Isaac Newton Group

We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the Gaia space mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 (~77%) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 (~58%) are hydrogen-rich sdBs, 65 are sdOBs (~21%), 32 are sdOs (~11%), and 30 are He-sdO/Bs (~10%). Among them, 48 (~16%) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the Gaia colour-magnitude diagram. The remaining sources in the sample include cataclysmic variables, blue horizontal branch stars, hot white dwarfs, and MS stars. We derived the mid-plane density ρ0 and scale height hz for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech2). The best-fit values are ρ0 = 5.17 ± 0.33 × 10−7 stars pc−3 and hz = 281 ± 62 pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to ρ0 = 6.15−0.53+1.16 × 10−7 stars pc−3. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates.

ABSTRACT AM CVn-type systems are ultracompact, helium-accreting binary systems that are evolutionarily linked to the progenitors of thermonuclear supernovae and are expected to be strong Galactic sources of gravitational waves detectable to upcoming space-based interferometers. AM CVn binaries with orbital periods ≲20–23 min exist in a constant high state with a permanently ionized accretion disc. We present the discovery of TIC 378898110, a bright (G = 14.3 mag), nearby (309.3 ± 1.8 pc), high-state AM CVn binary discovered in TESS two-minute-cadence photometry. At optical wavelengths, this is the third-brightest AM CVn binary known. The photometry of the system shows a 23.07172(6) min periodicity, which is likely to be the ‘superhump’ period and implies an orbital period in the range 22–23 min. There is no detectable spectroscopic variability. The system underwent an unusual, year-long brightening event during which the dominant photometric period changed to a shorter period (constrained to 20.5 ± 2.0 min), which we suggest may be evidence for the onset of disc-edge eclipses. The estimated mass transfer rate, $\log (\dot{M} / \mathrm{M_\odot } \, \mathrm{yr}^{-1}) = -6.8 \pm 1.0$, is unusually high and may suggest a high-mass or thermally inflated donor. The binary is detected as an X-ray source, with a flux of $9.2 ^{+4.2}_{-1.8} \times 10^{-13}$ erg cm−2 s−1 in the 0.3–10 keV range. TIC 378898110 is the shortest-period binary system discovered with TESS, and its large predicted gravitational-wave amplitude makes it a compelling verification binary for future space-based gravitational wave detectors.

We present the optical spectroscopic evolution of SN 2023ixf seen in subnight cadence spectra from 1.18 to 15 days after explosion. We identify high-ionization emission features, signatures of interaction with material surrounding the progenitor star, that fade over the first 7 days, with rapid evolution between spectra observed within the same night. We compare the emission lines present and their relative strength to those of other supernovae with early interaction, finding a close match to SN 2020pni and SN 2017ahn in the first spectrum and SN 2014G at later epochs. To physically interpret our observations, we compare them to CMFGEN models with confined, dense circumstellar material around a red supergiant (RSG) progenitor from the literature. We find that very few models reproduce the blended N iii (λλ4634.0,4640.6)/C iii (λλ4647.5,4650.0) emission lines observed in the first few spectra and their rapid disappearance thereafter, making this a unique diagnostic. From the best models, we find a mass-loss rate of 10−3–10−2 M ⊙ yr−1, which far exceeds the mass-loss rate for any steady wind, especially for an RSG in the initial mass range of the detected progenitor. These mass-loss rates are, however, similar to rates inferred for other supernovae with early circumstellar interaction. Using the phase when the narrow emission features disappear, we calculate an outer dense radius of circumstellar material R CSM,out ≈ 5 × 1014 cm, and a mean circumstellar material density of ρ = 5.6 × 10−14 g cm−3. This is consistent with the lower limit on the outer radius of the circumstellar material we calculate from the peak Hα emission flux, R CSM,out ≳ 9 × 1013 cm.

Context. Near-Earth asteroids (NEAs) that may evolve into impactors deserve detailed threat assessment studies. Early physical characterization of a would-be impactor may help in optimizing impact mitigation plans. We first detected NEA 2023 DZ2 on 27 February 2023. After that, it was found to have a minimum orbit intersection distance (MOID) with Earth of 0.00005 au as well as an unusually high initial probability of becoming a near-term (in 2026) impactor. Aims. We perform a rapid but consistent dynamical and physical characterization of 2023 DZ2 as an example of a key response to mitigating the consequences of a potential impact. Methods. We used a multi-pronged approach, drawing from various methods (observational-computational) and techniques (spectroscopy-photometry from multiple instruments), and bringing the data together to perform a rapid and robust threat assessment. Results. The visible reflectance spectrum of 2023 DZ2 is consistent with that of an X-type asteroid. Light curves of this object obtained on two different nights give a rotation period P = 6.2743 ± 0.0005 min with an amplitude A = 0.57 ± 0.14 mag. We confirm that although its MOID is among the smallest known, 2023 DZ2 will not impact Earth in the foreseeable future as a result of secular near-resonant behaviour. Conclusions. Our investigation shows that coordinated observation and interpretation of disparate data provides a robust approach from discovery to threat assessment when a virtual impactor is identified. We prove that critical information can be obtained within a few days after the announcement of the potential impactor.

