HST's hunt for intermediate-mass black holes in star clusters

Establishing or ruling out, either through solid mass measurements or upper limits, the presence of intermediate-mass black holes (IMBHs) at the centers of star clusters would profoundly impact our understanding of problems ranging from the formation and long-term dynamical evolution of stellar systems, to the nature of the seeds and the growth mechanisms of supermassive black holes. While there are sound theoretical arguments both for and against their presence in today's clusters, observational studies have so far not yielded truly conclusive IMBH detections nor upper limits. We argue that the most promising approach to solving this issue is provided by the combination of measurements of the proper motions of stars at the centers of Galactic globular clusters and dynamical models able to take full advantage of this type of data set. We present a program based on HST observations and recently developed tools for dynamical analysis designed to do just that.Comment: 7 pages; Invited talk at IAU Symposium 266 "Star clusters: basic galactic building blocks" (Rio de Janeiro, 10-14 August 2009), R. de Grijs and R.D. Lepine, eds; v2 reflects a better citation of some references and corrected typo

Astrometry and Photometry for ≈\approx1000 L, T, and Y Dwarfs from the UKIRT Hemisphere Survey

We present positions, proper motions, and near-infrared photometry for 966 known objects with spectral types later than M observed as part of the the UKIRT Hemisphere Survey (UHS). We augment the photometry and astrometry from UHS with information from Gaia DR3, Pan-STARRS DR2, and CatWISE 2020 to produce a database of homogeneous photometry and astrometry for this sample. The multi-epoch survey strategy of UHS allows us to determine proper motions for most sources, with a median proper motion uncertainty of $\sim$3.6 mas yr$^{-1}$. Our UHS proper motion measurements are generally in good agreement with those from Gaia DR3, Pan-STARRS, and CatWISE 2020, with UHS proper motions typically more precise than those from CatWISE 2020 and Pan-STARRS but not Gaia DR3. We critically analyze publicly available spectra for 406 members of this sample and provide updated near-infrared spectral types for $\sim$100 objects. We determine typical colors as a function of spectral type and provide absolute magnitude vs. spectral type relations for UHS $J$- and $K$-band photometry. Using newly determined proper motions, we highlight several objects of interest, such as objects with large tangential velocities, widely separated co-moving companions, and potential members of young nearby associations.Comment: Accepted to A

USING THE 1.64 MICRON [FeII] EMISSION LINE TO DETECT SUPERNOVA REMNANTS IN NGC 6946

In the infrared (IR), shock models indicate, and observations show that supernova remnants (SNRs) emit strongly in [Fe II] at 1.64 μm. This thesis reports the results of a search for SNRs in NGC 6946 relying on [Fe II] 1.64 μm line emission, where we employed an adjacent [Fe II]Off filter to accurately assess the local continuum levels. In this study, we used the WIYN High Resolution Infrared Camera (WHIRC) on the WIYN 3.5m telescope to image NGC 6946 in broad bands J and H and narrow bands [Fe II], [Fe II]Off, Paβ and PaβOff. From our search, we have identified 72 supernova remnant candidates (SNRcs), 11 of which are coincident with sources found in prior radio, optical and/or x-ray studies. Six of the 11 coincident sources were specifically classified as SNRs in their respective studies. The measured [Fe II] luminosities of our SNRcs range from 4.02×10^35 to 2.09×10^37 erg s^−1 and are among the highest of previously published extragalactic SNR [Fe II] luminosities. Using the measured [Fe II] luminosities, we calculate an estimated SN rate of 0.078 yr^−1 for NGC 6946, which is comparable to the observed rate in the last 100 years of 0.09 yr^−1. We also compare our SNR candidates with SNRs that have been found at radio, optical and X-ray wavelengths. All of the candidates now need to be confirmed spectroscopically. However, the fact that we detect as many objects as we did, suggests that [Fe II] can be used as an effective search tool to find extragalactic SNRs

Redder than Red: Discovery of an Exceptionally Red L/T Transition Dwarf

We present the discovery of CWISE J050626.96+073842.4 (CWISE J0506+0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 citizen science project. Photometry from UKIRT and CatWISE give a ( J − K ) _MKO color of 2.97 ± 0.03 mag and a J _MKO − W2 color of 4.93 ± 0.02 mag, making CWISE J0506+0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506+0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity, late-type L/T transition dwarf. The spectrum of CWISE J0506+0738 shows possible signatures of CH _4 absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8 γ –T0 γ . We tentatively find that CWISE J0506+0738 is variable at 3–5 μ m based on multiepoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in the β Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object