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arxiv: 2605.24871 · v1 · pith:YRJ5I7NU · submitted 2026-05-24 · astro-ph.SR

High-precision K2/TESS Photometry and HERMES Spectroscopy of Four Bright Field Stars

Otto Trust , Santosh Joshi , Benard Nsamba , Sharon Aol , Owen Vermeulen , Ronald Ssembatya , Peter De Cat , Patricia Lampens
show 8 more authors
Sydney A. Barnes Partha P. Goswami Athul Dileep Surath C. Ghosh Pramod S. Kumar Edward Jurua Mrinmoy Sarkar Sarabjeet S. Bedi
This is my paper

Reviewed by Pith2026-06-30 00:15 UTCgrok-4.3pith:YRJ5I7NUopen to challenge →

classification astro-ph.SR
keywords stellar variabilitygamma Doradus starsAm starsK2 photometryTESS photometryHERMES spectroscopystellar abundancesevolutionary modeling
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The pith

Space photometry and spectroscopy identify a new gamma Doradus g-mode pulsator and a rotating Am star among four bright field stars.

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

This paper combines legacy ground-based photometry with K2 and TESS time-series data and high-resolution HERMES spectroscopy to determine the variability, chemical abundances, and evolutionary parameters of four previously poorly characterized bright stars. Frequency analysis shows distinct low-amplitude behaviors across the sample, including a 1.5-day signal in one star and multiple low-frequency signals in another. Spectroscopic results classify one star as chemically peculiar with an Am pattern while the others appear normal, after which grid-based modeling supplies masses, radii, and ages. These updated classifications turn ambiguous objects into usable targets for further stellar astrophysics work.

Core claim

The frequency analysis of the photometric data reveals a diversity of low-amplitude variability across the sample. HD 73135 shows a persistent modulation near 1.5 days that is most consistent with rotation, although an ellipsoidal-binary interpretation cannot yet be excluded, while BD +19° 2045 exhibits multiple low-frequency signals and is identified as a new candidate gamma Doradus g-mode pulsator. BD +19° 2046 and TYC 1395-855-1 are non-variable or only marginally variable in K2 but display coherent low-frequency modulation in TESS of uncertain origin. Spectroscopically, HD 73135 is the only chemically peculiar star in the sample and shows a clear Am abundance pattern, whereas the other t

What carries the argument

Frequency analysis of combined K2 and TESS photometric time series, paired with high-resolution spectroscopic abundance determination and grid-based evolutionary modeling.

If this is right

  • HD 73135 supplies a reference case for studying rotational effects in Am stars.
  • BD +19° 2045 adds a new target for detailed g-mode asteroseismology in gamma Doradus stars.
  • The derived masses, radii, and ages for the sample can be added to catalogs of field stars for population studies.
  • The reclassification shows how space photometry resolves variability questions left open by ground-based surveys.

Where Pith is reading between the lines

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

  • The same photometry-plus-spectroscopy workflow could be applied to remaining targets from the original ground-based survey to find additional variables.
  • Direct comparison of the TESS and K2 light curves for the same stars offers a test of whether low-frequency signals are consistent across different instruments.
  • If the g-mode assignment holds, the star provides an opportunity to constrain mixing processes in intermediate-mass stellar interiors.

Load-bearing premise

The observed photometric signals are assumed to arise from intrinsic single-star variability rather than undetected binary companions or instrumental artifacts.

What would settle it

Radial-velocity time series spanning multiple weeks that would detect or exclude orbital motion at the 1.5-day period in HD 73135.

Figures

Figures reproduced from arXiv: 2605.24871 by Athul Dileep, Benard Nsamba, Edward Jurua, Mrinmoy Sarkar, Otto Trust, Owen Vermeulen, Partha P. Goswami, Patricia Lampens, Peter De Cat, Pramod S. Kumar, Ronald Ssembatya, Santosh Joshi, Sarabjeet S. Bedi, Sharon Aol, Surath C. Ghosh, Sydney A. Barnes.

