Thank you to my advisor, Gregg Hallinan, for being an enthusiastic and generous advisor, a champion of all your students, and an example of the kind of mentor I would like to be. Thanks to Lynne Hillenbrand, for chairing my thesis committee, for helpful conversations, and for your support of astronomy graduate students.
INTRODUCTION
80 MHz radio observation of an expulsion of a dense plasma cloud from the Sun on March 1, 1969.
CORONAL MASS EJECTIONS
The interpretation of the dynamical spectra of coherent radio stellar bursts depends on three interrelated questions: The emission is produced in low harmonics of the plasma density upstream of the CME shock.
FIRST DETECTION OF THERMAL RADIO EMISSION FROM SOLAR-TYPE STARS WITH THE KARL G. JANSKY VERY
LARGE ARRAY
To detect the analogue of the thermal radiation that dominates stationary microwave emission from the Sun, we turn to nearby solar-type stars (at 4 to 6 per cent) with solar-like X-ray luminosities. Analysis of the time series showed no statistically significant evidence of variability, consistent with rest.
As noted above, coronal radiation is unlikely to contribute a significant fraction of the observed 34.5 GHz emission. In the case of the Sun, the corona is optically thin to free emission above a few GHz.
DESIGN AND IMPLEMENTATION OF THE STARBURST SYSTEM
With the sensitivity of the cooled receivers, these observations would only take a few minutes a day, leaving the 27m antennas available outside of this time for use with the Starburst backend. Since the frontend deployment was on a similar timeline to the Starburst backend deployment, I also participated in the frontend deployment. I worked with Dale Gary to do some of the commissioning observations for the ant14 frontend, using the OVSA backend instead of the Starburst backend, as the OVSA backend was already well tested from use with the small antennas.
Many of the parameters of the Starburst downconverter and correlator design are given above in Section 3.2, but a brief supplementary description follows. I spearheaded the development of software for the Starburst system, which was a significant fraction of the work I did on the Starburst project. Commissioning of the Starburst project also included optical photometry test runs of the Starburst targets.
EQ Peg produces a small burst during this period that is not seen in the field star time series. This article will publish a description of the design and testing of the Starburst downconverter and correlator.
A WIDEBAND SURVEY FOR COHERENT RADIO BURSTS ON ACTIVE M DWARFS
The end product of the data reduction is dynamical spectra (such as in Figure 4.1) for all the observations showing the evolution of stellar emission in the time-frequency plane. A point source at the phase center produces a positive signal in the real component of the dynamic spectrum and no signal in the imaginary component. The second requirement for identifying a burst is the lack of comparable features in the imaginary component of the dynamic spectrum, which would indicate contamination by RFI or background sources.
These electrons flow ahead of the shock front, generating plasma emission at a frequency corresponding to the ambient plasma density upstream of the shock. The RCP component of the burst shows vertical features in the time-frequency plane, attributed to fast frequency drift that is not resolved by the 1 second integration time. If all 1-4 GHz emission is at the fundamental cyclotron frequency, this implies magnetic field strengths in the source region of 0.36 to 1.4 kG.
The time series for the imaginary component of the baseline mean visibility are shown on the graph, offset by -0.5 Jy. The time series is the true component of the baseline average visibility; shown with an offset of -0.5. The color scale saturates at 6 times the median RMS per channel in the imaginary component of the dynamic spectrum.
These plots are in the style of Figure 4.6a, but binned to 300 sec and 4 MHz and the color scale saturates at 6 times the median channel RMS of the imaginary component of the spectrum.
ULTRA-WIDEBAND OBSERVATIONS OF UV CETI’S RADIO AURORA
In contrast, Route (2016) infers magnetic polarity reversals on many ultracool dwarfs based on changes in the sense of circular polarization of radio bursts. The sense of circular polarization of coherent radio bursts depends on the longitudinal (line-of-sight) polarity of the magnetic field in the region of origin, making coherent radio bursts a means of tracking the evolution of the magnetic field. Based on the similar luminosity and frequency range, the burst in the first epoch of 2013 may also originate from UV Cet.
The shared sense of circular polarization (right hand) at all frequencies implies that the emission at all frequencies is in the same mode, x-mode or o-mode. The emission in the range of observed frequencies is most likely all in the same harmonic, based on the lack of clear harmonic structure and the consistent sense of circular polarization. If the emission is all in the same harmonic, the ratio of the strongest to the weakest magnetic field strengths in the source region is Bmax/Bmin = νmax/νmin.
This is evidence that UV Cet did not undergo any magnetic polarity reversals in the period from 2011 to 2015. Possible causes of the evolving dynamic spectrum morphology and intensity of the bursts include: evolution over time in the underlying magnetic field structure or in the source of high speed electrons.
