arXiv subjects

Constantine P. Deliyannis

Publications and source records attributed to Constantine P. Deliyannis.

At least 19 recordsLinked to original sources

arXiv2025

WOCS XCIII: NGC 7789: the Evolution of Li, Stellar Rotation, and Extended Main Sequence Turnoffs

Precision UBVRI photometry of NGC 7789 is combined with Gaia data to map reddening variations across the cluster face. HYDRA spectra, Gaia astrometry, and isochrone fitting constrain the absolute reddening, apparent modulus, and age to E(B-V) = 0.30 +/- 0.02, (m-M)=12.51 +/- 0.06, and 1.46 +/- 0.02 Gyr for [Fe/H] between -0.2 and solar; the spectroscopic [Fe/H] = -0.13 +/- 0.068 (MAD) from 156 single-star members. Corrections for variable reddening reduce the scatter in the unevolved main sequence below the turnoff. A(Li) is derived for only single star members from the G-dwarf Li-Plateau to the tip of the red giant branch. Giants separate into two distinct groups, probable first-ascent giants with detectable Li that declines with evolution toward the red giant tip and stars within the clump and the asymptotic giant branch which only exhibit upper limits. A(Li) structure from the turnoff to the unevolved main sequence, including the Li-Dip, and the presence of an extended color spread among the upper main sequence stars are attributed to the V_ROT distribution, indicating the wall of the Li-Dip as the true hot boundary of the Kraft break. Differences in the color-magnitude diagram topology of NGC 7789 and NGC 752 are explored and attributed to differences in the individual cluster V_ROT distributions. Prior indications that main sequence stars more massive than the Li-Dip evolve redward across the Li-Wall, undergoing rotational spindown and Li depletion like stars within the Li-Dip, are confirmed.

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arXiv2025

Lithium Abundances of Main-Sequence Stars in the Old Open Cluster NGC 188: Probes of Stellar Evolution Beyond the Solar Age

We present lithium abundances for 119 main-sequence stars in the old open cluster NGC 188 (age = 6.3 Gyr), using high-resolution, high signal-to-noise ratio spectra from WIYN/Hydra. We observe the stars over multiple nights and measure radial velocities for each night, which we combine with Gaia proper motions to identify multiplicity and cluster membership. We identify 95 single members, 14 binary members, 9 members with uncertain multiplicity, and 1 single likely member. We determine effective temperatures using empirical color-temperature relations, surface gravities from isochrones, and microturbulence from empirical relations for main-sequence stars. Our sample includes G and K dwarfs with temperatures between 6000 and 5300 K, which expands significantly on earlier observations. We find that lithium abundances in NGC 188 are lower than predictions from standard stellar evolution theory. As expected, stars in NGC 188 are more lithium-depleted than those in the younger Hyades and Praesepe clusters (650 Myr). However, their abundances are higher, or at least comparable, to those in the slightly younger cluster M67 (4.5 Gyr), challenging the idea that older stars have lower lithium than younger ones. Lithium depletion may depend on factors beyond age and mass, such as metallicity.

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arXiv2025

Subgiants in NGC 188 Reveal that Rotationally Induced Mixing Creates the Main Sequence Li-Dip

The "Li-Dip" is an unexpected, striking, and highly non-standard anomaly of severe lithium depletion observed in mid-F dwarf stars, which has puzzled astronomers for nearly 40 years. Mechanisms proposed to explain the Li-Dip include effects related to rotation, magnetic fields, diffusion, gravity waves, and mass loss. The critical question became, which, if any, might be realistic? Here we show that mixing due to shear induced by stellar angular momentum loss is the unique mechanism driving the Li depletion. Each mechanism leaves a different signature in the subsurface Li distribution. The deepening surface convection zones of subgiants of NGC 188 evolving out of the Li-Dip dredge up the sub-surface material and thus reveal the signature of the responsible mechanism, rotation. Beryllium and boron data have also favored rotational mixing; however, these elements can be extremely difficult or impossible to observe. Our highly complementary approach provides fresh and very feasible perspectives on using Li to probe poorly understood physical mechanisms acting below the stellar surface, thereby improving fundamental understanding of stellar evolution. Rotational mixing may be the dominant mechanism that depletes Li in a wide range of Solar-type stars, including in the Sun. Possible connections to Big Bang Nucleosynthesis are discussed.

