Mark S. Giampapa

Bio: Mark S. Giampapa is an academic researcher from Association of Universities for Research in Astronomy. The author has contributed to research in topics: Stars & Brown dwarf. The author has an hindex of 29, co-authored 72 publications receiving 3376 citations. Previous affiliations of Mark S. Giampapa include Max Planck Society.

The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets

Abstract: National Science Foundation Graduate Research Fellowship Program [DGE-1143953]; Max Planck Institute for Astronomy, Heidelberg; NASA's Science Mission Directorate

Rotational Velocities and Chromospheric/Coronal Activity of Low-Mass Stars in the Young Open Clusters IC 2391 and IC 2602

Abstract: We have obtained high-resolution, moderate signal-to-noise ratio spectra for approximately 80 candidate low-mass members of the nearby, very young open clusters IC 2391 and IC 2602. Most of the stars observed are confirmed as cluster members based on a combination of photometric and spectroscopic criteria. We provide radial velocities, rotational velocities, and H? equivalent widths for these stars. From comparison to theoretical pre-main-sequence (PMS) evolutionary isochrones from D'Antona and Mazzitelli, we derive an estimated age of the two clusters of ~25 Myr. By contrast, the usually quoted upper main-sequence turnoff age for the clusters is ~35 Myr. We do not believe that this provides evidence for noncoeval star formation within these clusters, but rather that the best age estimate for them given the uncertainties is ~30 ? 5 Myr. In principle, the scatter of stars about the PMS isochrone provides a measure of the age spread among the low-mass stars in these clusters; however, with the data presently available, we are able to derive only a relatively uninteresting upper limit for an age spread of order 20 Myr. We compare the rotational velocity distribution for IC 2391/2602 to that observed for the Pleiades. For the G dwarfs in the IC clusters, we resolve rotation in all but one of the probable cluster members, and thus except for inclination effects, our data provide the complete distribution of rotational velocities for solar mass stars on their arrival on the ZAMS. The projected rotational velocities (v sin i) of the G dwarfs in the two IC clusters span the range from ~8 to ~200 km s-1. Comparison of the distribution of rotational velocities for the G dwarfs of the Pleiades and the IC clusters indicates that both the slow and the rapid rotators lose of order half their angular momentum during the first ~35 Myr on the main sequence if they rotate as solid bodies. The low-mass stars in these two clusters exhibit a similar correlation between rotation and coronal activity as is found in several other young open clusters. That is, there is a large spread in coronal activity for stars with v sin i < 25 km s-1, where we assume there is an intrinsic link between increasing rotation and increasing activity superimposed upon which are a variety of observational and physical mechanisms that act to smear out this relation; above v sin i ~ 25 km s-1, all of the low-mass stars have log (LX/Lbol) ~ -3.0, the canonical saturation limit. Our measurements of the H? equivalent widths are consistent with a similar relationship holding for chromospheric activity. One and possibly two of our spectra for M dwarf members of the IC clusters show broad wings for the H? profile, which we attribute to a flare event or to microflares. Since spectra of a small sample of late-type M dwarfs in the Pleiades also showed similarly broad H? wings, this suggests that flare frequencies for very young M dwarfs may be quite high.

SIMULTANEOUS MULTI-WAVELENGTH OBSERVATIONS OF MAGNETIC ACTIVITY IN ULTRACOOL DWARFS. III. X-RAY, RADIO, AND Hα ACTIVITY TRENDS IN M AND L DWARFS