Figure 1
Figure 1. Figure 1: Lomb–Scargle periodograms of the ground‑based Johnson 𝐵 (left) and 𝑉 (right) light curves. Top panels: HD 73135; bottom panels: BD +19◦ 2045. The ordinate is amplitude (magnitudes); the dashed line marks the SNR = 4.0 detection threshold, computed from a running￾median noise estimate. Only the B-band spectra show peaks that reach the threshold, clustered near ∼1–2 d−1 (and harmonics) consistent with diurna… view at source ↗
Figure 2
Figure 2. Figure 2: Left panels: Long cadence PDC light curves (blue dots) are overplotted with models including the significant frequencies (red line) for the four stars studied in this paper during the 2015 K2 Campaign 5. Middle panel: Lomb-Scargle periodograms. The period distribution (blue) has been plotted on a log-linear plot to aid visualization. A signal-to-noise ratio (SNR) of 4.0 is indicated by the black dashed hor… view at source ↗
Figure 3
Figure 3. Figure 3: Period diagnostics for the K2 C05 light curve of HD 73135. Top: light curve flux (left) and autocorrelation function (ACF; right). Middle-left: Morlet wavelet power spectrum (log10 power) computed on 200 logarithmically spaced periods spanning 0.5–30 d using symmetric reflection padding; the dashed curve marks the cone of influence (COI; Torrence & Compo 1998) and the hatched region indicates where edge ef… view at source ↗
Figure 4
Figure 4. Figure 4: The H𝛽 (left) and Mg I triplet (right) regions of the spectra for the target stars. In each panel, the observed spectrum (black) is overplotted with the best-fitting synthetic spectrum (red). All spectra have been shifted to the stellar rest frame using the measured radial velocities ( [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: Hertzsprung–Russell diagram showing the positions of the sam￾ple stars. Solid green lines represent MIST evolutionary tracks for masses between 1.2 M⊙ and 2.4 M⊙. Blue and red dashed lines mark the empiri￾cal boundaries of the 𝛿 Scuti instability strip, as defined by Murphy et al. (2019). REFERENCES Aerts C., Christensen-Dalsgaard J., Kurtz D. W., 2010, Asteroseismology Aigrain S., Hodgkin S. T., Irwin M. … view at source ↗
read the original abstract

Space-based photometry from K2 and TESS, coupled with high-resolution spectroscopy, provides a unique opportunity to re-examine the variability and chemical nature of four bright stars (HD 73135, BD +19$^\circ$ 2045, BD +19$^\circ$ 2046, and TYC 1395-855-1), which were poorly characterised by the original ground-based campaign under the Nainital-Cape survey. We aim to establish the nature of the variability, chemical properties, and evolutionary status of these stars. We analysed ground-based Johnson $BV$ photometry combined with K2 and TESS time-series data to establish the nature of their variability. Using high-resolution HERMES spectroscopy, we determined their spectral classifications and chemical abundances. In addition, we used the inferred spectroscopic constraints with grid-based evolutionary modelling to derive their corresponding masses, radii, and ages. The frequency analysis reveals a diversity of low-amplitude variability across the sample. HD 73135 shows a persistent modulation near 1.5 d that is most consistent with rotation, although an ellipsoidal-binary interpretation cannot yet be excluded, while BD +19$^\circ$ 2045 exhibits multiple low-frequency signals and is identified as a new candidate $\gamma$ Doradus $g$-mode pulsator. BD +19$^\circ$ 2046 and TYC 1395-855-1 are non-variable or only marginally variable in K2 but display coherent low-frequency modulation in TESS of uncertain origin. Spectroscopically, HD 73135 is the only chemically peculiar star in the sample and shows a clear Am abundance pattern, whereas the other three stars are chemically normal. These results demonstrate the value of combining legacy survey data with contemporary photometric and spectroscopic analysis.

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 / 2 minor

Summary. The paper re-examines four bright field stars (HD 73135, BD +19° 2045, BD +19° 2046, TYC 1395-855-1) using combined K2/TESS photometry and HERMES spectroscopy. It reports a 1.5 d modulation in HD 73135 most consistent with rotation (ellipsoidal binary not excluded), multiple low-frequency signals in BD +19° 2045 interpreted as γ Dor g-modes, uncertain low-frequency signals in the remaining two stars, and chemically peculiar (Am) status only for HD 73135, with masses/radii/ages from spectroscopic constraints and grid-based modeling.

Significance. If the variability classifications are robust, the work adds four well-characterized bright stars to the literature on low-amplitude variability and demonstrates the value of legacy survey follow-up with space photometry and high-resolution spectroscopy. The new γ Dor candidate and confirmation of the Am star are useful additions, though the provisional nature of two of the four classifications reduces the overall impact.

major comments (2)
  1. [Frequency analysis] Frequency analysis section (abstract and main text): The interpretation of the 1.5 d signal in HD 73135 as rotation rests on the assumption that it is intrinsic single-star variability, yet the text explicitly states an ellipsoidal-binary origin cannot be excluded; no radial-velocity time series, binary modeling, or artifact-injection tests are described to discriminate scenarios.
  2. [Frequency analysis] Frequency analysis section: The classification of BD +19° 2045 as a γ Doradus g-mode pulsator is based on multiple low-frequency signals, but the manuscript provides no details on frequency extraction (e.g., pre-whitening procedure), amplitude uncertainties, data exclusion criteria, or quantitative comparison to g-mode templates versus alternative origins such as instrumental artifacts or undetected companions.
minor comments (2)
  1. The abstract refers to 'grid-based evolutionary modelling' without naming the specific grids, codes, or input physics used to derive masses, radii, and ages from the spectroscopic constraints.
  2. No error bars or formal uncertainties are mentioned for the reported frequencies, amplitudes, or abundance values in the summary of results.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the constructive comments on the frequency analysis. We respond to each major comment below and indicate where revisions will be made.

read point-by-point responses

  1. Referee: [Frequency analysis] Frequency analysis section (abstract and main text): The interpretation of the 1.5 d signal in HD 73135 as rotation rests on the assumption that it is intrinsic single-star variability, yet the text explicitly states an ellipsoidal-binary origin cannot be excluded; no radial-velocity time series, binary modeling, or artifact-injection tests are described to discriminate scenarios.