Context. Realistic three-dimensional time-dependent simulations of stellar near-surface convection employ the opacity binning method for the efficient and accurate computation of the radiative energy exchange. The method provides several orders of magnitude of speedup, but its implementation includes a number of free parameters. Aims. Our aim is to evaluate the accuracy of the opacity binning method as a function of the choice of these free parameters. Methods. The monochromatic opacities computed with the SYNSPEC code were used to construct opacity distribution function (ODF) that was then verified through detailed comparison with the results of the ATLAS code. The opacity binning method was implemented with the SYNSPEC opacities for four representative cool main-sequence stellar spectral types (F3V, G2V, K0V, and M2V). Results. The ODFs from SYNSPEC and ATLAS show consistent results for the opacity and bolometric radiative energy exchange rate Q in the case of the F-, G-, and K-type stars. Significant differences, coming mainly from the molecular line lists, are found for the M-type star. It is possible to optimise a small number of bins to reduce the deviation of the results coming from the opacity grouping with respect to the ODF for the F-, G-, and K-type stars. In the case of the M-type star, the inclusion of splitting in wavelength is needed in the grouping to get similar results, with a subsequent increase in computing time. In the limit of a large number of bins, the deviation for all the binning configurations tested saturates and the results do not converge to the ODF solution. Due to this saturation, the Q rate cannot be improved by increasing the number of bins to more than about 20 bins. The more effective strategy is to select the optimal location of fewer bins.

Aims. In addition to the large surveys and catalogs of massive young stellar objects (MYSOs) and outflows, dedicated studies are needed of particular sources in which high angular observations, mainly at near-IR and (sub)millimeter wavelengths, are analyzed in depth, to shed light on the processes involved in the formation of massive stars. The galactic source G079.1272+02.2782 is a MYSO at a distance of about 1.4 kpc that appears in several catalogs, and is hereafter referred to as MYSO G79. It is an ideal source to carry out this kind of study because of its relatively close distance and the intriguing structures that the source shows at near-IR wavelengths. Methods. Near-IR integral field spectroscopic observations were carried out using NIFS at Gemini North. The spectral and angular resolutions, about 2.4–4.0 Å, and 0.″15–0.″22, allow us to perform a detailed study of the source and its southern jet, resolving structures with sizes between 200 and 300 au. As a complement, millimeter data retrieved from the James Clerk Maxwell Telescope and the IRAM 30 m telescope databases were analyzed to study the molecular gas around the MYSO on a larger spatial scale. Results. The detailed analysis of a jet extending southward from MYSO G79 shows corkscrew-like structures at 2.2 μm continuum, strongly suggesting that the jet is precessing. The jet velocity is estimated at between 30 and 43 km s−1 and its kinematics indicates that it is blueshifted, that the jet is coming to us along the line of sight. We suggest that the precession may be produced by the gravitational tidal effects generated in a probable binary system, and we estimate a jet precession period of about 103 yr, indicating a slow-precessing jet, which is in agreement with the observed helical features. An exhaustive analysis of H2 lines at the near-IR band along the jet allows us to investigate in detail a bow shock produced by this jet. We find that this bow shock is indeed generated by a C-type shock and it is observed coming to us, at an inclination angle, along the line of sight. This is confirmed by the analysis of molecular outflows on a larger spatial scale. A brief analysis of several molecular species at millimeter wavelengths indicates a complex chemistry developing at the external layers of the molecular clump in which MYSO G79 is embedded. We note that we are presenting interesting observational evidence that can give support to theoretical models of bow shocks and precessing jets.

