Mark S. Giampapa

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

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.

PERIODIC RADIO AND Hα EMISSION FROM THE L DWARF BINARY 2MASSW J0746425+200032: EXPLORING THE MAGNETIC FIELD TOPOLOGY AND RADIUS OF AN L DWARF

We present an 8.5 hr simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746425+200032. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P = 124.32 ± 0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B ≈ 1.7 kG magnetic field. A quiescent nonvariable component is also detected, likely due to emission from a uniform large-scale field. The Hα emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating misaligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (1) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (2) the Hα and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (v sin i ≈ 27 km s–1), and the binary orbital inclination (i ≈ 142°), to derive a radius for the primary star of 0.078 ± 0.010 R ☉. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085 ± 0.010 M ☉ it provides a constraint on the mass-radius relation below 0.1 M ☉. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr. The origin of this discrepancy is either a breakdown of the models at the bottom of the main sequence, or a significant misalignment between the rotational and orbital axes.

The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X-Ray, and Hα Observations

We present the first simultaneous, multiwavelength observations of an L dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with the Very Large Array, the Chandra X-Ray Observatory, and the Kitt Peak 4 m telescope. We detect strongly variable and periodic radio emission (P = 3 hr) with a fraction of about 60% circular polarization. No X-ray emission is detected to a limit of LX/Lbol 2 × 10-5, several hundred times below the saturation level observed in early M dwarfs. Similarly, we do not detect Hα emission to a limit of LHα/Lbol 2 × 10-7, the deepest for any L dwarf observed to date. The ratio of radio to X-ray luminosity is at least 4 orders of magnitude in excess of that observed in a wide range of active stars (including M dwarfs), providing the first direct confirmation that late-M and L dwarfs violate the radio/X-ray correlation. The radio emission is due to gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which is maintained on timescales longer than 3 yr. The detected 3 hr period may be due to (1) the orbital motion of a companion at a separation of about 5 stellar radii, similar to the configuration of RS CVn systems, (2) an equatorial rotation velocity of about 37 km s-1 and an anchored, long-lived magnetic field, or (3) periodic release of magnetic stresses in the form of weak flares. In the case of orbital motion, the magnetic activity may be induced by the companion, possibly explaining the unusual pattern of activity and the long-lived signal. We conclude that fully convective stars can maintain a large-scale and stable magnetic field, but the lack of X-ray and Hα emission indicates that the atmospheric conditions are markedly different than in early-type stars and even M dwarfs. Similar observations are therefore invaluable for probing both the internal and external structure of low-mass stars and substellar objects, and for providing constraints on dynamo models.

Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM 513–46546

We present the first simultaneous radio, X-ray, ultraviolet, and optical spectroscopic observations of the M8.5 dwarf TVLM 513–46546, with a duration of 9 hr. These observations are part of a program to study the origin of magnetic activity in ultracool dwarfs, and its impact on chromospheric and coronal emission. Here we detect steady quiescent radio emission superposed with multiple short-duration, highly polarized flares; there is no evidence for periodic bursts previously reported for this object, indicating their transient nature. We also detect soft X-ray emission, with -->LX/Lbol ≈ 10−5.1, the faintest to date for any object later than M5, and a possible X-ray flare. TVLM 513–46546 continues the trend of severe violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4 orders of magnitude. From the optical spectroscopy we find that the Balmer line luminosity exceeds the X-ray luminosity by a factor of a few, ruling out chromospheric heating by coronal X-ray emission. More importantly, we detect sinusoidal Hα and Hβ equivalent width light curves with a period of 2 hr, matching the rotation period of TVLM 513–46546. This behavior points to a corotating chromospheric hot spot or an extended magnetic structure, with a covering fraction of about 50%. This feature may be transitory based on the apparent decline in light-curve peak during the four observed maxima. From the radio data we infer a large-scale and steady magnetic field of ~102 G. A large-scale field is also required by the sinusoidal Balmer line emission. The radio flares, on the other hand, are produced in a component of the field with a strength of ~3 kG and a likely multipolar configuration. The overall lack of correlation between the various activity indicators suggests that the short-duration radio flares do not have a strong influence on the chromosphere and corona, and that the chromospheric emission is not the result of coronal heating.

Stellar coronae at the end of the main sequence - A ROSAT survey of the late M dwarfs

We present X-ray data, both detections and upper limits, from the ROSAT all-sky survey for most known M dwarfs later than type MS, as well as from selected ROSAT pointed observations of some of these stars. We compare these data with similar data for early M dwarfs in an attempt to probe the nature or the magnetic dynamo and coronal heating mechanism for the very late M dwarfs, which are presumably totally convective. Our results indicate that late M dwarfs can have coronae which are just as active as those for the early M dwarfs and that coronal heating efficiency for saturated stars (as evidenced from L X /L bol ) does not drop at spectral type M6

Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. II. Mixed Trends in VB 10 and LSR 1835+32 and the Possible Role of Rotation