Abstract: As part of our on-going investigation into the magnetic field properties of ultracool dwarfs, we present simultaneous radio, X-ray, and H? observations of three M9.5-L2.5 dwarfs (BRI?0021-0214, LSR?060230.4+391059, and 2MASS?J052338.2?140302). We do not detect X-ray or radio emission from any of the three sources, despite previous detections of radio emission from BRI?0021 and 2M0523?14. Steady and variable H? emission are detected from 2M0523?14 and BRI?0021, respectively, while no H? emission is detected from LSR?0602+39. Overall, our survey of nine M8-L5 dwarfs doubles the number of ultracool dwarfs observed in X-rays, and triples the number of L dwarfs, providing in addition the deepest limits to date, log(L X/L bol) ?5. With this larger sample we find the first clear evidence for a substantial reduction in X-ray activity, by about two orders of magnitude, from mid-M to mid-L dwarfs. We find that the decline in H? roughly follows L H?/L bol 10?0.4?(SP-6) for SP ? 6, where SP = 0 for spectral type M0. In the radio band, however, the luminosity remains relatively unchanged from M0 to L4, leading to a substantial increase in L rad/L bol. Our survey also provides the first comprehensive set of simultaneous radio/X-ray/H? observations of ultracool dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond spectral type M7, evolving smoothly from L ?,rad/L X 10?15.5 to ~10?11.5 Hz?1 over the narrow spectral-type range M7-M9. This breakdown reflects the substantial reduction in X-ray activity beyond M7, but its physical origin remains unclear since, as evidenced by the uniform radio emission, there is no drop in the field dissipation and particle acceleration efficiency. Based on the results of our survey, we conclude that a further investigation of magnetic activity in ultracool dwarfs will benefit from a two-pronged approach: multi-rotation observations of nearby known active sources and a snapshot survey of a large sample within ~50 pc to uncover rare flaring objects.

Simultaneous Multi-Wavelength Observations of Magnetic Activity in Ultracool Dwarfs. III. X-ray, Radio, and H-alpha Activity Trends in M and L Dwarfs

Abstract: [Abridged] As part of our on-going investigation into the magnetic field properties of ultracool dwarfs, we present simultaneous radio, X-ray, and H-alpha observations of three M9.5-L2.5 dwarfs (BRI0021-0214, LSR060230.4+391059, and 2MASSJ052338.2-140302). We do not detect X-ray or radio emission from any of the three sources, despite previous detections of radio emission from BRI0021 and 2M0523-14. Steady and variable H-alpha emission are detected from 2M0523-14 and BRI0021, respectively, while no H-alpha emission is detected from LSR0602+39. Overall, our survey of nine M8-L5 dwarfs doubles the number of ultracool dwarfs observed in X-rays, and triples the number of L dwarfs, providing in addition the deepest limits to date, log(L_X/L_bol) M6. In the radio band, however, the luminosity remains relatively unchanged from M0 to L4, leading to a substantial increase in L_rad/L_bol. Our survey also provides the first comprehensive set of simultaneous radio/X-ray/H-alpha observations of ultracool dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond spectral type M7, evolving smoothly from L_{ u,rad}/L_X ~ 10^-15.5 to ~10^-11.5 Hz^-1 over the narrow spectral type range M7-M9. This breakdown reflects the substantial reduction in X-ray activity beyond M7, but its physical origin remains unclear since, as evidenced by the uniform radio emission, there is no drop in the field dissipation and particle acceleration efficiency.