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arXiv2024

Li Evolution Among Stars of Low/Intermediate Mass: The Metal-Deficient Open Cluster, NGC 2204

We have analyzed high-dispersion spectra in the Li 6708 Angstrom region for 167 stars within the anticenter cluster, NGC 2204. From 105 probable members, abundance analysis of 45 evolved stars produces [Fe/H] = -0.40 +/- 0.12, [Si/Fe] = 0.14 +/- 0.12, [Ca/Fe] = 0.29 +/- 0.07, and [Ni/Fe] = -0.12 +/- 0.10, where quoted errors are standard deviations. With E(B-V) = 0.07 and (m-M)_0 = 13.12, appropriate isochrones provide an excellent match from the main sequence through the tip of the giant branch for an age of 1.85 +/- 0.05 Gyr. Li spectrum synthesis produces A(Li) below 1.4 at the base of the red giant branch to a detectable value of -0.4 at the tip. Six probable AGB stars and all but one red clump star have only Li upper limits. A rapidly rotating red giant is identified as a possible Li-rich giant, assuming it is a red clump star. Main sequence turnoff stars have a well-defined A(Li) = 2.83 +/- 0.03 (sem) down to the Li-dip wall at the predicted mass of 1.29 solar masses. Despite the same isochronal age as the more metal-rich NGC 2506, the red giant luminosity distribution reflects a younger morphology similar to NGC 7789, possibly indicating a deeper metallicity impact on stellar structure and A(Li) than previously assumed. As in NGC 2506 and NGC 7789, the NGC 2204 turnoff exhibits a broad range of rotation speeds, making abundance estimation impossible for some stars. The cluster place within Galactic A(Li) evolution is discussed.

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arXiv2024

Primordial Lithium from Globular Cluster Turn-Off Stars: M13 AND M71

During Big Bang nucleosynthesis (BBN) in the first 15 minutes of the Universe some $^7$Li was created along with isotopes of H and He. The determination of that primordial value of Li can help constrain the conditions at that time. The oldest stars with known ages can be found in globular clusters which have well-determined ages through stellar evolution models. High-resolution spectra have been obtained with the Keck I telescope and HIRES of Li in several unevolved stars in the clusters M13 and M71 with V magnitudes of 17.6 -- 17.9. Abundances of Li have been determined with spectrum synthesis techniques and show a range of a factor of 4. We attribute that spread to differences in initial angular momentum resulting in different amounts of spin-down, related mixing, and destruction of Li. Our results are compared with similar results for main-sequences and turn-off stars in other globular clusters. The range in age of these clusters is 11.2 to 14.2 Gyr for an age span of 3 Gyr. These clusters range in [Fe/H] from $-$0.75 to $-$2.24 corresponding to a factor of 30 in metallicity. The maximum in the Li abundance for these unevolved stars in all eight clusters is the same corresponding to Li/H = 3.16 x 10$^{-10}$ while the predicted Li abundance, based on the deuterium abundance and the BBN predictions, is 5.24 x 10$^{-10}$.

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arXiv2023

Lithium in Turn-off Stars in the Globular Cluster M5: A Quest for Primordial Lithium

The light element lithium is formed by nucleosynthesis during the Big Bang. Its abundance can help to define the parameters of the early universe. To find this primordial value, it is necessary to determine Li abundances in the oldest stars because it is readily destroyed by nuclear reactions in stellar interiors. We have made high-resolution (~45,000) spectroscopic observations of five identical unevolved turn-off stars in the 13 Gyr old globular cluster M5. In our analysis we find a range in Li abundance of a factor of two; the spread is five times the individual error. The comparison of these results with those for turn-off stars from five other globular clusters reveals similarly large range in Li. Lithium in M5 and the other clusters all have stars above the field star Li plateau, but none are as high as the predictions for primordial Li. The maximum values for Li are the same in all six clusters. Multiple generations of stars are found in many globular clusters; those later generations are expected to have formed from Li-depleted gas. Such second- and later-generation stars would have no Li. However, only one of the six clusters has a few unevolved stars with upper limits on the Li abundance.

