Blanco 1, a 100Myr open cluster in the solar neighborhood, is well known for its two 50pc-long tidal tails. Taking Blanco 1 as a reference, we find evidence of early-stage tidal disruption in two other open clusters of ~120Myr: the Pleiades and NGC 2
516, via Gaia EDR3 data. These two clusters have a total mass of 2-6 times that of Blanco 1. Despite having a similar age as Blanco 1, the Pleiades and NGC 2516 have a larger fraction of their members bound: 86% of their mass is inside the tidal radius, versus 63% for Blanco 1. However, a correlation between Blanco 1s 50pc-long tidal tails and the kinematic tails in velocity space is also found for the Pleiades and NGC 2516. This evidence supports the idea that the modest elongation seen in the spatial distribution for the Pleiades and NGC 2516 is a result of early-stage tidal disruption.
Brown dwarfs are essential targets for understanding planetary and sub-stellar atmospheres across a wide range of thermal and chemical conditions. As surveys continue to probe ever deeper, and as observing capabilities continue to improve, the number
of known Y dwarfs -- the coldest class of sub-stellar objects, with effective temperatures below about 600 K -- is rapidly growing. Critically, this class of ultra-cool objects has atmospheric conditions that overlap with Solar System worlds and, as a result, tools and ideas developed from studying Earth, Jupiter, Saturn and other nearby worlds are well-suited for application to sub-stellar atmospheres. To that end, we developed a one-dimensional (vertical) atmospheric structure model for ultra-cool objects that includes moist adiabatic convection, as this is an important process for many Solar System planets. Application of this model across a range of effective temperatures (350, 300, 250, 200 K), metallicities ([M/H] of 0.0, 0.5, 0.7, 1.5), and gravities (log $g$ of 4.0, 4.5, 4.7, 5.0) demonstrates strong impacts of water latent heat release on simulated temperature-pressure profiles. At the highest metallicities, water vapor mixing ratios reach an Earth-like 3%, with associated major alterations to the thermal structure in the atmospheric regions where water condenses. Spectroscopic and photometric signatures of metallicity and moist convection should be readily detectable at near- and mid-infrared wavelengths, especially with James Webb Space Telescope observations, and can help indicate the formation history of an object.
Application of the radial velocity (RV) technique in the near infrared is valuable because of the diminished impact of stellar activity at longer wavelengths, making it particularly advantageous for the study of late-type stars but also for solar-typ
e objects. In this paper, we present the IGRINS RV open source python pipeline for computing infrared RV measurements from reduced spectra taken with IGRINS, a R ~ 45,000 spectrograph with simultaneous coverage of the H band (1.49--1.80 $mu$m) and K band (1.96--2.46 $mu$m). Using a modified forward modeling technique, we construct high resolution telluric templates from A0 standard observations on a nightly basis to provide a source of common-path wavelength calibration while mitigating the need to mask or correct for telluric absorption. Telluric standard observations are also used to model the variations in instrumental resolution across the detector, including a yearlong period when the K band was defocused. Without any additional instrument hardware, such as a gas cell or laser frequency comb, we are able to achieve precisions of 26.8 $rm m,s^{-1}$ in the K band and 31.1 $rm m,s^{-1}$ in the H band for narrow-line hosts. These precisions are empirically determined by a monitoring campaign of two RV standard stars as well as the successful retrieval of planet-induced RV signals for both HD 189733 and $tau$ Boo A; furthermore, our results affirm the presence of the Rossiter-McLaughlin effect for HD 189733. The IGRINS RV pipeline extends another important science capability to IGRINS, with publicly available software designed for widespread use.
