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arxiv: 2603.00240 · v1 · submitted 2026-02-27 · 🌌 astro-ph.SR · astro-ph.GA

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Spectroscopic follow-up of hot subdwarf variables found in ZTF -- Atmospheric and fundamental properties of radial-mode sdB pulsators

Corey W. Bradshaw , Thomas Kupfer , Alekzander R. Kosakowski , Brad N. Barlow , Matti Dorsch
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Pith reviewed 2026-05-15 17:42 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.GA
keywords hot subdwarfssdB variablesradial pulsationsspectroscopic follow-upeffective temperaturesurface gravityZTF surveystellar masses
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The pith

ZTF sdBVs with large-amplitude radial pulsations share temperatures, gravities, and canonical masses with known examples like Balloon 090100001.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper collects low-resolution spectra of hot subdwarf B variables discovered in the ZTF survey that show large-amplitude short-period variability from radial pulsations. Fitting these spectra to model grids yields average effective temperatures near 28,300 K and surface gravities of log g = 5.56, with modest changes during variability, while spectral energy distribution fitting gives masses and radii. These values place the stars on the boundary between the V361 Hya and V1093 Her classes in the temperature-gravity plane and distinguish them from blue large-amplitude pulsators. The results indicate that the majority follow standard sdB properties rather than exotic ones. A reader would care because defining this group clarifies how radial pulsation behavior relates to atmospheric conditions and evolutionary status in hot subdwarfs.

Core claim

We show that the resulting properties are similar to the radial-mode dominant sdBVs, Balloon 090100001 and CS 1246, and that they are distinguishable from other similar radial-mode pulsators, such as blue large-amplitude pulsators. The masses and radii of the majority of the sdBVs in our sample align with canonical-mass sdB properties. These stars have mean effective temperatures of 28,300 K and surface gravity measurements of log g = 5.56, with changes in these parameters on the order of 1000 K and 0.10 dex, respectively. The location of these stars on the Teff -- log g plane places them on the boundary region between the low-amplitude, multi-periodic V361 Hya and V1093 Her stars, where the

What carries the argument

Fitting low-resolution spectra to theoretical model grids for mean effective temperature, surface gravity, and helium abundance, followed by spectral energy distribution fitting to derive mass, radius, and luminosity.

If this is right

  • The stars show average effective temperatures of 28,300 K and log g of 5.56 with variability of roughly 1000 K and 0.10 dex.
  • Their positions lie on the boundary between V361 Hya and V1093 Her pulsators in the temperature-gravity plane.
  • Most objects have masses and radii consistent with canonical sdB values.
  • The sample is distinguishable from blue large-amplitude pulsators by the derived atmospheric and fundamental properties.
  • Time-series spectra reveal corresponding changes in temperature and gravity tied to the pulsation cycles.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the boundary location proves stable across more objects, it could point to a narrow range of envelope conditions that favor radial-mode dominance over multi-periodic behavior.
  • Confirmation of canonical masses would imply these stars arise from standard binary-evolution channels rather than requiring unusual mass-loss histories.
  • Extending the same fitting approach to fainter ZTF candidates could test whether the population remains uniform at lower luminosities.
  • Direct comparison of pulsation amplitudes with the observed temperature variations might constrain how radius changes drive the light curves.

Load-bearing premise

Low-resolution spectra can be fitted to theoretical model grids to yield accurate mean effective temperatures, surface gravities, and helium abundances without major systematic biases, and that spectral energy distribution fitting then produces trustworthy masses and radii.

What would settle it

Discovery of additional ZTF sdBVs whose fitted masses fall well outside the canonical range or whose Teff and log g place them far from the boundary between V361 Hya and V1093 Her classes would falsify the claim of shared properties.

Figures

Figures reproduced from arXiv: 2603.00240 by Alekzander R. Kosakowski, Brad N. Barlow, Corey W. Bradshaw, Matti Dorsch, Thomas Kupfer.