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arXiv2023

WIYN Open Cluster Study 89. M48 (NGC 2548) 2: Lithium Abundances in the 420 Myr Open Cluster M48 From Giants Through K Dwarfs

We consider WIYN/Hydra spectra of 329 photometric candidate members of the 420-Myr-old open cluster M48, and report Lithium detections or upper limits for 234 members and likely members. The 171 single members define a number of notable Li-mass trends, some delineated even more clearly than in Hyades/Praesepe: The giants are consistent with subgiant Li dilution and prior MS Li depletion due to rotational mixing. A dwarfs (8600-7700K) have upper limits higher than the presumed initial cluster Li abundance. Two of five late A dwarfs (7700- 7200K) are Li-rich, possibly due to diffusion, planetesimal accretion, and/or engulfment of hydrogen-poor planets. Early F dwarfs already show evidence of Li depletion seen in older clusters. The Li-$T_{\rm eff}$ trends of the Li Dip (6675-6200K), Li Plateau (6200-6000K), and G and K dwarfs (6000-4000K) are very clearly delineated and are intermediate to those of the 120-Myr-old Pleiades and 650-Myr-old Hyades/Praesepe, which suggests a sequence of Li depletion with age. The cool side of the Li Dip is especially well-defined with little scatter. The Li-$T_{\rm eff}$ trend is very tight in the Li Plateau and early G dwarfs, but scatter increases gradually for cooler dwarfs. These patterns support and constrain models of the universally dominant Li depletion mechanism for FGK dwarfs, namely rotational mixing due to angular momentum loss; we discuss how diffusion and gravity-wave driven mixing may also play roles. For late-G/K dwarfs, faster rotators show higher Li than slower rotators, and we discuss possible connections between angular momentum loss and Li depletion.

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arXiv2023

A uvbyCaHbeta CCD Analysis of the Open Cluster Standard, M67,and its Relation to NGC 752

Precision CCD uvbyCaHbeta photometry is presented of the old cluster, M67, covering one square degree with typical internal precision at the 0.005-0.020 mag level to V~17. The photometry is calibrated using standards over a wide range in luminosity and temperature from NGC 752 and zeroed to the standard system via published photoelectric observations. Relative to NGC 752, differential offsets in reddening and metallicity are derived using astrometric members, supplemented by radial-velocity information. From single-star members, offsets in the sense (M67 - NGC 752) are Delta E(b-y) = -0.005 +/-0.001 (sem) mag from 327 F/G dwarfs and Delta [Fe/H] = 0.062 +/- 0.006 (sem) dex from the combined m1 and hk indices of 249 F dwarfs, leading to E(b-y) = 0.021 +/- 0.004 (sem), and [Fe/H] = +0.030 +/- 0.016 (sem) for M67, assuming [Fe/H]{Hyades} = +0.12. With probable binaries eliminated using c1,(b-y) indices, 83 members with relative parallax errors < 0.02 generate (m-M)_0 = 8.220 +/- 0.005 (sem) for NGC 752 and an isochronal age of 1.45 +/- 0.05 Gyr. Using the same parallax restriction for 312 stars, M67 has (m-M) = 9.77 +/- 0.02 (sem), leading to an age tied solely to the luminosity of the subgiant branch of 3.70 +/- 0.03 Gyr. The turnoff color spread implies +/- 0.1 Gyr, but the turnoff morphology defines a younger age/higher mass for the stars, consistent with recent binary analysis and broad-band photometry indicating possible missing physics in the isochrones. Anomalous stars positioned blueward of the turnoff are discussed.