We present the stellar population, using {it Gaia},DR2 parallax, kinematics, and photometry, of the young ($sim 100$~Myr), nearby ($sim 230$~pc) open cluster, Blanco1. A total of 644 member candidates are identified via the unsupervised machine learn
ing method textsc{StarGO} to find the clustering in the 5-dimensional position and proper motion parameter ($X$, $Y$, $Z$, $mu_alpha cosdelta$, $mu_delta$) space. Within the tidal radius of $10.0 pm 0.3$~pc, there are 488 member candidates, 3 times more than those outside. A leading tail and a trailing tail, each of 50--60~pc in the Galactic plane, are found for the first time for this cluster, with stars further from the cluster center streaming away faster, manifest stellar stripping. Blanco1 has a total detected mass of $285pm32$~M$_odot $ with a mass function consistent with a slope of $alpha=1.35pm0.2$ in the sense of $dN/dm propto m^{-alpha}$, in the mass range of 0.25--2.51~M$_odot $, where $N$ is the number of members and $m$ is stellar mass. A Minimum Spanning Tree ($Lambda_{rm MSR}$) analysis shows the cluster to be moderately mass segregated among the most massive members ($gtrsim 1.4$~M$_odot$), suggesting an early stage of dynamical disintegration.
We report the discovery of tidal structures around the intermediate-aged ($sim$ 700--800~Myr), nearby ($sim85$~pc) star cluster Coma Berenices. The spatial and kinematic grouping of stars is determined with the {it Gaia} DR2 parallax and proper motio
n data, by a clustering analysis tool, textsc{StarGO}, to map 5D parameters ($X, Y, Z$, $mu_alpha cosdelta, mu_delta$) onto a 2D neural network. A leading and a trailing tails, each with an extension of $sim50$~pc are revealed for the first time around this disrupting star cluster. The cluster members, totaling to $sim115^{+5}_{-3},rm {M_odot}$, are clearly mass segregated, and exhibit a flat mass function with $alpha sim 0.79pm0.16$, in the sense of $dN/dm propto m^{-alpha}$, where $N$ is the number of member stars and $m$ is stellar mass, in the mass range of $m=0.25$--$2.51~{rm M_odot}$. Within the tidal radius of $sim$6.9~pc, there are 77 member candidates with an average position, i.e., as the cluster center, of R.A.= 186.8110~deg, and decl.= 25.8112~deg, and an average distance of 85.8~pc. Additional 120 member candidates reside in the tidal structures, i.e., outnumbering those in the cluster core. The expansion of escaping members lead to an anisotropy in the velocity field of the tidal tails. Our analysis also serendipitously uncovers an adjacent stellar group, part of which has been cataloged in the literature. We identify 218 member candidates, 10 times more than previously known. This star group is some 65~pc away from, and $sim400$~Myr younger than, Coma Ber, but is already at the final stage of disruption.
We have identified stellar and substellar members in the nearby star cluster Coma Berenices, using photometry, proper motions, and distances of a combination of 2MASS, UKIDSS, URAT1, and {it Gaia}/DR2 data. Those with {it Gaia}/DR2 parallax measureme
nts provide the most reliable sample to constrain the distance, averaging 86.7~pc with a dispersion 7.1~pc, and the age $sim800$~Myr, of the cluster. This age is older than the 400--600~Myr commonly adopted in the literature. Our analysis, complete within 5deg of the cluster radius, leads to identification of 192 candidates, among which, after field contamination is considered, about 148 are true members. The members have $Jsim3$~mag to $sim17.5$~mag, corresponding to stellar masses 2.3--0.06~$M_odot$. The mass function of the cluster peaks around 0.3~$M_odot$ and, in the sense of $dN/dm = m^{-alpha}$, where $N$ is the number of members and $m$ is stellar mass, has a slope $alphaapprox 0.49pm0.03$ in the mass range 0.3--2.3~$M_odot$. This is much shallower than that of the field population in the solar neighborhood. The slope $alpha=-1.69pm0.14$ from 0.3~$M_odot$ to 0.06~$M_odot$, the lowest mass in our sample. The cluster is mass segregated and has a shape elongated toward the Galactic plane. Our list contains nine substellar members, including three new discoveries of an M8, an L1 and an L4 brown dwarfs, extending from the previously known coolest members of late-M types to even cooler types.