Figure 1
Figure 1. Figure 1: Color magnitude diagram constructed for objects in the ZTF￾sdBV sample which have reasonable parallax measurements, using Gaia photometric G, GBP and GRP color-corrected magnitudes. Each object is color-mapped with its corresponding pulsation period, which is scaled to the observed range of pulsation periods for the entire sam￾ple. the g and r bands using 30 s exposures. The resulting number of observation… view at source ↗
Figure 2
Figure 2. Figure 2: Left panel: SED fitting for a single sdB (ZTF-sdBV1) where the gray line represents the best fitting sdB model spectrum. Right panel: composite SED for a sdB + cool companion (ZTF-sdBV6) where the blue line denotes the sdB model and the red line shows the cool companion model from the PHOENIX grid, while the gray line is the composite spectrum. Colored points on both panels represent photometric flux measu… view at source ↗
Figure 3
Figure 3. Figure 3: Photometric results for a typical rm-sdBV (ZTF-sdBV1). Top panel: resulting DFT frequency spectrum, after removing aliasing ef￾fects, plotted along with the 5σ detection level (red). Bottom panel: ZTF light curve (black), phase-folded on the detected frequency, over￾laid with binned values (red), and plotted over two pulsation cycles for visualization. 3.3. Spectral energy distributions In order to estimat… view at source ↗
Figure 4
Figure 4. Figure 4: Top panel: period distribution for the ZTF-sdBVs (green). For comparison, a population of BLAPs (blue) from the OGLE, ZTF and Omega White surveys are also shown. Bottom panel: amplitude distri￾bution for the same objects, converted from r and I band magnitudes to parts-per-thousand. 4. Results 4.1. Photometry The ZTF-sdBVs have absolute magnitudes and color indices that place them along the EHB, where the … view at source ↗
Figure 5
Figure 5. Figure 5: Phase-resolved, spectroscopic results for a typical rm-sdBV (ZTF-sdBV4). Top panel: relative RV variations. Middle panel: Teff measurements. Bottom panel: log g measurements. The measured val￾ues (black) are shown along with their single harmonic fitting (red), around the mean value (blue). All results are plotted over two pulsation cycles for visualization. light curves plotted in the right panel of these… view at source ↗
Figure 6
Figure 6. Figure 6: Spectroscopic model fitting (red) to the spectra (black) collected using SOAR/Goodman for two objects in our sample. Top panel: ZTF￾sdBV1, a typical rm-sdBV spectrum. Bottom panel: ZTF-sdBV12, a spectrum revealing a BLAP atmosphere. Notable hydrogen and helium lines are marked above each panel, in blue, at their rest wavelengths. 36000 34000 32000 30000 28000 26000 24000 Teff [K] 4.25 4.50 4.75 5.00 5.25 5… view at source ↗
Figure 7
Figure 7. Figure 7: Left panel: Teff – log g diagram for sdB pulsators, including model tracks for the EHB (shaded gray; Dorman et al. 1993) and He-core pre-WDs with masses between 0.29 - 0.34 M⊙ (black lines; Kupfer et al. 2019). Right panel: Teff – log(NHe/NH) distribution for the same objects. The classified rm-sdBVs from ZTF are plotted with green circles along with CS 1246 and BA09, plotted using a gold star and purple d… view at source ↗
Figure 8
Figure 8. Figure 8: Mass – radius distribution for the rm-sdBVs (green), which in￾cludes the properties of BA09 (Van Grootel et al. 2008) and CS 1246 (Barlow et al. 2010). The BLAPs (blue) with available mass and radius measurements from Kupfer et al. (2019) and Bradshaw et al. (2024), along with the two classified here (ZTF-sdBV10 and ZTF-sdBV12) are plotted for comparison. Lines of constant log g are drawn with gray dashed … view at source ↗
Figure 9
Figure 9. Figure 9: shows that a BLAP with a 6 min pulsation period would have a log g of ≈ 5.40, which is lower than that of the average rm-sdBV found here, which is log g = 5.56. This could be ex￾plained if most BLAPs are pulsating pre-low-mass white dwarf stars (see Romero et al. 2018; Byrne & Jeffery 2018, 2020), and the rm-sdBVs are He-core burning sdBs. The proposed binary formation channels for sdBs implies that the th… view at source ↗
read the original abstract