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arXiv2022

Evolved Eclipsing Binaries and the Age of the Open Cluster NGC 752

We present analyses of improved photometric and spectroscopic observations for two detached eclipsing binaries at the turnoff of the open cluster NGC 752: the 1.01 day binary DS And and the 15.53 d BD $+$37 410. For DS And, we find $M_1 = 1.692\pm0.004\pm0.010 M_\odot$, $R_1 = 2.185\pm0.004\pm0.008 R_\odot$, $M_2 = 1.184\pm0.001\pm0.003 M_\odot$, and $R_2 = 1.200\pm0.003\pm0.005 R_\odot$. We either confirm or newly identify unusual characteristics of both stars in the binary: the primary star is found to be slightly hotter than the main sequence turn off and there is a more substantial discrepancy in its luminosity compared to models (model luminosities are too large by about 40%), while the secondary star is oversized and cooler compared to other main sequence stars in the same cluster. The evidence points to non-standard evolution for both stars, but most plausible paths cannot explain the low luminosity of the primary star. BD $+$37 410 only has one eclipse per cycle, but extensive spectroscopic observations and the TESS light curve constrain the stellar masses well: $M_1 = 1.717\pm0.011 M_\odot$ and $M_2 = 1.175\pm0.005 M_\odot$. The radius of the main sequence primary star near $2.9R_\odot$ definitively requires large convective core overshooting ($> 0.2$ pressure scale heights) in models for its mass, and multiple lines of evidence point toward an age of $1.61\pm0.03\pm0.05$ Gyr (statistical and systematic uncertainties). Because NGC 752 is currently undergoing the transition from non-degenerate to degenerate He ignition of its red clump stars, BD $+$37 410 A directly constrains the star mass where this transition occurs.

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arXiv2022

Lithium and Beryllium in One Solar Mass Stars

The surface content of lithium (Li) and beryllium (Be) in stars can reveal important information about the temperature structure and physical processes in their interior regions. This study focuses on solar-type stars with a sample that is more precisely defined than done previously. Our selection of stars studied for Be is constrained by five parameters: mass, temperature, surface gravity, metallicity, and age to be similar to the Sun and is focussed on stars within +-0.02 of 1 M_sun. We have used the Keck I telescope with HIRES to obtain spectra of the Be II spectral region of 52 such stars at high spectral resolution ($\sim$45,000) and high signal-to-noise ratios. While the spread in Li in these stars is greater than a factor of 400, the spread in Be is only 2.7 times. Two stars were without any Be, perhaps due to a merger or a mass transfer with a companion. We find a steep trend of Li with temperature but little for Be. While there is a downward trend in Li with [Fe/H] from -0.4 to +0.4 due to stellar depletion, there is a small increase in Be with Fe from Galactic Be enrichment. While there is a broad decline in Li with age, there may be a small increase in Be with age, though age is less well-determined. In the subset of stars closest to the Sun in temperature and other parameters we find that the ratio of the abundances of Be to Li is much lower than predicted by models; there may be other mixing mechanisms causing additional Li depletion.

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arXiv2022

WIYN Open Cluster Study: The Old Open Cluster, NGC 188, and a Re-evaluation of Lithium-Richness Among Red Giants

We present WIYN/Hydra spectra of 34 red giant candidate members of NGC 188, which, together with WOCS and Gaia data yield 23 single members, 6 binary members, 4 single nonmembers, and 1 binary nonmember. We report [Fe/H] for 29 members and derive [Fe/H]$_{\rm{NGC188}}$ = +0.064 $\pm$ 0.018 dex ($σ_μ$) (sky spectra yield A(Fe)$_{\odot}$ = 7.520 $\pm$ 0.015 dex ($σ_μ$)). We discuss effects on the derived parameters of varying Yale-Yonsei isochrones to fit the turnoff. We take advantage of the coolest, lowest-gravity giants to refine the line list near Li 6707.8 Å. Using synthesis we derive detections of A(Li) = 1.17, 1.65, 2.04, and 0.60 dex for stars 4346, 4705, 5027, and 6353, respectively, and 3$σ$ upper-limits for the other members. Whereas only two of the detections meet the traditional criterion for "Li-richness" of A(Li) > 1.5 dex, we argue that since the cluster A(Li) vanish as subgiants evolve to the base of the RGB, all four stars are Li-rich in this cluster's context. An incidence of even a few Li-rich stars in a sample of 29 stars is far higher than what recent large surveys have found in the field. All four stars lie either slightly or substantially away from the cluster fiducial sequence, possibly providing clues about their Li-richness. We discuss a number of possibilities for the origin for the Li in each star, and suggest potentially discriminating future observations.

