This paper uses the multi-epoch astrometry from the Wide-field Infrared Survey Explorer (WISE) to demonstrate a method to measure proper motions and trigonometric parallaxes with precisions of ∼4 mas ...yr−1 and ∼7 mas, respectively, for low-mass stars and brown dwarfs. This method relies on WISE single exposures (Level 1b frames) and a Markov Chain Monte Carlo method. The limitations of Gaia in observing very low-mass stars and brown dwarfs are discussed, and it is shown that WISE will be able to measure astrometry past the 95% completeness limit and magnitude limit of Gaia (L, T, and Y dwarfs fainter than G 19 and G = 21, respectively). This method is applied to WISE data of 20 nearby ( 17 pc) dwarfs with spectral types between M6-Y2 and previously measured trigonometric parallaxes. Also provided are WISE astrometric measurements for 23 additional low-mass dwarfs with spectral types between M6-T7 and estimated photometric distances <17 pc. Only nine of these objects contain parallaxes within Gaia Data Release 2.
The Orion Nebula Cluster (ONC) is the nearest site of ongoing massive star formation, which allows us to study the kinematics and dynamics of the region in detail and constrain star formation ...theories. Using HST ACS/WFPC2/WFC3IR and Keck II NIRC2 data, we have measured the proper motions of 701 stars within an ∼6′ × 6′ field of view around the center of the ONC. We have found more than 10 escaping star candidates, concentrated predominantly at the core of the cluster. The proper motions of the bound stars are consistent with a normal distribution, albeit elongated north-south along the Orion filament, with proper-motion dispersions of mas yr−1 or intrinsic velocity dispersions of km s−1 assuming a distance of 400 pc to the ONC. The cluster shows no evidence for tangential-to-radial anisotropy. Our velocity dispersion profile agrees with the prediction from the observed stellar + gas density profile from Da Rio et al., indicating that the ONC is in virial equilibrium. This finding suggests that the cluster was formed with a low star formation efficiency per dynamical timescale based on comparisons with current star formation theories. Our survey also recovered high-velocity IR sources BN, x and n in the BN/KL region. The estimated location of the first two sources ∼500 yr ago agrees with that of the radio source I, consistent with their proposed common origin from a multistellar disintegration. However, source n appears to have a small proper motion and is unlikely to have been involved in the event.
Abstract
We report the detection of large-amplitude, rapid radial velocity (RV) variations and line-splitting in high-resolution Keck/NIRSPEC spectra of the M9 dwarf LP 413-53. We attribute these ...features to binary motion. Analyzing data spanning 15 yr, we infer a preliminary orbital period of 0.7106156 ± 0.0000002 days, an eccentricity of 0.0088 ± 0.0017, a primary RV semiamplitude of 23.70 ± 0.05 km s
−1
, and a secondary RV semiamplitude of 28.41 ± 0.06 km s
−1
, implying a system mass ratio
M
secondary
/
M
primary
= 0.8340 ± 0.0017. These measurements identify LP 413-53 as the shortest-period ultracool binary discovered to date, and one of the smallest separation main-sequence binaries known. The position and velocity of the system rule out previously reported membership in the Hyades Moving Group, and indicate that this is likely a pair of evolved (age ≳1 Gyr), very-low-mass stars. Assuming masses consistent with evolved late-M and L dwarfs, we estimate an orbital separation of 0.0081–0.0084 au or 17–19 stellar radii, and an orbital inclination angle of 24°, making it unlikely that this system exhibits eclipse events. The larger radii of these stars at young ages would have put them near contact at the system’s current separation, and we speculate that this system has undergone dynamical evolution, either through orbital angular momentum loss or ejection of a third component followed by tidal circularization. While further observations are needed to fully constrain the orbital and physical parameters of LP 413-53, this ultra-short-period ultracool dwarf binary system serves as a new test bed for formation and dynamical evolution models of very-low-mass multiples.
Abstract
The kinematics and dynamics of stellar and substellar populations within young, still-forming clusters provide valuable information for constraining theories of formation mechanisms. Using ...Keck II NIRSPEC+AO data, we have measured radial velocities for 56 low-mass sources within 4′ of the core of the Orion Nebula Cluster (ONC). We also remeasure radial velocities for 172 sources observed with SDSS/APOGEE. These data are combined with proper motions measured using HST ACS/WFPC2/WFC3IR and Keck II NIRC2, creating a sample of 135 sources with all three velocity components. The velocities measured are consistent with a normal distribution in all three components. We measure intrinsic velocity dispersions of (
σ
v
α
,
σ
v
δ
,
σ
v
r
) = (1.64 ± 0.12, 2.03 ± 0.13,
2.56
−
0.17
+
0.16
) km s
−1
. Our computed intrinsic velocity dispersion profiles are consistent with the dynamical equilibrium models from Da Rio et al. (2014) in the tangential direction but not in the line-of-sight direction, possibly indicating that the core of the ONC is not yet virialized, and may require a nonspherical potential to explain the observed velocity dispersion profiles. We also observe a slight elongation along the north–south direction following the filament, which has been well studied in previous literature, and an elongation in the line-of-sight to tangential velocity direction. These 3D kinematics will help in the development of realistic models of the formation and early evolution of massive clusters.
