Rene A. Mendez

The new observational data obtained during the 2020, 2021, 2022, 2023, and 2024 speckle runs with the 4.1-m Southern Astrophysical Research (SOAR) telescope located at Cerro Pachón (Chile) has been used to improve the orbits of the following 22 visual binaries with components of spectral types F and G, most of them from the main sequence: WDS 00271-0753 (A 431 AB), WDS 00463-0634 (HDS 101), WDS 03189-0101 (BU 1177), WDS 03596-1019 (HU 29), WDS 04502-3113 (B 1474), WDS 05276-2055 (SEE 53), WDS 05352-4657 (RST 141), WDS 06380-6132 (I 5 AB), WDS 07303-5657 (FIN 105), WDS 08075-1212 (A 2749), WDS 08331-2436 (BU 205 AB), WDS 11068-7050 (B 2006), WDS 13005-3330 (HDS 1824 Aa,Ab), WDS 13040-1738 (BU 798 AB), WDS 14565-3438 (I 227 AB), WDS 15548-6554 (NZO 65), WDS 17240-0921 (RST 3972 Aa,Ab), WDS 20073-5127 (RST 1059), WDS 20507-3116 (B 997), WDS 21395-0003 (BU 1212 AB), WDS 22586-4531 (HU 1335), WDS 23218-1217 (HU 95). In the cases that it was feasible, we have calculated the dynamical parallaxes and individual masses of the components and compared them with those in the literature.

We have used new observational data obtained during the 2020, 2021, 2022, 2023, and 2024 speckle runs with the 4.1 m Southern Astrophysical Research (SOAR) telescope located at Cerro Pachón (Chile) to improve the orbits of the following 21 visual binaries with components of spectral types B and A: WDS 03124-4425 (JC 8 AB), WDS 05010-1112 (A 2629), WDS 05474-1032 (MCA 22), WDS 07374-3458 (FIN 324 AB,C), WDS 08277-0425 (A 550), WDS 13123-5955 (SEE 170), WDS 13320-6519 (FIN 369), WDS 14567-6247 (FIN 372), WDS 15351-4110 (HJ 4786 AB), WDS 16094-3103 (I 557), WDS 16589-3737 (SEE 315), WDS 17104-1544 (BU 1118AB), WDS 17400-0038 (BU 631), WDS 17471-3807 (I 1336), WDS 18150-5018 (I 429, two solutions), WDS 19026-2953 (HDO 150 AB), WDS 19172-6640 (GLE 3), WDS 19253-2431 (FIN 327), WDS 21147-0050 (A 883 AB), WDS 21273-3218 (B 1007), and WDS 21579-5500 (FIN 307). Most of the systems are comprised of main sequence or subgiant stars. In the cases that it is feasible, we have calculated the dynamical parallaxes and individual masses of the components. These parallaxes have been compared with those in the literature.

Results of speckle interferometry observations at the 4.1 m Southern Astrophysical Research telescope obtained during 2024–2025 are presented. We present 5316 measurements of relative positions and magnitude differences in 3532 pairs (including 524 unpublished measures made before 2024) with median and minimum separations of 0 . ″ 19 and 12 mas, respectively; nonresolutions of 1723 stars are documented as well. More than 400 pairs have been resolved here for the first time and not resolved by Gaia; among those are 222 TESS objects of interest, 46 inner subsystems in known wider binaries within 100 pc, and 43 subdwarfs. Positional measurements are used to compute or improve binary orbits; elements of 202 orbits with meaningful errors are given here, while preliminary and tentative orbits are published elsewhere. Of special note are orbits with large and accurately measured eccentricities (e.g., e = 0.9866 ± 0.0014 for J13038−2035) and orbits of pre-main-sequence binaries. The Appendix contains parameters of 86 binaries used for calibration of pixel scale and orientation.

We present astrometric measurements for 13 cold brown dwarfs in the solar neighborhood ( d < 20 pc). By combining archival Spitzer data with our own Hubble Space Telescope (HST) observations, we achieve parallax uncertainties typically around 10%. Using Spitzer and HST photometry we compare our targets with other known late T and Y dwarfs in the Solar neighborhood, confirming that there is large intrinsic scatter in the near- and mid-infrared absolute magnitudes and colors of this population, further highlighting the diversity observed spectroscopically by several James Webb Space Telescope programs. This scatter makes photometric distance estimates highly unreliable and, therefore, makes astrometric parallax measurements fundamental for a meaningful characterization of even the nearest cold brown dwarfs.

Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2 is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra ( R ∼ 2700, 2.87−5.13 μ m) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2 b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major moving group (414 ± 23 Myr) using BANYAN Σ and its full kinematics. We also reestimate the astrometry of COCONUTS-2b using the MIRI data. We support this membership with a comparison of the rotation period, metallicity, and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which, combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2 b is a planetary-mass object (7.5 ± 0.4 M _Jup ), which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7 ± 0.2 M _Jup ). We identify absorption feature differences, which could indicate that 0825+2805 has stronger vertical mixing.

Context. In point spread function (PSF) photometry, the selection of the fitting aperture radius plays a critical role in determining the precision of flux and background estimations. Traditional methods often rely on maximizing the signal-to-noise ratio (S/N) as a criterion for aperture selection. However, S/N-based approaches do not necessarily provide the optimal precision for joint estimation problems as they do not account for the statistical limits imposed by the Fisher information in the context of the Cramér-Rao lower bound (CRLB). Aims. This study aims to establish an alternative criterion for selecting the optimal fitting radius based on Fisher information rather than S/N. Fisher information serves as a fundamental measure of estimation precision, providing theoretical guarantees on the achievable accuracy for parameter estimation. By leveraging Fisher information, we seek to define an aperture selection strategy that minimizes the loss of precision. Methods. We conducted a series of numerical experiments that analyze the behavior of Fisher information and estimator performance as a function of the PSF aperture radius. Specifically, we revisited fundamental photometric models and explored the relationship between aperture size and information content. We compared the empirical variance of classical estimators, such as maximum likeli-hood and stochastic weighted least squares, against the theoretical CRLB derived from the Fisher information matrix. Results. Our results indicate that aperture selection based on the Fisher information provides a more robust framework for achieving optimal estimation precision. The findings reveal that S/N-based aperture selection may lead to significant discrepancies, with potential precision losses of up to 70%. In contrast, Fisher information-based selection allows a more accurate and consistent estimation process, ensuring that the empirical variance closely aligns with the theoretical limits.

We present measurements in the context of a survey of southern hemisphere binary and multiple stellar systems observed with the Zorro Speckle dual diffraction-limited optical imaging camera on the 8.1 m Gemini South telescope carried out between 2019 and 2023. The overall motivation of our survey and some initial results of these observations are outlined to demonstrate the capabilities—and limitations—of Zorro. We report on the astrometric characterization of the instrument in terms of the precision and accuracy of our measurements and provide details of our custom-made data reduction pipeline. For targets with separations smaller than 0 . ″ 4, an overall precision of 1 mas in the radial and tangential directions is obtained, while the uncertainty in position angle is 0 . ° 2. Relative astrometry and contrast brightness in the two Zorro filters at 562 and 832 nm are reported for 70 pairs on 64 distinct systems (six are triples). Eleven new binaries are found, mostly of small separations (down to 15 mas) and large brightness contrast (up to Δm = 6 in the red channel). Our results indicate that the Zorro instrument, when properly calibrated, delivers excellent-quality data for visual binary studies of tight and/or faint companions.

We present measurements in the context of a survey of southern hemisphere binary and multiple stellar systems observed with the Zorro Speckle dual diffraction-limited optical imaging camera on the 8.1m Gemini-South telescope carried out between 2019 and 2023. The overall motivation of our survey, as well as some initial results of these observations, are outlined to demonstrate the capabilities - and limitations - of Zorro. We report on the astrometric characterization of the instrument in terms of the precision and accuracy of our measurements and provide details of our custom-made data reduction pipeline. For targets with separations smaller than 0.4 arcsec, an overall precision of 1 mas in the radial and tangential directions is obtained, while the uncertainty in position angle is 0.2 deg. Relative astrometry and contrast brightness in the two Zorro filters at 562 and 832 nm are reported for 70 pairs on 64 distinct systems (six are triples). Eleven new binaries are found, mostly of small separations (down to 15 mas), and large brightness contrast (up to Delta m=6 in the red channel). Our results indicate that the Zorro instrument, when properly calibrated, delivers excellent quality data for visual binary studies of tight and/or faint companions.