arXiv subjects

Jerry Jun-Yan Zhang

Publications and source records attributed to Jerry Jun-Yan Zhang.

7 recordsLinked to original sources

arXiv2026

Halo stars harbour few wide ultracool companions

We conducted the first search for wide ultracool companions to metal-poor halo stars. A sample of nearby halo stars with spectroscopically determined metallicities and high proper motions was imaged in the $J$ band and examined for faint candidate companions. Follow-up imaging over baselines of two to four years enabled a search for common proper-motion sources. The survey reached average limiting magnitudes of $J_{\mathrm{lim}}=22.8$ and $23.0$ mag (Vega) in the first and second epochs, respectively, sufficient to detect extreme subdwarfs earlier than esdT0 out to 250 pc. No bona fide wide ultracool companion was identified over projected separations of a few hundred to a few thousand au. We therefore derive an upper limit of $4.0\%$ (at a $90\%$ confidence level) on the frequency of wide ultracool companions to metal-poor halo stars. Four wide stellar companions were recovered and confirmed with Gaia, yielding a wide stellar companion frequency of $6.1^{+7.2}_{-4.0}\%$ (at a $90\%$ confidence level). We conclude that wide ultracool companions are intrinsically rare around metal-poor halo stars and that their occurrence rate is, at most, comparable to that of wide stellar companions. Current observations provide no evidence for a metallicity dependence of the wide ultracool companion frequency around stars. Formation and retention processes in binary systems are likely to operate less efficiently for ultracool secondaries.

astro-ph.SR↗

arXiv2026

Integral field spectroscopy with no IFUs: combining wide-field rotational slitless spectroscopy with tomographic reconstruction

Among spectroscopic techniques, Integral Field Spectroscopy (IFS) is regarded as one of the most versatile and powerful, but it is limited by small FoVs, complex designs, and high costs. We hereby present ROSSINI: the ROtational Slitless Spectrograph for INtegral field spectroscopy and Imager, a novel spectrograph design aimed at performing IFS without IFUs but in a slitless fashion instead. The device relies on generating a series of independent detector images of the same field by rotating the whole telescope and/or the dispersion direction of the optical element, and on a postprocessing tomographic reconstruction algorithm, yielding a full IFS datacube with arbitrary angular and spectral pixelization defined by the user. We first develop the mathematical formulation of the problem and derive the solution as a linear matrix inversion. Then, we provide an interpretation of ROSSINI as a particular application of tomography and leverage this to propose a practical numerical solution based on iterative reconstruction. We test this novel conception through a series of numerical experiments: we first generate toy datacubes on the sky, then simulate the spectrograph and the corresponding detector images, and finally apply tomographic reconstruction, recovering the input datacube. The number of needed rotations can be easily computed from the relative pixelization of the datacube and the pixel number of the detector. From the numerical experiments, ROSSINI turns out to be able to reconstruct the benchmark datacubes with percent accuracy and in just a few hundred iterations, using negligible computational resources. ROSSINI is a promising way forward for future spectroscopic facilities, as it allows us to perform wide-field IFS in an efficient and cheap fashion by relying mostly on existing slitless spectrograph technologies.

astro-ph.IM↗

arXiv2025

$K_s$-band photometry of the Extreme T Subdwarf CWISE J221706.28$-$145437.6

We present deep $K_s$-band imaging of the extreme T subdwarf CWISE J221706.28$-$145437.6. Using the new photometry, we construct its spectral energy distribution and find this object exhibits exceptionally strong collision-induced absorption in the $H$ and $K$ bands. The comparison with the nearest benchmark extreme T subdwarf WISEA J181006.18$-$101000.5 suggests the object would be cooler and more metal-poor than the benchmark.

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arXiv2025

Optical constraints on the coldest metal-poor population

The coldest metal-poor population made of T and Y dwarfs are archaeological tracers of our Galaxy because they are very old and have kept the pristine material. The optical properties of these objects are important to characterise their atmospheric properties. We aim at characterising further the optical properties of ultracool metal-poor population with deep far-red optical images and parallax determinations. With a two-year baseline, we solved trigonometric parallaxes of the five metal-poor T dwarf candidates using Calar-Alto 3.5-m telescope. We obtained $z'$-band photometry for the other 12 metal-poor T dwarf candidates using 10.4-m Gran Telescopio Canarias, the 8.2-m European Southern Observatory Very Large Telescope, and the Dark Energy Survey, increasing the sample of T subdwarfs with optical photometry from 12 to 24. We report a 3-$σ$ limit for the only potential metal-poor Y dwarf, a.k.a., the Accident in five optical bands using the Gran Telescopio Canarias. We compared these objects with a known subdwarf benchmark and solar-metallicity dwarfs in colour-magnitude and colour-colour diagrams, as well as with state-of-the-art theoretical ultracool models. We confirm three more T subdwarfs and show that the Accident is subluminous compared to the current Y dwarf limit. Additionally, we propose two more Y subdwarf candidates. We emphasise that the $z_{PS1} - W1$ colour combining with the $W1 - W2$ colour could break the metallicity-temperature degeneracy for T and possibly for Y dwarfs. The $z_{PS1} - W1$ colour shifts redward when metallicity decreases for a certain temperature, which is not predicted by models. The Accident has the reddest $z_{PS1} - W1$ colour among our sample. The $z_{PS1} - W1$ colour will be useful to search for other examples of this cold and old population in upcoming and existing deep optical and infrared large-area surveys.