    Authors: We agree that the data do not permit a definitive exclusion of an ellipsoidal-binary origin, as already stated in the manuscript. The HERMES spectra are single-epoch only and provide no radial-velocity time series. No binary modeling or artifact-injection tests were performed because the 1.5 d signal is coherent and present in both K2 and TESS photometry. We will add an explicit statement in the discussion reiterating this limitation and the need for future multi-epoch RV observations. revision: partial

  2. Referee: [Frequency analysis] Frequency analysis section: The classification of BD +19° 2045 as a γ Doradus g-mode pulsator is based on multiple low-frequency signals, but the manuscript provides no details on frequency extraction (e.g., pre-whitening procedure), amplitude uncertainties, data exclusion criteria, or quantitative comparison to g-mode templates versus alternative origins such as instrumental artifacts or undetected companions.

    Authors: We accept that the frequency-analysis description is insufficiently detailed. In the revised manuscript we will expand the relevant section to specify the pre-whitening procedure (using PERIOD04), the adopted significance threshold (amplitudes exceeding four times the local noise), outlier rejection criteria, and a direct comparison of the observed frequencies and spacings against theoretical γ Dor g-mode expectations. These additions will better support the pulsational classification over alternative explanations. revision: yes

standing simulated objections not resolved
  • No multi-epoch radial-velocity time series are available from the existing HERMES observations to test the binary hypothesis for HD 73135.

Circularity Check

0 steps flagged

No significant circularity; empirical classification against external templates

full rationale

The paper performs direct frequency analysis of K2/TESS photometry and HERMES spectroscopy on four stars, then assigns variability types (rotation for HD 73135; candidate γ Dor g-modes for BD +19° 2045) by comparison to established observational templates. No equations, fitted parameters, or self-citations are used to generate predictions that reduce to the inputs by construction. The abstract explicitly flags the ellipsoidal-binary alternative as unexcluded, confirming the assignments are provisional comparisons rather than derived results. This is standard observational work with no load-bearing self-referential steps.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The abstract provides no explicit free parameters, axioms, or invented entities; all inferences rest on standard stellar classification templates and evolutionary grids from prior literature.

pith-pipeline@v0.9.1-grok · 5933 in / 1286 out tokens · 11341 ms · 2026-06-30T00:15:47.483670+00:00 · methodology

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Reference graph

Works this paper leans on

9 extracted references · 1 canonical work pages · 1 internal anchor

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    iSpec: An integrated software framework for the analysis of stellar spectra

    Aerts C., Christensen-Dalsgaard J., Kurtz D. W., 2010, Asteroseismology Aigrain S., Hodgkin S. T., Irwin M. J., Lewis J. R., Roberts S. J., 2015, MNRAS , 447, 2880 Aigrain S., Parviainen H., Pope B. J. S., 2016, MNRAS , 459, 2408 Andrievsky S. M., Spite M., Korotin S. A., Spite F., François P ., Bonifacio P ., Cayrel R., Hill V ., 2009, A&A , 494, 1083 As...

    work page internal anchor Pith review Pith/arXiv arXiv doi:10.1017/cbo9781316036570 2010

  2. [2]

    Figure A2

    Note that BD +19 ◦ 2046 was not observed during this campaign. Figure A2. Same as Fig. 2 but for BD+19 ◦ 2045 short cadence K2 data. MNRAS 000, 1–15 (2026) K2/TESS–HERMES study of bright field stars 17 Figure A3. Same as Fig. 2 but for TESS data. MNRAS 000, 1–15 (2026) 18 Trust et al. Figure B1. Same as for HD 73135 in Fig. 3 but for campaign 18 long cade...

    2046

  3. [3]

    Same as for HD 73135 in Fig

    MNRAS 000, 1–15 (2026) K2/TESS–HERMES study of bright field stars 21 Figure B7. Same as for HD 73135 in Fig. 3 but for TYC 1395-855-1 campaign

    2026

  4. [4]

    Figure B9

    MNRAS 000, 1–15 (2026) 22 Trust et al. Figure B9. Same as for HD 73135 in Fig. 3 but for TESS Sector

    2026

  5. [5]

    Same as for HD 73135 in Fig

    Figure B10. Same as for HD 73135 in Fig. 3 but for BD +19° 2045 TESS Sector

    2045

  6. [6]

    Same as for HD 73135 in Fig

    MNRAS 000, 1–15 (2026) K2/TESS–HERMES study of bright field stars 23 Figure B11. Same as for HD 73135 in Fig. 3 but for BD +19° 2046 TESS Sector

    2026

  7. [7]

    Same as for HD 73135 in Fig

    Figure B12. Same as for HD 73135 in Fig. 3 but for BD +19 ◦ 2046 TESS Sector

    2046

  8. [8]

    Figure B13

    MNRAS 000, 1–15 (2026) 24 Trust et al. Figure B13. Same as for HD 73135 in Fig. 3 but for TYC 1395-855-1 TESS Sector

    2026

  9. [9]

    MNRAS 000, 1–15 (2026)

    2026