As part of our ongoing investigation of magnetic activity in ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical observations of LSR1835+32 (M8.5), and simultaneous X-ray and UV observations of VB10 (M8), both with a duration of about 9 hr. LSR1835+32 exhibits persistent radio emission, and H? variability on timescales of 0.5-2 hr. The detected UV flux is consistent with photospheric emission, and no X-ray emission is detected to a deep limit of -->LX/Lbol 10?5.7. The H? and radio emission are temporally uncorrelated, and the ratio of radio to X-ray luminosity exceeds the correlation seen in F-M6 stars by > -->2 ? 104. The lack of radio variability during four rotations of LSR1835+32 requires a uniform stellar-scale field of ~10 G, and indicates that the H? variability is dominated by much smaller scales, vsin i > 25 km s?1, which could be the result of secondary effects such as inefficient heating or centrifugal stripping of extended coronal loops. These effects may underlie the severe violation of the radio/X-ray correlation in ultracool dwarfs.

An X-Ray Flare Detected on the M8 Dwarf VB 10

We have detected an X-ray flare on the very low mass star VB 10 (GL 752 B; M8V) using the ROSAT High Resolution Imager. VB 10 is the latest type (lowest mass) main-sequence star known to exhibit coronal activity. X-rays were detected from the star during a single 1.1 ks segment of an observation that lasted 22 ks in total. The energy released by this flare is on the order of 1027 ergs s-1. This is at least 2 orders of magnitude greater than the quiescent X-ray luminosity of VB 10, which has yet to be measured. This X-ray flare is very similar in nature to the far-ultraviolet flare that was observed by Linsky et al. using the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope. We discuss reasons for the extreme difference between the flare and quiescent X-ray luminosities, including the possibility that VB 10 has no quiescent (106 K) coronal plasma at all.

A Survey of Chromospheric Activity in the Solar-Type Stars in the Open Cluster M67*

We present the results of a spectroscopic survey of the Ca II H and K core strengths in a sample of 60 solar-type stars that are members of the solar-age and solar-metallicity open cluster M67. We adopt the HK index, defined as the summed H+K core strengths in 1 A bandpasses centered on the H and K lines, respectively, as a measure of the chromospheric activity that is present. We compare the distribution of mean HK index values for the M67 solar-type stars with the variation of this index as measured for the Sun during the contemporary solar cycle. We find that the stellar distribution in our HK index is broader than that for the solar cycle. Approximately 17% of the M67 Sun-like stars exhibit average HK indices that are less than solar minimum. About 7%-12% are characterized by relatively high activity in excess of solar maximum values, while 72%-80% of the solar analogs exhibit Ca II H+K strengths within the range of the modern solar cycle. The ranges given reflect uncertainties in the most representative value of the maximum in the HK index to adopt for the solar cycle variations observed during the period AD 1976-2004. Thus, ~20%-30% of our homogeneous sample of Sun-like stars have mean chromospheric H+K strengths that are outside the range of the contemporary solar cycle. Any cycle-like variability that is present in the M67 solar-type stars appears to be characterized by periods greater than ~6 yr. Finally, we estimate a mean chromospheric age for M67 in the range of 3.8-4.3 Gyr.

Chromospheric H-alpha and CA II lines in late-type stars

Observations of H-alpha and Ca II H and K in the chromospheres of 50 main-sequence K and M stars are reported. The photospheric contribution to the integrated core flux in these lines is found to be more important than previously thought, and earlier estimates of the basal flux of cool dwarf stars are revised. The H-alpha data confirm the presence of both an upper and a lower limit to the H-alpha equivalent width attained at any given spectral type. The maximum H-alpha absorption strength decreases gradually toward cooler stars while the maximum saturated quiescent emission strength increases. The Ca II emission strength is related to the strength of the emission or absorption feature at H-alpha, but there is not a one-to-one correlation. The main results may be explained in terms of photoionization and collisional control of the non-LTE H-alpha source function. Several aspects of the structure and heating of the outer atmospheres of dwarf K and M stars are discussed. 60 refs.

Stellar model chromospheres. XIII - M dwarf stars

Single-component, homogeneous model chromospheres that are consistent with high-resolution profiles of the Ca II K line calibrated in surface flux units for three dMe and 2 dM stars observed at quiescent times are constructed. The models reveal several systematic trends. Large values of the ratio of T(min) to T(eff) are derived, indicating a large amount of nonradiative heating present in the upper photospheres of M dwarf stars. It is also found that the lower chromospheric temperature gradient is similar for all the M dwarf stars. Since for the models here the chromospheric K line emission strength is most sensitive to the total amount of chromospheric material present within the approximate temperature range T(min)-6000 K, increasing the emission strength is not simply due to increasing chromospheric temperature gradients. It is also found that both the electron density and electron temperature at one thermalization length in the K line below the top of the chromospheres are greater in the dMe stars than in the dM stars. The M dwarf models here have microturbulent velocities between 1 and 2 km/sec, which are much smaller than for solar chromosphere models.