Monthly Notices of the Royal Astronomical Society

Abstract: Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

A Hipparcos Census of the Nearby OB Associations

Abstract: A comprehensive census of the stellar content of the OB associations within 1 kpc from the Sun is presented, based on Hipparcos positions, proper motions, and parallaxes. It is a key part of a long-term project to study the formation, structure, and evolution of nearby young stellar groups and related star-forming regions. OB associations are unbound moving groups, which can be detected kinematically because of their small internal velocity dispersion. The nearby associations have a large extent on the sky, which traditionally has limited astrometric membership determination to bright stars (V 6 mag), with spectral types earlier than ~B5. The Hipparcos measurements allow a major improvement in this situation. Moving groups are identified in the Hipparcos Catalog by combining de Bruijne's refurbished convergent point method with the Spaghetti method of Hoogerwerf & Aguilar. Astrometric members are listed for 12 young stellar groups, out to a distance of ~650 pc. These are the three subgroups Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2, α Persei (Per OB3), Cas–Tau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection procedure corrects the list of previously known astrometric and photometric B- and A-type members in these groups and identifies many new members, including a significant number of F stars, as well as evolved stars, e.g., the Wolf-Rayet stars γ2 Vel (WR 11) in Vel OB2 and EZ CMa (WR 6) in Col 121, and the classical Cepheid δ Cep in Cep OB6. Membership probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the expected number of interloper field stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri objects and other stars in the final pre–main-sequence phase. This provides a firm link between the classical high-mass stellar content and ongoing low-mass star formation. Detailed studies of these 12 groups, and their relation to the surrounding interstellar medium, will be presented elsewhere. Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive. OB associations do exist in many of these regions, but they are either at distances beyond ~500 pc where the Hipparcos parallaxes are of limited use, or they have unfavorable kinematics, so that the group proper motion does not distinguish it from the field stars in the Galactic disk. The mean distances of the well-established groups are systematically smaller than the pre-Hipparcos photometric estimates. While part of this may be caused by the improved membership lists, a recalibration of the upper main sequence in the Hertzsprung-Russell diagram may be called for. The mean motions display a systematic pattern, which is discussed in relation to the Gould Belt. Six of the 12 detected moving groups do not appear in the classical list of nearby OB associations. This is sometimes caused by the absence of O stars, but in other cases a previously known open cluster turns out to be (part of) an extended OB association. The number of unbound young stellar groups in the solar neighborhood may be significantly larger than thought previously.

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics

Abstract: While the strong anticorrelation between chromospheric activity and age has led to the common use of the Ca II H and K emission index (R'_(HK) = L_(HK)/L_(bol)) as an empirical age estimator for solar-type dwarfs, existing activity-age relations produce implausible ages at both high and low activity levels.We have compiled R'_(HK) HK data from the literature for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity age index,we derive an improved activity-age calibration for F7-K2 dwarfs (0:5 mag < B - V < 0.9 mag). We also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_(HK) values through the Rossby number to rotation periods and then makes use of improved gyrochronology relations. We demonstrate that our new activity-age calibration has typical age precision of ~0.2 dex for normal solar-type dwarfs aged between the Hyades and the Sun (~0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts for some color-dependent effects and systematically improves the age estimates (albeit only slightly). We demonstrate that coronal activity as measured through the fractional X-ray luminosity (R_X = L_X/L_(bol)) has nearly the same age- and rotation inferring capability as chromospheric activity measured through R'_(HK). As a first application of our calibrations, we provide new activity-derived age estimates for a volume-limited sample of the 108 solar-type field dwarfs within 16 pc.

Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors.

Abstract: Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high-transition-temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle-resolved photoemission spectroscopy to probe electron dynamics-velocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80 meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity.

The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs ?

Abstract: We present the results of a new study on the relationship between coronal X-ray emission and stellar rotation in late-type main-sequence stars. We have selected a sample of 259 dwarfs in the B V range 0.5-2.0, including 110 field stars and 149 members of the Pleiades, Hyades, Persei, IC 2602 and IC 2391 open clusters. All the stars have been observed with ROSAT, and most of them have photometrically-measured rotation periods available. Our results confirm that two emission regimes exist, one in which the rotation period is a good predictor of the total X-ray luminosity, and the other in which a constant saturated X-ray to bolometric luminosity ratio is attained; we present a quantitative estimate of the critical rotation periods below which stars of dierent masses (or spectral types) enter the saturated regime. In this work we have also empirically derived a characteristic time scale,e, which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number Re = Prot=e: we show that our empirical time scalee resembles the theoretical convective turnover time for 0:4 M=M 1:2, but it also has the same functional dependence on B V as L 1=2 bol in the color range 0:5 B V 1:5. Our results imply that - for non-saturated coronae - the Lx - Prot relation is equivalent to the Lx=Lbol vs. Re relation.