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arXiv2022

Lithium and Beryllium in NGC 752 -- An Open Cluster Twice the Age of the Hyades

The surface abundances of the light elements lithium (Li) and beryllium (Be) reveal information about the physical processes taking place in stellar interiors. The investigation of the amount of these two elements in stars in open clusters shows the effect of age on those mechanisms. We have obtained spectra of both Li and Be in main-sequence stars in NGC 752 at high spectral resolution and high signal-to-noise ratios with HIRES on the Keck I telescope. In order to make meaningful comparisons with other clusters, we have determined the stellar parameters on a common scale. We have found abundances of Li and Be by spectral synthesis techniques. NGC 752 is twice the age of the well-studied Hyades cluster. We find that 1) The Li dip centered near 6500 K is wider in NGC 752, having expanded toward cooler temperatures; 2) The Be dip is deeper in the older NGC 752; 3) The Li "peak" near 6200 K is lower by about 0.3 dex; 4) Although there is little Be depletion in the cooler stars, it is possible that Be may be lower in NGC 752 than in the Hyades; 5) The Li content in both clusters declines with decreasing temperature, but there is less Li in NGC 752 at a given temperature by $\sim$0.4 dex. These differences are consistent with the transport of the light-element nuclei below the surface convection zone as predicted by theory. That connection to rotational spin-down is indicated by the pattern of rotation with temperature in the two clusters.

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arXiv2020

WIYN Open Cluster Study LXXX: HDI CCD $UBVRI$ Photometry of the Old Open Cluster NGC 7142 and Comparison to M67

We present $UBVRI$ photometry of 8702 stars in a $0.5 ^\circ\times0.5 ^\circ$ field in the direction of NGC 7142, taken with the Half Degree Imager (HDI) at the WIYN 0.9-meter telescope, to improve knowledge of this cluster's basic parameters. Our photometry spans the ranges 10.6 - 20.4 mag in $U$, 10.6 - 22.0 mag in $B$, 10.0 - 21.8 mag in $V$, 9.2 - 20.7 mag in $R$, and 8.5 - 19.9 mag in $I$. Using color-color diagrams that employ all four color combinations that include $U$, versus $B-V$, we derive a reddening-metallicity relation for the cluster, with preferred values $E(B-V$) = 0.338 $\pm$ 0.031 mag for the left-edge fiducial of the main sequence and [Fe/H] = 0.0 $\pm$ 0.1 dex, where the Hyades cluster has been used as an unreddened reference cluster, the extinction relations of Cardelli have been employed, and the metallicity dependence of the Yonsei-Yale ($Y^2$) isochrones has been assumed. Comparison of our data to the $Y^2$ isochrones in multiple color-magnitude diagrams (CMDs) yields distance-metallicity and age-metallicity relations, with preferred values of $m-M\ =\ 12.65\ \pm\ 0.23$ mag and age = $4.0 ^{-0.7}_{+1.3}$ Gyr. Re-evaluation of the parameters of M67 using Stetson's $UBVI$ photometry yields [Fe/H] = -0.02 $\pm$ 0.05 dex, $E(B-V$) = 0.04 $\pm$ 0.01 mag, $m-M$ = 9.75 $\pm$ 0.03 mag, and age = 3.85 $\pm$ 0.17 Gyr; we thus find the metallicity and age of the two clusters to be indistinguishable. A semi-independent analysis adopting the parameters of M67 and shifting the fiducials of the two clusters in six CMDs until they match strongly corroborates the values listed above. The differences between our inferred parallaxes and the Gaia DR2 values are $87\ \pm\ 60$ $μ$as for NGC 7142 and 48 $\pm$ 15 $μ$as for M67, consistent with previous studies.