<p class="western" align="justify"><strong>Introduction</strong></p> <p class="western" align="justify">Asteroid (594913) 'Aylo'chaxnim, formerly designated as 2020 AV<sub>2</sub>, is the only one known to orbit inside Venus orbit. It was first observed by the Zwicky Transient Facility on January 4, 2020 (Bolin et al. 2020) and it may represent the largest member of a new population of small bodies with the aphelion distance smaller than 0.718 a.u., called Vatiras (Greenstreet et al. 2012). Because of its orbit, the surface of this asteroid is being constantly modified by the high temperature, by the strong solar wind irradiation that characterizes the innermost region of the Solar system, and by high-energy micrometeorite impacts. Thus, it represents an extreme case when compared with the near-Earth asteroids.</p> <p class="western" align="justify">Based on the observations obtained on January 13, and 14, 2020, we found that 'Aylo'chaxnim has a wide spectral absorption band around 1 μm that represents the signature of an olivine-rich composition. The estimated absorption band center BIC = 1.08 ± 0.02 μm corresponds to a ferroan olivine mineralogy (Popescu et al. 2020 MNRAS). By comparing these spectra with the taxonomic types from Bus-DeMeo taxonomy, we found Sa types as the best fit for the (0.5 - 1.5 ) μm spectral interval. The classification as an S-complex asteroid was also reported by Bolin et al. (2020 EPSC) based on a visible spectrum obtained on January 23, 2020 with the Keck I telescope. They also reported a reddish surface with g-r=0.65 mag and r – i = 0.23 mag and i-z = 0.11 mag.</p> <p class="western" align="justify">Here we report new near-infrared spectra, in the range 0.8-2.5 μm, obtained using the 3m NASA Infrared Telescope Facility (NASA IRTF). The observations were performed during August 2021, when the object was close to its maximum solar elongation of 40 deg.</p> <p class="western" align="justify">Table 1. Observational circumstances for (594913) 'Aylo'chaxnim.</p> <table width="644" cellspacing="0" cellpadding="2"> <tbody> <tr valign="bottom"> <td width="66" height="14"> <p class="western">Spec. ID</p> </td> <td width="165"> <p class="western" align="center">Average UT</p> </td> <td width="115"> <p class="western" align="center">Average airmass</p> </td> <td width="118"> <p class="western" align="center">Total exp.[sec]</p> </td> <td width="160"> <p class="western" align="center">No. of exp.</p> </td> </tr> <tr valign="bottom"> <td width="66" height="14"> <p class="western" align="center">S1</p> </td> <td width="165"> <p class="western">2021-08-11T14:38:38</p> </td> <td width="115"> <p class="western" align="center">2.8</p> </td> <td width="118"> <p class="western" align="center">1912.7</p> </td> <td width="160"> <p class="western" align="center">16 x 120 sec</p> </td> </tr> <tr valign="bottom"> <td width="66" height="14"> <p class="western" align="center">S2</p> </td> <td width="165"> <p class="western">2021-08-12T14:37:21</p> </td> <td width="115"> <p class="western" align="center">2.9</p> </td> <td width="118"> <p class="western" align="center">1793.2</p> </td> <td width="160"> <p class="western" align="center">15 x 120 sec</p> </td> </tr> <tr valign="bottom"> <td width="66" height="14"> <p class="western" align="center">S3</p> </td> <td width="165"> <p class="western">2021-08-14T15:05:12</p> </td> <td width="115"> <p class="western" align="center">2.3</p> </td> <td width="118"> <p class="western" align="center">836.8</p> </td> <td width="160"> <p class="western" align="center">7 x 120 sec</p> </td> </tr> </tbody> </table> <p> </p> <p class="western" align="justify"><strong>Observations</strong></p> <p class="western" align="justify">We used the SpeX instrument, with the PRISM mode and a slit of 0.8 x 15 arcsec. This configuration allowed us to cover the 0.82 – 2.5 μm spectral interval. The spectral images were acquired in the ABBA mode. Table 1 shows the observation log for the three sets of data. The object was observable at an air-mass of ~2.5-3 for about 30 min before the start of the morning twilight. We noticed a wide apparent magnitude variation between the nights.</p> <p class="western" align="justify">In order to obtain the reflectance spectrum, the asteroid’s observed spectral data must be divided by that of a solar analog. We could observe only one solar analog, a G2V star, namely GSC 01881-01236, which was the best suited for our work in terms of apparent vicinity and time constraints. The data reduction was performed with the Spextool package (Cushing et al. 2004).</p> <p class="western" align="justify"><img src="" alt="" width="300" height="225" /></p> <p class="western" align="justify"><img src="" alt="" width="300" height="225" /></p> <p class="western" align="justify"><img src="" alt="" width="299" height="224" /></p> <p class="western" align="justify">Fig.1 The spectral data obtained with IRTF/SpeX during August 2021 for (594913) 'Aylo'chaxnim. The gray points represent the data obtained as provided by Spextool (the pipeline used to reduce the spectral images). The red points were obtained by binning every 9 points from the original data (the gray ones). The observational details associated with S1, S2, S3 are described in Table 1. The data are normalized at 1 μm.</p> <p class="western" align="justify"> </p> <p class="western" align="justify"><strong>Results</strong></p> <p class="western" align="justify">The spectra obtained during the three nights are identical up to 1.3 μm. They also match the spectrum obtained in 2020 (Fig. 2). The S1 and S3 spectra are redder compared with S2, for wavelengths longer than 1.3 μm. A thermal tail is identifiable for S2.</p> <p class="western" align="justify"><img src="" alt="" width="300" height="225" /></p> <p