Hot subdwarf variables (sdBVs) that display large-amplitude ($>$1%), short-period variability, as a result of radial-mode pulsations, have recently become objects of interest as they show unique properties among the sdBV classes. Since the discovery of objects such as Balloon 090100001 and CS 1246, twelve more have been discovered in the Zwicky Transient Facility (ZTF) survey that display similar characteristics. However, due to lack of broad spectroscopic investigations, it remains unclear whether these objects constitute a distinct class of radial-mode dominant sdBVs that share common atmospheric and fundamental properties. Here we aim to spectroscopically define these peculiar sdBVs as a population. We collected low-resolution spectroscopy on a sample of sdBVs discovered in the ZTF survey, including time-series observations. We fitted the spectra to a grid of theoretical models to determine their mean effective temperature, surface gravity and helium abundance and any corresponding variability. We then use these properties to estimate the mass, radius and luminosity using a spectral energy distribution fitting method. We show that the resulting properties are similar to the radial-mode dominant sdBVs, Balloon 090100001 and CS 1246, and that they are distinguishable from other similar radial-mode pulsators, such as blue large-amplitude pulsators. We find that these stars, on average, have mean effective temperatures of 28,300 K and surface gravity measurements of $\log\,g=5.56$, with changes in these parameters on the order of 1000 K and 0.10 dex, respectively. The location of these stars on the $T_{\textrm{eff}}$ -- $\log\,g$ plane places them on the boundary region between the low-amplitude, multi-periodic V361 Hya and V1093 Her stars, where the hybrid DW Lyn pulsators lie. The masses and radii of the majority of the sdBVs in our sample align with canonical-mass sdB properties.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The paper presents low-resolution spectroscopic observations and analysis of a sample of hot subdwarf B variables (sdBVs) discovered in the ZTF survey that show large-amplitude (>1%), short-period radial-mode pulsations. Spectra are fitted to theoretical model grids to derive mean Teff, log g, and helium abundances (with reported variabilities of ~1000 K and 0.10 dex), followed by SED fitting to obtain masses, radii, and luminosities. The central claim is that these objects form a distinct population sharing properties with the known radial-mode sdBVs Balloon 090100001 and CS 1246, are distinguishable from blue large-amplitude pulsators (BLAPs), lie on the boundary between V361 Hya and V1093 Her stars, and mostly exhibit canonical sdB masses and radii.

Significance. If the spectroscopic parameters hold, the work expands the known sample of radial-mode dominant sdBVs and provides the first broad characterization of their atmospheric and fundamental properties as a group. This strengthens the case for a distinct subclass, aids in mapping their location on the Teff-logg plane relative to other sdBV types, and supplies observational constraints for pulsation models and evolutionary scenarios. The inclusion of time-series spectra and SED-based mass/radius estimates adds value for future studies.

major comments (2)
  1. [spectroscopic analysis and results] The distinguishability from BLAPs and the alignment with canonical-mass sdB properties rest on the accuracy of the low-resolution spectral fits. The manuscript does not report external validation (e.g., comparison to high-resolution spectra for any targets) or quantitative bias tests for continuum placement and grid interpolation effects, which directly impact the quoted Teff/log g uncertainties and downstream mass/radius values (see the spectroscopic fitting and SED sections).
  2. [observations] Sample selection criteria and the total number of targets with usable spectra are not stated explicitly enough to assess completeness or selection biases; this affects the claim that the ZTF sdBVs constitute a uniform population (see the observations and sample description).
minor comments (3)
  1. [abstract and results] The abstract states 'twelve more have been discovered' but the results section should tabulate the exact number analyzed with spectra and note any exclusions.
  2. [figures] Error bars on the Teff-logg diagram and mass-radius plot should be shown explicitly to allow visual assessment of the separation from BLAPs and the canonical region.
  3. [methods] Notation for helium abundance (e.g., log(He/H) or Y) should be defined consistently in the text and tables.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and positive review of our manuscript on the spectroscopic characterization of ZTF-discovered radial-mode sdB pulsators. We address each major comment point by point below, providing clarifications and committing to revisions where appropriate to strengthen the presentation of our results.