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arXiv2020

The effects of rotation on the lithium depletion of G- and K-dwarfs in Messier 35

New fibre spectroscopy and radial velocities from the WIYN telescope are used to measure photospheric lithium in 242 high-probability, zero-age-main-sequence (ZAMS) F- to K-type members of the rich cluster M35. Combining these with published rotation periods, the connection between lithium depletion and rotation is studied in unprecedented detail. At $T_{\rm eff}<5500$ K there is a strong relationship between faster rotation and less Li depletion, although with a dispersion larger than measurement uncertainties. Components of photometrically identified binary systems follow the same relationship. A correlation is also established between faster rotation rate (or smaller Rossby number), decreased Li depletion and larger stellar radius at a given $T_{\rm eff}$. These results support models where starspots and interior magnetic fields lead to inflated radii and reduced Li depletion during the pre main sequence (PMS) phase for the fastest rotators. However, the data are also consistent with the idea that all stars suffered lower levels of Li depletion than predicted by standard PMS models, perhaps because of deficiencies in those models or because saturated levels of magnetic activity suppress Li depletion equally in PMS stars of similar $T_{\rm eff}$ regardless of rotation rate, and that slower rotators subsequently experience more mixing and post-PMS Li depletion.

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arXiv2020

WIYN Open Cluster Study LXXIX. M48 (NGC 2548) I. Radial Velocities, Rotational Velocities, and Metallicities of Stars in the Open Cluster M48 (NGC 2548)

WIYN/Hydra spectra (R $\sim$ 13,500, signal-to-noise pixel$^{-1}$ = 50-1000) of a 400 $\unicode{x212B}$ region around Li 6708 $\unicode{x212B}$ are used to determine radial and rotational velocities for 287 photometrically selected candidate members of the open cluster M48. The sample ranges from turnoff A stars to late-K dwarfs and eight giants. We combine our $V_{\rm{RAD}}$ measurements and power spectrum analysis with parallax and proper motion data from Gaia DR2 to evaluate membership and multiplicity. We classify 152 stars as single cluster members, 11 as binary members, 16 as members of uncertain multiplicity, 56 as single nonmembers, 28 as single "likely" nonmembers, two as single "likely" members, one as a binary "likely" member, five as binary nonmembers, 10 as "likely" members of uncertain multiplicity, three as nonmembers of uncertain multiplicity, and three as "likely" nonmembers of uncertain multiplicity. From a subsample of 95 single members, we derive $V_{\rm{RAD}}$ = 8.512 $\pm$ 0.087 km s$^{-1}$ ($σ_μ$, and $σ$ = 0.848 km s$^{-1}$). Using 16 isolated Fe I lines for a subsample of 99 single members (that have $σ_{T_{\rm{eff}}}$ $<$ 75 K (from 10 colors from $UBVRI$), $v$ sin $i$ $<$ 25 km s$^{-1}$, and well-behaved Fe I lines), [Fe/H]$_{\rm{M48}}$ = -0.063 $\pm$ 0.007 dex ($σ_μ$). [Fe/H] is independent of $T_{\rm{eff}}$ over an unprecedentedly large range of 2500 K. The minimum cluster binary fraction is 11%-21%. M48 exhibits a clear but modest broadening of the main-sequence turnoff, and there is no correlation between color and $v$ sin $i$.