Stokes V
The second panel shows the fractional circular polarization spectrum, the third panel shows the duration of the burst, and the fourth panel shows the total energy radiated in the burst. The time-averaged U-band flare luminosity of the UV Cet-BL Cet system is 3.26x1026 erg/s (Panagi & Andrews 1995), which is probably dominated by UV Cet. Wolk (2015) calculates that for active stars the U-band flare energy averages 11% of the bolometric flare energy, so we estimate UV Cet's bolometric time-averaged flare luminosity to be 3.0x1027erg/s.
Is it burning, typically interpreted as the source of the non-thermal incoherent radio emission and thermal X-ray emission, whose correlation is captured by the Güdel–Benz relation ( Benz & Güdel 1994a ). The consistency of the time-frequency structure of bursts observed over 4 months in 2015 provides evidence that UV Cet produces a rotationally modulated radio light; this hypothesis will be tested by ongoing 1.5 GHz observations with the Deep Space Network. Is this a consequence of UV Cet's stronger burning activity, or is it another symptom of the "hidden variable" that also causes UV Cet to flare more frequently than BL Cet and maintain a stronger non-thermal radio corona.
In the latter scenario, explored for star–planet interaction by Hess & Zarka (2011) , the evolution of auroral pulses over time depends on the orbital phase of the satellite. The National Radio Astronomy Observatory is a National Science Foundation facility operated under a cooperative agreement by Associated Universities, Inc.
COHERENT RADIO STORMS AND RESOLVED FLARES ON AD LEONIS
A 1034 severe flare on AD Leo was observed spectroscopically and photometrically in the ultraviolet and optical (Hawley & Pettersen 1991), providing a template for theoretical predictions of the effects of M dwarf activity on planetary atmospheres and habitability (Buccino et al. Modeling of these data provides estimates of the magnetic field coverage: Saar & Linsky (1985) estimate that 73% of the star is covered with an average field strength of 3.8 kG, and Shulyak et al.Spectropolarimetric observations of Stokes V line profiles, which is sensitive to the large-scale line-of-sight magnetic flux (opposite polarity cancels out small-scale magnetic structures), detects 7% of the magnetic flux seen in Stokes I (Reiner.
The similar frequency structure, frequency drift characteristics, and intensity of the 1.1–1.6 GHz emission in the first two epochs suggest that this luminous event may be ongoing between the 2 epochs during at least 2 weeks. The sense of circular polarization of coherent radio bursts depends on the orientation of the magnetic field in the source region relative to the line of sight. If this emission is continuous between the two epochs and rotationally modulated, it is detected for >30% of the star's rotational period.
In epoch 2, two features are seen in Figure 6.2 occurring approximately 140 minutes from the start of observations, an RCP burst from 1.6-2 GHz and. If the frequency shift is due to source motion, then ν/ν˙ provides an estimate of the time scale for the source to traverse a scale length in density or magnetic field strength (depending on whether the emission is at the plasma frequency or at the cyclotron frequency).
CONCLUSIONS
I am still refining the calibration of the VLBA data for imaging, but initial results suggest that the 8.4 GHz coherent radio bursts in the UV Cet, which must originate near the stellar photosphere, are offset by the quiescent radio emission by about 2 photospheric diameters. I would like to perform radio dynamical spectroscopy of the fastest rotating M dwarfs to search for more auroral emitters, targeting stars whose large-scale fields have been mapped by Zeeman Doppler Imaging, and follow up with VLBI of the stellar magnetosphere for measure the locations of coherent radio bursts and characterize the non-thermal electron populations in the large-scale magnetosphere of these stars, providing constraints on the mechanisms that can accelerate the high-velocity electrons that power the auroras and other long-lived ones. coherent radio bursts. The ALMA observations open a new window into the high-energy tail of the non-thermal electron population, with bright radio emission detected by some young M dwarfs (Hallinan, private communication; MacGregor et al. 2013).
The ability of VLBA to resolve coronal structures on length scales smaller than the stellar photosphere will be complemented in the next few years by the sub-mass astrometry of the Gaia mission, which allows the measurement of the relative locations of the stellar photosphere and the radio corona. UV Cet and AD Leo are not in the Tycho-Gaia catalog, so they do not have the corresponding motion- and parallax-corrected positions in Gaia's recent data release 1, but I am contacting members of the Gaia team to see if positions could be calculated for these stars. Combining Gaia positions with large-scale magnetic fields measured by Zeeman Doppler imaging will allow the calculation of the stellar magnetic field in the emitting regions of the stellar radio corona, allowing confirmation of the emission mechanism (supposed to be a mildly relativistic synchrotron at s~10) and shedding light on the question , why in the coronas of active stars persistent non-thermal radio radiation occurs on a large scale without a similar one in the Sun.
One of the planned benefits of the Starburst program was the large amount of observation time, with the ability to observe approximately 20 hours per day for many years. This framework and the diagnostic power of stellar coherent radio bursts should prove very fruitful in the era of the Square Kilometer Array.
BIBLIOGRAPHY
