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astro-ph.SR · Solar and Stellar Astrophysicsastro-ph.EP · Earth and Planetary Astrophysicsastro-ph.GA · Astrophysics of Galaxies

Rotation alone does not explain CPV youth: M dwarfs (P < 2 d, d < 150 pc) in groups older than 60 Myr have 3 complex periodic variables where 30 are expected for their period, colour, T and sectors

Question: Complex periodic variables (CPVs) are M dwarfs with sharp flux dips that repeat at the rotation period. Almost all known CPVs are younger than about 150 Myr. Bouma et al. (2024; B24) suggested that CPV occurrence decreases from about 3% at 10 Myr to 0% at 700 Myr. CPVs rotate fast, and young M dwarfs rotate fast. We tested if the decrease is only a rotation effect. Approach: We used our TESS FFI census (153 CPVs among 46,947 M dwarfs with P < 2 d within 150 pc; sci:sha256:1d6fc89a165c50c6965c1ad5986d078f5391ad141e23ee7afb530925104a8f0d). BANYAN Sigma gave moving-group membership and age for 6,000 stars (77 CPVs). We fit a logistic model of CPV detection against period, BP-RP, T magnitude and number of TESS sectors to members younger than 60 Myr. Then we calculated the expected CPV count in older groups and did Poisson-binomial tests. Findings: The raw CPV fraction is 1.70% (55/3244) at < 20 Myr, 1.52% (19/1252) at 20-60 Myr, 0.22% (3/1348) at 60-200 Myr and 0% (0/156) at > 200 Myr. Groups older than 60 Myr have 3 CPVs, but the model expects 30.1 (95%: 23.5-41.2). O/E = 0.10 (0.02-0.29), p = 1e-8. Each old group with 2 or more expected CPVs (Pleiades, Psc-Eri, AB Dor, Carina-Near, Hyades) has O/E < 1. At P = 0.25-0.5 d, without a model, the fraction is 6.4%, 3.1% and 0.2% in the first three age bins. Direct standardisation, a group bootstrap, 14 model and sample variants and 2 membership-purity checks all give O/E <= 0.19. At fixed rotation, the fraction also decreases at 20-60 Myr (O/E = 0.37, p = 3e-4). The raw fractions are almost equal only because 20-60 Myr stars rotate faster. Field stars have O/E = 0.11, and 65 of the 76 field CPVs are more than 1 mag above the main sequence. Thus fast rotation is necessary but not sufficient for a CPV. A second property that decreases with age also controls CPV occurrence. This agrees with the B24 idea of a finite supply of dust. Limitations: We measure the detectable fraction, not the true fraction. Memberships and ages come from catalogues. Field stars in the groups can decrease O/E, but members that a second catalogue confirms give the same result. With only radial-velocity members, the 20-60 Myr decrease is not significant (p = 0.11). The model fit is poor at the brightest and faintest T. The CPV vetting had only one vetter. The package contains the code, tables, figures and reproduction steps.

complex periodic variables · M dwarfs · stellar age · stellar rotation · young moving groups · BANYAN Sigma · Pleiades · Hyades · AB Doradus moving group · Sco-Cen · TESS full-frame images · TARS light curves · Gaia DR3 · occurrence rate · logistic regression · Poisson-binomial test · direct standardisation · circumstellar dust · scallop-shell stars · selection effects

Admitted 4 citing publications

References

16
  1. supportsdoi:10.3847/1538-3881/ad0c4c

    B24 wrote that CPV occurrence seems to decrease from about 3% at 10 Myr to about 1% at 100 Myr and 0% at 700 Myr. They noted that M dwarfs spin up over the first 100 Myr, thus rotation alone predicts more CPVs at 100 Myr, and they suggested a finite supply of dust. Our test at fixed period, colour, T and sectors shows that rotation does not explain the decrease (O/E = 0.10 above 60 Myr). Our 60-200 Myr fraction (0.22%, 95%: 0.06-0.59%) is lower than their 1%, but the denominators and detection methods are different.

  2. usesTESS FFI search of 46,947 fast-rotating red dwarfs within 150 pc finds 104 new complex periodic variable candidates; the census grows 1.2-1.5x within 50 pc and 2.7-2.9x within 100 pc, not 2x and 10x sci:sha256:1d6fc89a165c50c6965c1ad5986d078f5391ad141e23ee7afb530925104a8f0d

    Our TESS FFI census. We use its 46,947 parent stars, their periods, sectors and T magnitudes, and its 153 CPVs.

  3. usesdoi:10.3847/1538-4365/ae6657

    TARS paper. Source of the FFI light curves and rotation periods of the parent sample.

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    TARS HLSP at MAST. The parent census read its FFI light curves.

  5. usesdoi:10.5281/zenodo.19917941

    TARS Table 2. Source of the periods, Gaia identifiers and contamination flags of the parent stars.

  6. usesdoi:10.1117/1.JATIS.1.1.014003

    TESS mission. Source of all photometry.

  7. usesdoi:10.1051/0004-6361/202243940

    Gaia DR3. We use positions, parallaxes, proper motions, radial velocities, RUWE, G and BP-RP.

  8. usesdoi:10.3847/1538-4357/aaae09

    BANYAN Sigma. We use its code and models for group membership, and its Table 1 ages for 27 groups.

  9. usesdoi:10.1051/0004-6361/202346285

    Hunt and Reffert 2023 cluster members and ages. We use them to confirm members and for one robustness variant.

  10. usesdoi:10.3847/1538-4357/ac0251

    Kerr et al. 2021 young groups. We use them to confirm members.

  11. usesdoi:10.1088/0067-0049/208/1/9

    Pecaut and Mamajek 2013 dwarf sequence (version 2022.04.16). We use it to calculate the height above the main sequence.

  12. usesdoi:10.3847/1538-4357/aadaed

    Age of the Volans-Carina group (89 Myr).

  13. usesdoi:10.3847/1538-4357/aaee66

    Ages of the Argus association (45 Myr) and the chi1 For cluster (35 Myr).

  14. usesdoi:10.3847/1538-4357/abb77e

    Age of the mu Tau association (62 Myr).

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    Age of the Pisces-Eridanus stream (120 Myr).

  16. usesdoi:10.1051/0004-6361/202141366

    chi1 For cluster DANCe study. One of our two sources for the chi1 For cluster age (35 Myr); the BANYAN Sigma table gives about 500 Myr.

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