 4 Myr for AF Lep b. Comparing this planet’s age to the isochrone age (24 ± 3 Myr) of its host association, the
β
Pictoris moving group (BPMG), suggests AF Lep b formed 12 ± 5 Myr later than its host star. Alternatively, the BPMG’s updated kinematic age (
16.3
−
2.1
+
3.4
Myr) implies that AF Lep b formed 5 ± 5 Myr after its host star’s formation, aligning more closely with protoplanetary disk dispersal timescales. The sensitivities of our findings to the planet’s mass and luminosity are discussed. AF Lep b uniquely facilitates the constraints of its initial entropy and formation epoch, paving the way for similar insights into forthcoming exoplanet discoveries.
Abstract
The orientation between a star’s spin axis and a planet’s orbital plane provides valuable information about the system’s formation and dynamical history. For non-transiting planets at wide ...separations, true stellar obliquities are challenging to measure, but lower limits on spin–orbit orientations can be determined from the difference between the inclination of the star’s rotational axis and the companion’s orbital plane (Δ
i
). We present results of a uniform analysis of rotation periods, stellar inclinations, and obliquities of cool stars (SpT ≳ F5) hosting directly imaged planets and brown dwarf companions. As part of this effort, we have acquired new
v
sin
i
*
values for 22 host stars with the high-resolution Tull spectrograph at the Harlan J. Smith telescope. Altogether our sample contains 62 host stars with rotation periods, most of which are newly measured using light curves from the Transiting Exoplanet Survey Satellite. Among these, 53 stars have inclinations determined from projected rotational and equatorial velocities, and 21 stars predominantly hosting brown dwarfs have constraints on Δ
i
. Eleven of these (52
−
11
+
10
% of the sample) are likely misaligned, while the remaining 10 host stars are consistent with spin–orbit alignment. As an ensemble, the minimum obliquity distribution between 10 and 250 au is more consistent with a mixture of isotropic and aligned systems than either extreme scenario alone—pointing to direct cloud collapse, formation within disks bearing primordial alignments and misalignments, or architectures processed by dynamical evolution. This contrasts with stars hosting directly imaged planets, which show a preference for low obliquities. These results reinforce an emerging distinction between the orbits of long-period brown dwarfs and giant planets in terms of their stellar obliquities and orbital eccentricities.
Abstract We derive the bolometric luminosities ( L bol ) of 865 field-age and 189 young ultracool dwarfs (spectral types M6–T9, including 40 new discoveries presented here) by directly integrating ...flux-calibrated optical to mid-infrared (MIR) spectral energy distributions (SEDs). The SEDs consist of low-resolution ( R ∼ 150) near-infrared (NIR; 0.8–2.5 μ m) spectra (including new spectra for 97 objects), optical photometry from the Pan-STARRS1 survey, and MIR photometry from the CatWISE2020 survey and Spitzer/IRAC. Our L bol calculations benefit from recent advances in parallaxes from Gaia, Spitzer, and UKIRT, as well as new parallaxes for 19 objects from CFHT and Pan-STARRS1 presented here. Coupling our L bol measurements with a new uniform age analysis for all objects, we estimate substellar masses, radii, surface gravities, and effective temperatures ( T eff ) using evolutionary models. We construct empirical relationships for L bol and T eff as functions of spectral type and absolute magnitude, determine bolometric corrections in optical and infrared bandpasses, and study the correlation between evolutionary model-derived surface gravities and NIR gravity classes. Our sample enables a detailed characterization of BT-Settl and ATMO 2020 atmospheric model systematics as a function of spectral type and position in the NIR color–magnitude diagram. We find the greatest discrepancies between atmospheric and evolutionary model-derived T eff (up to 800 K) and radii (up to 2.0 R Jup ) at the M/L spectral type transition boundary. With 1054 objects, this work constitutes the largest sample to date of ultracool dwarfs with determinations of their fundamental parameters.
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