Context. It is well known that cool star atmospheres depart from local thermodynamic equilibrium (LTE). Making an accurate abundance determination requires taking those effects into account, but the necessary non-LTE (hereafter NLTE) calculations are often lacking. Aims. Our goal is to provide detailed estimates of NLTE effects for FGK type stars for all spectral lines from the ultraviolet (UV) to the near infrared (NIR) that are potentially useful as abundance diagnostics. The first paper in this series focusses on the light elements Na, Mg, and Al. Methods. The code PySME was used to compute curves of growth for 2158 MARCS model atmospheres in the parameter range 3800 < Teff < 8000 K, 0.0 < log(g) < 5.5, and −5 < [Fe/H] < +0.5. Two microturbulence values, 1 and 2 kms−1, and nine abundance points spanning −1 < [X/Fe] < 1 for element X, are used to construct individual line curves of growth by calculating the equivalent widths of 35 Na lines, 134 Mg lines, and 34 Al lines. The lines were selected in the wavelength range between 2000 Å and 3 µm. Results. We demonstrate the power of the new grids with LTE and NLTE abundance analysis by means of equivalent width measurements of five benchmark stars; the Sun, Arcturus, HD 84937, HD 140283 and HD 122563. For Na, the NLTE abundances are lower than in LTE and show markedly reduced line-to-line scatter in the metal-poor stars. For Mg, we confirm previous reports of a significant ~0.25 dex LTE ionisation imbalance in metal-poor stars that is only slightly improved in NLTE (~0.18 dex). LTE abundances based on Mg II lines agree better with models of Galactic chemical evolution. For Al, NLTE calculations strongly reduce an ~0.6 dex ionisation imbalance seen in LTE for the metal-poor stars. The abundance corrections presented in this work are in good agreement with previous studies for the subset of lines that overlap, with the exception of strongly saturated lines. Conclusions. A consensus between different abundance diagnostics is the most powerful tool available to stellar spectroscopists to assess the accuracy of the models. Here we report that NLTE abundance analysis in general leads to improved agreement, in particular for metal-poor stars. The residual scatter is believed to be caused mainly by unresolved blends and/or poor atomic data, with the notable exception of Mg, which calls for further investigation.

WEAVE is the new wide-field spectroscopy facility for the prime focus of the William Herschel Telescope in La Palma, Spain. Its fibre positioner is essential for the accurate placement of the spectrograph's 960 fibre multiplex. We provide an overview of the recent maintenance, flexure modifications, and calibration measurements conducted at the observatory prior to the final top-end assembly. This work ensures that we have a complete understanding of the positioner's behaviour as it changes orientation during observations. All fibre systems have been inspected and repaired, and the tumbler structure contains new clamps to stiffen both the internal beam and the retractor support disk onto which the field plates attach. We present the updated metrology procedures and results that will be verified on-sky.

ABSTRACT Photometric and spectroscopic data for two Low Luminosity Type IIP Supernovae (LL SNe IIP) 2020cxd and 2021aai are presented. SN 2020cxd was discovered 2 d after explosion at an absolute magnitude of Mr = −14.02 ± 0.21 mag, subsequently settling on a plateau which lasts for ∼120 d. Through the luminosity of the late light curve tail, we infer a synthesized 56Ni mass of (1.8 ± 0.5) × 10−3 M⊙. During the early evolutionary phases, optical spectra show a blue continuum ($T\, \gt $8000 K) with broad Balmer lines displaying a P Cygni profile, while at later phases, Ca ii, Fe ii, Sc ii, and Ba ii lines dominate the spectra. Hydrodynamical modelling of the observables yields $R\, \simeq$ 575 R⊙ for the progenitor star, with Mej = 7.5 M⊙ and $E\, \simeq$ 0.097 foe emitted during the explosion. This low-energy event originating from a low-mass progenitor star is compatible with both the explosion of a red supergiant (RSG) star and with an Electron Capture Supernova arising from a super asymptotic giant branch star. SN 2021aai reaches a maximum luminosity of Mr = −16.57 ± 0.23 mag (correcting for AV = 1.92 mag), at the end of its remarkably long plateau (∼140 d). The estimated 56Ni mass is (1.4 ± 0.5) × 10−2 M⊙. The expansion velocities are compatible with those of other LL SNe IIP (few 103 km s−1). The physical parameters obtained through hydrodynamical modelling are $R\, \simeq$ 575 R⊙, Mej = 15.5 M⊙, and E = 0.4 foe. SN 2021aai is therefore interpreted as the explosion of an RSG, with properties that bridge the class of LL SNe IIP with standard SN IIP events.