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arXiv2019

Correlated Depletion and Dilution of Lithium and Beryllium Revealed by Subgiants in M 67

The surface content of lithium (Li) and beryllium (Be) provide insights into the mixing and circulation mechanisms in stellar interiors. The old open cluster, M 67, has been well-studied for Li abundances in both main-sequence and evolved stars. The Be abundances give us a probe to a deeper level in stars. We have taken high-resolution spectra with Keck I with HIRES to determine Be abundances along the subgiant branch of M 67, where there are dramatic depletions of Li. These subgiants range in mass from 1.26 to 1.32 M$_{\odot}$ and have evolved from main-sequence stars that would have occupied the region of the Li-Be dip found in younger clusters. Lithium abundances have been adjusted to the same scale for 103 stars in M 67 by Pace et al. The more massive stars $-$ now the coolest and furthest-evolved from the main-sequence $-$ show a drop in Li by a factor of 400 across the subgiant branch. Our new Be abundances also show a decline, but by a factor of $\sim$50. The two elements decline together with Li showing a steeper decline in these subgiants than it does in the Li-Be dip stars. The relative decline in Be abundance compared to Li is remarkably well fit by the models of Sills & Deliyannis, made specifically for the subgiants in M 67. Those models include the effects of mixing induced by stellar rotation. These M 67 subgiants show the effects of both main-sequence depletion and post-main-sequence dilution of both Li and Be.

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arXiv2018

A search for radius inflation among active M-dwarfs in Praesepe

Rotation periods from Kepler K2 are combined with projected rotation velocities from the WIYN 3.5-m telescope, to determine projected radii for fast-rotating, low-mass ($0.15 \leq M/M_{\odot} \leq 0.6$) members of the Praesepe cluster. A maximum likelihood analysis that accounts for observational uncertainties, binarity and censored data, yields marginal evidence for radius inflation -- the average radius of these stars is $6\pm4$ per cent larger at a given luminosity than predicted by commonly-used evolutionary models. This over-radius is smaller (at 2-sigma confidence) than was found for similar stars in the younger Pleiades using a similar analysis; any decline appears due to changes occurring in higher mass ($>0.25 M_{\odot}$) stars. Models incorporating magnetic inhibition of convection predict an over-radius, but do not reproduce this mass dependence unless super-equipartition surface magnetic fields are present at lower masses. Models incorporating flux-blocking by starspots can explain the mass dependence but there is no evidence that spot coverage diminishes between the Pleiades and Praesepe samples to accompany the decline in over-radius. The fastest rotating stars in both Praesepe and the Pleiades are significantly smaller than the slowest rotators for which a projected radius can be measured. This may be a selection effect caused by more efficient angular momentum loss in larger stars leading to their progressive exclusion from the analysed samples. Our analyses assume random spin-axis orientations; any alignment in Praesepe, as suggested by Kovacs (2018), is strongly disfavoured by the broad distribution of projected radii.

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arXiv2018

The inflated radii of M-dwarfs in the Pleiades

Rotation periods obtained with the Kepler satellite have been combined with precise measurements of projected rotation velocity from the WIYN 3.5-m telescope to determine the distribution of projected radii for several hundred low-mass ($0.1 \leq M/M_{\odot} \leq 0.8$), fast-rotating members of the Pleiades cluster. A maximum likelihood modelling technique, that takes account of observational uncertainties, selection effects and censored data, and considers the effects of differential rotation and unresolved binarity, has been used to find that the average radius of these stars is $14 \pm 2$ per cent larger at a given luminosity than predicted by the evolutionary models of Dotter et al. (2008) and Baraffe et al. (2015). The same models are a reasonable match to the interferometric radii of older, magnetically inactive field M-dwarfs, suggesting that the over-radius may be associated with the young, magnetically active nature of the Pleiades objects. No evidence is found for any change in this over-radius above and below the boundary marking the transition to full convection. Published evolutionary models that incorporate either the effects of magnetic inhibition of convection or the blocking of flux by dark starspots do not individually explain the radius inflation, but a combination of the two effects might. The distribution of projected radii is consistent with the adopted hypothesis of a random spatial orientation of spin axes; strong alignments of the spin vectors into cones with an opening semi-angle $<30^{\circ}$ can be ruled out. Any plausible but weaker alignment would increase the inferred over-radius.

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