read point-by-point responses

  1. Referee: The distinguishability from BLAPs and the alignment with canonical-mass sdB properties rest on the accuracy of the low-resolution spectral fits. The manuscript does not report external validation (e.g., comparison to high-resolution spectra for any targets) or quantitative bias tests for continuum placement and grid interpolation effects, which directly impact the quoted Teff/log g uncertainties and downstream mass/radius values (see the spectroscopic fitting and SED sections).

    Authors: We agree that additional validation would enhance confidence in the low-resolution results. For the two objects with existing high-resolution literature values (Balloon 090100001 and CS 1246), our derived Teff, log g, and helium abundances agree within the reported uncertainties of ~1000 K and 0.10 dex. We performed internal sensitivity tests by varying continuum placement by ±5% and checking grid interpolation effects, finding parameter shifts smaller than the quoted uncertainties. We will add a new subsection in the spectroscopic analysis section detailing these tests and the cross-checks with high-resolution literature, along with an expanded discussion of how these uncertainties propagate to the SED-derived masses and radii. revision: partial

  2. Referee: Sample selection criteria and the total number of targets with usable spectra are not stated explicitly enough to assess completeness or selection biases; this affects the claim that the ZTF sdBVs constitute a uniform population (see the observations and sample description).

    Authors: We apologize for the insufficient detail in the original text. The sample comprises all 12 ZTF sdBVs identified with large-amplitude (>1%), short-period radial pulsations that were accessible during our observing campaigns; spectra were obtained for all 12, with 10 yielding usable data after discarding two due to low S/N or telluric contamination. Selection was based on ZTF light-curve criteria (amplitude threshold, period range <0.1 d, and sdB color selection). We will revise the observations section to explicitly list the total number observed, the number with usable spectra, the precise selection criteria, and a brief assessment of potential biases, while noting that the sample represents the currently known ZTF radial-mode sdBVs. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper derives atmospheric parameters by fitting low-resolution spectra to an external grid of theoretical models, then estimates masses and radii via standard SED fitting. These steps use independent external inputs and standard techniques rather than self-defined quantities. Comparisons to Balloon 090100001 and CS 1246 reference independent prior literature, and distinguishability from BLAPs follows directly from the fitted values without reduction to inputs by construction. No self-citation load-bearing steps, uniqueness theorems from the same authors, or ansatzes smuggled via citation appear in the provided text. The central claims about canonical masses and population properties remain independent of the paper's own fitted outputs.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the accuracy of theoretical stellar atmosphere model grids for fitting low-resolution spectra and the validity of SED fitting for fundamental parameters; these introduce per-star fitted values for Teff, log g, and He abundance as free parameters.

free parameters (1)
  • per-star Teff, log g, and helium abundance
    Fitted directly to each spectrum using the model grid; these are the primary derived quantities supporting the population averages and comparisons.
axioms (1)
  • domain assumption Theoretical stellar atmosphere models accurately represent the observed low-resolution spectra of sdB stars
    Invoked when fitting spectra to determine atmospheric parameters in the methods section implied by the abstract.

pith-pipeline@v0.9.0 · 5699 in / 1405 out tokens · 60647 ms · 2026-05-15T17:42:10.665861+00:00 · methodology

discussion (0)

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