arXiv subjects

R. J. Jackson

Publications and source records attributed to R. J. Jackson.

44 records · Page 3Linked to original sources

arXiv2013

On the relationship between the size and surface coverage of starspots on magnetically active low-mass stars

We present a model that predicts the light curve amplitude distribution for an ensemble of low-mass magnetically active stars, under the assumptions that stellar spin axes are randomly orientated and that cool starspots have a characteristic scale length and are randomly distributed across the stellar surfaces. The model is compared with observational data for highly magnetically active M-dwarfs in the young cluster NGC 2516. We find that the best fitting starspot scale length is not constrained by these data alone, but requires assumptions about the overall starspot filling factor and starspot temperature. Assuming a spot coverage fraction of 0.4+/-0.1 and a starspot to unspotted photosphere temperature ratio of 0.7+/-0.05, as suggested by the inflated radii of these stars compared to evolutionary model predictions and by TiO band measurements on other active cool stars of earlier spectral type, the best-fitting starspot angular scale length is 3.5(+2)(-1) degrees, or a linear scale length of \sim 25000 km. This linear scale length is similar to large sunspot groups, but 2--5 times smaller than the starspots recently deduced on an active G-dwarf using eclipse mapping by a transiting exoplanet. However, the best-fitting spot scale length in the NGC 2516 M-dwarfs increases with the assumed spot temperature ratio and with the inverse square root of the assumed spot filling factor. Hence the light curve amplitude distribution might equally well be described by these larger spot scale lengths if the spot filling factors are <0.1 or the spot temperature ratio is >0.9.

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arXiv2012

Why do some young cool stars show spot modulation while others do not?

We present far-red, intermediate resolution spectroscopy of 572 photometrically selected, low-mass stars (0.2<M/M_sun<0.7) in the young open cluster NGC 2516, using the FLAMES spectrograph at the Very Large Telescope. Precise radial velocities confirm membership for 210 stars that have published rotation periods from spot-modulated light curves and for another 144 stars in which periodic modulation could not be found. The two sub-samples are compared and no significant differences are found between their positions in colour-magnitude diagrams, the distribution of their projected equatorial velocities or their levels of chromospheric activity. We rule out differing observational sensitivity as an explanation and conclude that otherwise similar objects, with equally high levels of chromospheric activity, do not exhibit spot-induced light curve modulation because their significant spot coverage is highly axisymmetric. We propose that the spot coverage consists of large numbers of small, dark spots with diameters of about 2 degrees. This explains why about half of cluster members do not exhibit rotationally modulated light curves and why the light curve amplitudes of those that do have mean values of only 0.01-0.02 mag.

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arXiv2010

Investigating coronal saturation and super-saturation in fast-rotating M-dwarf stars

At fast rotation rates the coronal activity of G- and K-type stars has been observed to "saturate" and then decline again at even faster rotation rates -- a phenomenon dubbed "super-saturation". In this paper we investigate coronal activity in fast-rotating M-dwarfs using deep XMM-Newton observations of 97 low-mass stars of known rotation period in the young open cluster NGC 2547, and combine these with published X-ray surveys of low-mass field and cluster stars of known rotation period. Like G- and K-dwarfs, we find that M-dwarfs exhibit increasing coronal activity with decreasing Rossby number N_R, the ratio of period to convective turnover time, and that activity saturates at L_x/L_bol ~ 10^-3 for log N_R < -0.8. However, super-saturation is not convincingly displayed by M-dwarfs, despite the presence of many objects in our sample with log N_R < -1.8, where super-saturation is observed to occur in higher mass stars. Instead, it appears that a short rotation period is the primary predictor of super-saturation; P <=0.3d for K-dwarfs and perhaps P <=0.2d for M-dwarfs. These observations favour the "centrifugal stripping" model for super-saturation, where coronal structures are forced open or become radiatively unstable as the Keplerian co-rotation radius moves inside the X-ray emitting coronal volume.

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arXiv2010

Chromospheric activity among fast rotating M-dwarfs in the open cluster NGC 2516

We report radial velocities (RVs), projected equatorial velocities (v sin i) and CaII triplet (CaT) chromospheric activity indices for 237 late-K to mid-M candidate members of the young open cluster NGC 2516. These stars have rotation periods between 0.1 and 15 days. Intermediate resolution spectra were obtained using the Giraffe spectrograph at the Very Large Telescope. Membership was confirmed on the basis of RVs for 210 targets. We see a marked increase in the fraction of rapidly rotators as we move to cooler spectral types. About 20 per cent of M0-M1 stars have v sin i >15km/s, increasing to 90 per cent of M4 stars. Activity indices derived from the first two lines of the CaT (8498A and 8542A) show differing dependencies on rotation period and mass for stars lying above and below the fully convective boundary. Higher mass stars, of spectral type K3-M2.5, show chromospheric activity which increases with decreasing Rossby number (the ratio of period to convective turnover time), saturating for Rossby numbers <0.1. For cooler stars, which are probably fully convective and almost all of which have Rossby numbers <0.1, there is a clear decrease in chromospheric activity as (V-I)_0 increases, amounting to a fall of about a factor of 2-3 between spectral types M2.5 and M4. This decrease in activity levels at low Rossby numbers is not seen in X-ray observations of the coronae of cluster M-dwarfs or of active field M-dwarfs. There is no evidence for supersaturation of chromospheric activity for stars of any spectral type at Rossby numbers <0.01. We suggest that the fall in the limiting level of chromospheric emission beyond spectral type M3 in NGC~2516 is, like the simultaneous increase in rotation rates in field stars, associated with a change in the global magnetic topology as stars approach the fully convective boundary and not due to any decrease in dynamo-generated magnetic flux.