Abstract We present the kinematic analysis of 246 stars within 4 ′ from the center of Orion Nebula Cluster (ONC), the closest massive star cluster with active star formation across the full mass ...range, which provides valuable insights in the formation and evolution of star cluster on an individual-star basis. High-precision radial velocities and surface temperatures are retrieved from spectra acquired by the NIRSPEC instrument used with adaptive optics (NIRSPAO) on the Keck II 10 m telescope. A 3D kinematic map is then constructed by combining with the proper motions previously measured by the Hubble Space Telescope Advanced Camera for Surveys/WFPC2/WFC3IR and Keck II NIRC2. The measured root-mean-squared velocity dispersion is 2.26 ± 0.08 km s −1 , significantly higher than the virial equilibrium’s requirement of 1.73 km s −1 , suggesting that the ONC core is supervirial, consistent with previous findings. Energy equipartition is not detected in the cluster. Most notably, the velocity of each star relative to its neighbors is found to be negatively correlated with stellar mass. Low-mass stars moving faster than their surrounding stars in a supervirial cluster suggests that the initial masses of forming stars may be related to their initial kinematic states. Additionally, a clockwise rotation preference is detected. A weak sign of inverse mass segregation is also identified among stars excluding the Trapezium stars, although it could be a sample bias. Finally, this study reports the discovery of four new candidate spectroscopic binary systems.
Abstract
The carbon-to-oxygen (C/O) ratio in an exoplanet atmosphere has been suggested as a potential diagnostic of planet formation. Now that a number of exoplanets have measured C/O ratios, it is ...possible to examine this diagnostic at a population level. Here, we present an analysis of currently measured C/O ratios of directly imaged and transit/eclipse planets. First, we derive atmospheric parameters for the substellar companion HD 284149 AB b using data taken with the OSIRIS integral field spectrograph at the W.M. Keck Observatory and report two non-detections from our ongoing imaging spectroscopy survey with Keck/OSIRIS. We find an effective temperature of
T
eff
= 2502 K, with a range of 2291–2624 K,
log
g
=
4.52
, with a range of 4.38–4.91, and M/H = 0.37, with a range of 0.10–0.55. We derive a C/O of
0.59
−
0.30
+
0.15
for HD 284149 AB b. We add this measurement to the list of C/O ratios for directly imaged planets and compare them with those from a sample of transit/eclipse planets. We also derive the first dynamical mass estimate for HD 284149 AB b, finding a mass of ∼28
M
Jup
. There is a trend in C/O ratio with companion mass (
M
Jup
), with a break seen around 4
M
Jup
. We run a Kolmogorov–Smirnov and an Anderson–Darling test on planets above and below this mass boundary, and find that they are two distinct populations. This could be additional evidence of two distinct populations possibly having two different formation pathways, with companion mass as an indicator of most likely formation scenario.
Abstract
We report multiepoch radial velocities, rotational velocities, and atmospheric parameters for 37 T-type brown dwarfs observed with Keck/NIRSPEC. Using a Markov Chain Monte Carlo ...forward-modeling method, we achieve median precisions of 0.5 and 0.9 km s
−1
for radial and rotational velocities, respectively. All of the T dwarfs in our sample are thin-disk brown dwarfs. We confirm previously reported moving group associations for four T dwarfs. However, the lack of spectral indicators of youth in two of these sources suggests that these are chance alignments. We confirm two previously unresolved binary candidates, the T0+T4.5 2MASS J11061197+2754225 and the L7+T3.5 2MASS J21265916+7617440, with orbital periods of 4 and 12 yr, respectively. We find a kinematic age of 3.5 ± 0.3 Gyr for local T dwarfs, consistent with nearby late M dwarfs (4.1 ± 0.3 Gyr). Removal of thick-disk L dwarfs in the local ultracool dwarf sample gives a similar age for L dwarfs (4.2 ± 0.3 Gyr), largely resolving the local L dwarf age anomaly. The kinematic ages of local late M, L, and T dwarfs can be accurately reproduced with population simulations incorporating standard assumptions of the mass function, star formation rate, and brown dwarf evolutionary models. A kinematic dispersion break is found at the L4–L6 subtypes, likely reflecting the terminus of the stellar main sequence. We provide a compilation of precise radial velocities for 172 late M, L, and T dwarfs within ∼20 pc of the Sun.