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arXiv2025

Detection of Methane in the Closest Extreme Metal-poor T Dwarf WISEA J181006.18-101000.5

WISEA J181006.18-101000.5 (WISE1810) is the nearest metal-poor ultracool dwarf to the Sun. It has a low effective temperature and has been classified as extreme early-T subdwarf. However, methane, the characteristic molecule of the spectral class T, was not seen in the previous low-resolution spectrum. Using the 10.4-m Gran Telescopio Canarias, we collected a high-quality JHK-band intermediate-resolution R~5000 spectrum of WISE1810, in which a 17+/-6 ppm of methane is clearly detected, while carbon monoxide is absent. Based on customly computed ATMO2020++ model, we estimated an effective temperature of 1000+/-100 K, a high surface gravity of log g = 5.5+/-0.5 dex, a carbon abundance [C/H]=-1.5+/-0.2 dex, inferring [Fe/H]=-1.7+/-0.2 dex. Potassium is not seen in our data, and the upper limits of pseudo-equivalent width of J-band atomic lines are at least 25 to 60 times weaker than those measured from solar-metallicity early-T counterparts. We measured a heliocentric radial velocity of -83+/-13 km/s, inferring that WISE1810 is more likely a thick disk member.

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arXiv2024

Reconnaissance ultracool spectra in the Euclid Deep Fields

Context. Euclid will carry out a deep survey benefiting the discovery and characterisation of ultracool dwarfs (UCDs), especially in the Euclid Deep Fields (EDFs), which the telescope will scan repeatedly throughout its mission. The photometric and spectroscopic standards in the EDFs are important benchmarks, crucial for the classification and characterisation of new UCD discoveries and for the calibration of the mission itself. Aims. We aim to provide a list of photometric UCD candidates and collect near-infrared reconnaissance spectra for M, L, and T-type UCDs in the EDFs as future Euclid UCD references. Methods. In EDF North, we cross-matched public optical and infrared surveys with certain photometric criteria to select UCDs. In EDF Fornax and EDF South, we used photometrically classified samples from the literature. We also include UCDs identified by Gaia DR2. We selected 7 UCD targets with different spectral types from the lists and obtained low-resolution 0.9-2.5 μm spectra of them using GTC/EMIR and the VLT/X-shooter. We also selected a young, bright L dwarf near EDF Fornax to test the coherence of these two facilities. We included an extra T dwarf in EDF North with its published J-band spectrum. Results. We retrieved a list of 81 (49, 231) M, 8 (29, 115) L, and 1 (0, 2) T dwarf candidates in EDF North, Fornax, and South, respectively. They are provided to guide future UCD discoveries and characterisations by Euclid. In total, we collected near-infrared spectra for 9 UCDs, including 2 M types, 3 L types, and 4 T types in or close to the 3 EDFs. The Euclidised spectra show consistency in their spectral classification, which demonstrates that slitless Euclid spectroscopy will recover the spectral types with high fidelity for UCDs, both in the EDFs and in the wide survey. We also demonstrate that Euclid will be able to distinguish different age groups of UCDs.

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arXiv2023

Optical Properties of Metal-poor T Dwarf Candidates

Context. Metal-poor brown dwarfs are poorly understood because they are extremely faint and rare. Only a few candidates have been identified as T-type subdwarfs in infrared surveys and their optical properties remain unconstrained. Aims. We aim to improve the knowledge of the optical properties of T subdwarf candidates to break the degeneracy between metallicity and temperature and to investigate their atmospheric properties. Methods. Deep $z$-band images of 10 known T subdwarf candidates were collected with the 10.4-m Gran Telescopio Canarias. Low-resolution optical spectra for two of them were obtained with the same telescope. Photometric measurements of the $z$-band flux were performed for all the targets and they were combined with infrared photometry in $J, H, K, W1$ and $W2$-bands from the literature to obtain the colours. The spectra were compared with solar-metallicity T dwarf templates and with laboratory spectra. Results. We found that the targets segregate into three distinct groups in the $W1 - W2$ vs. $z - W1$ colour-colour diagram. Group I objects are mixed with solar-metallicity T dwarfs. Group III objects have $W1 - W2$ colours similar to T dwarfs but very red $z - W1$ colours. Group II objects lie between Group I and III. The two targets for which we obtained spectra are located in Group I and their spectroscopic properties resemble normal T dwarfs but with water features that are deeper and have a shape akin to pure water. Conclusions. We conclude that the $W1 - W2$ vs. $z - W1$ colour-colour diagram is excellent to break the metallicity-temperature degeneracy for objects cooler than L-type. A revision of the spectral classification of T subdwarf might be needed in the future, according to the photometric and spectroscopic properties of WISE1810 and WISE0414 in Group III discussed in this work.

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