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arXiv2009

Are the spin axes of stars randomly aligned within a cluster?

We investigate to what extent the spin axes of stars in young open clusters are aligned. Assuming that the spin vectors lie uniformly within a conical section, with an opening half-angle between lambda=0 (perfectly aligned) and lambda=90 degrees(completely random), we describe a Monte-Carlo modelling technique that returns a probability density for this opening angle given a set of measured sin i values, where i is the unknown inclination angle between a stellar spin vector and the line of sight. Using simulations we demonstrate that although azimuthal information is lost, it is easily possible to discriminate between strongly aligned spin axes and a random distribution, providing that the mean spin-axis inclination lies outside the range 45--75 degrees. We apply the technique to G- and K-type stars in the young Pleiades and Alpha Per clusters. The sin i values are derived using rotation periods and projected equatorial velocities, combined with radii estimated from the cluster distances and a surface brightness/colour relationship. For both clusters we find no evidence for spin-axis alignment: lambda=90 degrees is the most probable model and lambda>40 degrees with 90 per cent confidence. Assuming a random spin-axis alignment, we re-determine the distances to both clusters, obtaining 133+/-7pc for the Pleiades and 182+/-11 pc for Alpha Per. If the assumption of random spin-axis alignment is discarded however, whilst the distance estimate remains unchanged, it has an additional +18/-32 percent uncertainty.

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arXiv2009

Low-mass members of the young cluster IC 4665 and pre-main-sequence lithium depletion

We have used fibre spectroscopy to establish cluster membership and examine pre-main-sequence (PMS) lithium depletion for low-mass stars (spectral types F to M) in the sparse young (~30 Myr) cluster IC 4665. We present a filtered candidate list of 40 stars that should contain 75 per cent of single cluster members with V of 11.5 to 18 in the central square degree of the cluster. Whilst F- and G-type stars in IC 4665 have depleted little or no lithium, the K- and early M-type stars have depleted more Li than expected when compared with similar stars in other clusters of known age. An empirical age estimate based on Li-depletion among the late-type stars of IC 4665 would suggest it is older than 100 Myr. This disagrees entirely with ages determined either from the nuclear turn-off, from isochronal matches to low-mass stars or from the re-appearance of lithium previously found in much lower mass stars (the ``lithium depletion boundary''). We suggest that other parameters besides age, perhaps composition or rotation, are very influential in determining the degree of PMS Li-depletion in stars with M greater than 0.5 Msun. Further work is required to identify and assess the effects of these additional parameters, particularly to probe conditions at the interface between the sub-photospheric convection zone and developing radiative core. Until then, PMS Li depletion in F- to early M-type stars cannot be confidently used as a precise age indicator in young clusters, kinematic groups or individual field stars.

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arXiv2009

The radii of M-dwarfs in the young open cluster NGC 2516

Using a novel technique, which combines previously determined rotation periods with new spectroscopic determinations of projected rotation velocity, we have determined radii for fast rotating, low-mass (0.2-0.7 Msun) M-dwarfs in the young, solar-metallicity open cluster, NGC 2516. The mean radii are larger than model predictions at a given absolute I or K magnitude and also larger than the measured radii of magnetically inactive M-dwarfs; the difference increases from a few per cent, to 50 per cent for the lowest luminosity stars in our sample. We show that a simple two-temperature starspot model is broadly capable of explaining these observations, but requires spot coverage fractions of about 50 per cent in rapidly rotating M-dwarfs.

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arXiv2008

A survey for low mass spectroscopic binary stars in the young clusters around sigma Orionis and lambda Orionis

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs in the young clusters around sigma Ori and lambda Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their NaI 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its line width and shape. The sample covers the magnitude range Ic=14-18.9 (mass =~ 0.55-0.03 Msun), but all of the binary stars are brighter than Ic=16.6 (mass =~ 0.12Msun) and 10 are brighter than Ic=15.5 (mass =~ 0.23Msun). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of Ic magnitude with more than 99% confidence. The spectroscopic binary fraction for stars more massive than about 0.1Msun (Ic < 16.9) is f_bright=0.095(+0.012)(-0.028). The 90% confidence upper limit to the spectroscopic binary fraction for very low mass (VLM) stars (mass < 0.1Msun) and brown dwarfs (BDs) is f_faint < 7.5%. The hypothesis that f_bright and f_faint are equal can be rejected with 90% confidence. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and brown dwarfs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semi-major axes, or both. (Abridged)

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