Abstract
We present six epochs of optical spectropolarimetry of the Type II supernova (SN) 2023ixf ranging from ∼2 to 15 days after the explosion. Polarimetry was obtained with the Kast double ...spectrograph on the Shane 3 m telescope at Lick Observatory, representing the earliest such observations ever captured for an SN. We observe a high continuum polarization
p
cont
≈ 1% on days +1.4 and +2.5 before dropping to 0.5% on day +3.5, persisting at that level up to day +14.5. Remarkably, this change coincides temporally with the disappearance of highly ionized “flash” features. The decrease of the continuum polarization is accompanied by a ∼70° rotation of the polarization position angle (PA) as seen across the continuum. The early evolution of the polarization may indicate different geometric configurations of the electron-scattering atmosphere as seen before and after the disappearance of the emission lines associated with highly ionized species (e.g., He
ii
, C
iv
, and N
iii
), which are likely produced by elevated mass loss shortly prior to the SN explosion. We interpret the rapid change of polarization and PA from days +2.5 to +4.5 as the time when the SN ejecta emerge from the dense asymmetric circumstellar material (CSM). The temporal evolution of the continuum polarization and the PA is consistent with an aspherical SN explosion that exhibits a distinct geometry compared to the CSM. The rapid follow-up spectropolarimetry of SN 2023ixf during the shock ionization phase reveals an exceptionally asymmetric mass-loss process leading up to the explosion.
Using the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) spectroscopic catalog, we searched the WISE AllWISE catalog to investigate the occurrence of warm dust, as inferred from IR excesses, ...around field M dwarfs (dMs). We developed SDSS/WISE color selection criteria to identify 175 dMs (from 70,841) that show IR flux greater than the typical dM photosphere levels at 12 and/or 22 mu m, including seven new stars within the Orion OB 1 footprint. We characterize the dust populations inferred from each IR excess and investigate the possibility that these excesses could arise from ultracool binary companions by modeling combined spectral energy distributions. Our observed IR fluxes are greater than levels expected from ultracool companions (>3sigma). We also estimate that the probability the observed IR excesses are due to chance alignments with extragalactic sources is <0.1%. Using SDSS spectra we measure surface gravity-dependent features (K, Na, and CaH 3) and find <15% of our sample indicates low surface gravities. Examining tracers of youth (H alpha , UV fluxes, and Li absorption), we find <3% of our sample appear young, indicating we are observing a population of field stars gap1 Gyr, likely harboring circumstellar material. We investigate age-dependent properties probed by this sample, studying the disk fraction as a function of Galactic height. The fraction remains small and constant to |Z| ~ 700 pc and then drops, indicating little to no trend with age. Possible explanations for disks around field dMs include (1) collisions of planetary bodies, (2) tidal disruption of planetary bodies, or (3) failed planet formation.
Abstract
We present the direct-imaging discovery of a giant planet orbiting the young star AF Lep, a 1.2
M
⊙
member of the 24 ± 3 Myr
β
Pic moving group. AF Lep was observed as part of our ongoing ...high-contrast imaging program targeting stars with astrometric accelerations between Hipparcos and Gaia that indicate the presence of substellar companions. Keck/NIRC2 observations in
L
′
with the vector vortex coronagraph reveal a point source, AF Lep b, at ≈340 mas, which exhibits orbital motion at the 6
σ
level over the course of 13 months. A joint orbit fit yields precise constraints on the planet’s dynamical mass of
3.2
−
0.6
+
0.7
M
Jup
, semimajor axis of
8.4
−
1.3
+
1.1
au, and eccentricity of
0.24
−
0.15
+
0.27
. AF Lep hosts a debris disk located at ∼50 au, but it is unlikely to be sculpted by AF Lep b, implying there may be additional planets in the system at wider separations. The stellar inclination (
i
*
=
54
−
9
+
11
°
) and orbital inclination (
i
o
=
50
−
12
+
9
°
) are in good agreement, which is consistent with the system having spin–orbit alignment. AF Lep b is the lowest-mass imaged planet with a dynamical mass measurement and highlights the promise of using astrometric accelerations as a tool to find and characterize long-period planets.
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