Anthony B. Kaye

Driving the Gravity-Mode Pulsations in γ Doradus Variables

The γ Doradus stars are a newly discovered class of gravity-mode pulsators that lie just at or beyond the red edge of the δ Scuti instability strip. We present the results of calculations that the predict pulsation instability of high-order g-modes with periods between 0.4 and 3 days, as observed in these stars. The pulsations are driven by the modulation of the radiative flux by convection at the base of a deep envelope convection zone. Pulsation instability is predicted only for models with temperatures at the convection zone base between ~200,000 and ~480,000 K. The estimated shear dissipation that is due to turbulent viscosity within the convection zone or in an overshoot region below the convection zone can be comparable to or even exceed the predicted driving and is likely to reduce the number of unstable modes or possibly quench the instability. Additional refinements in the pulsation modeling are required to determine the outcome. At least one γ Doradus star has been observed that also pulsates in δ Scuti-type p-modes, and others have been identified as chemically peculiar. Since our calculated driving region is relatively deep, γ Doradus pulsations are not necessarily incompatible with surface abundance peculiarities or with δ Scuti p-mode pulsations driven by the H and He ionization κ-effect. Such stars will provide useful observational constraints on the proposed γ Doradus pulsation mechanism.

γ Doradus Stars: Defining a New Class of Pulsating Variables

ABSTRACT In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F‐type dwarf γ Doradus. These stars typically have between 1 and 5 periods ranging from 0.4 to 3 days with photometric amplitudes up to 0.1 mag in Johnson V. The mechanism for these observed variations is high‐order, low‐degree, nonradial, gravity‐mode pulsation.

Hubble Space Telescope Goddard High Resolution Spectrograph Observations of the Be + sdO Binary ϕ Persei*

Mass transfer during the evolution of intermediate-mass stars in a close binary system can result in a rejuvenated and spun-up secondary star (which may appear as a rapidly rotating Be star) orbiting an unseen, stripped-down, remnant companion. One of the best candidates for such a system is the long- period (127 days) binary / Per. Here we present new Hubble Space Telescope Goddard High Resolution Spectrograph spectra of / Per in several UV regions that show clearly for the -rst time the spectral signature of the faint remnant companion. We derive a double-lined solution for the radial velocity curve that yields masses of 9.3 ^ 0.3 and 1.14 ^ 0.04 for the Be star and companion, respectively. A M _ M _ Doppler tomographic reconstruction of the secondary spectrum shows a rich spectrum dominated by sharp Fe IV and Fe V lines, similar to those observed in hot sdO stars. Non-LTE spectrum synthesis indicates that the subdwarf has temperature kK and gravity log g \ 4.2 ^ 0.1 and that the T eff \ 53 ^ 3 subdwarfEtoEBe star Nux ratio is 0.165 ^ 0.006 and 0.154 ^ 0.009 for the 1374 and 1647 regions, Ae respectively. The spectrum of the Be primary appears normal for a very rapidly rotating early B-type star, but we argue that the star is overluminous for its mass (perhaps owing to accretion-induced mixing). Additional sharp lines of Fe IV appear when the companion is in the foreground, and we show that these form in a heated region of the Be starIs disk that faces the hot subdwarf. Subject headings: stars: binaries: spectroscopic E stars: emission-line, Be E stars: evolution E stars: individual (/ Persei, HD 10516) E stars: subdwarfs

A Theoretical γ Doradus Instability Strip

In this paper, we present the first theoretical γ Doradus instability strip. We find that our model instability strip agrees very well with the previously established, observationally based, instability strip of Handler & Shobbrook. We stress, as do Guzik et al., that the convection zone depth plays the major role in the determination of our instability strip. Once this depth becomes too deep or too shallow, the convection zone no longer allows for pulsational instability. Our theoretical γ Dor instability strip is bounded by ~6850 and 7360 K at the red and blue edge, respectively, on the zero-age main sequence and by ~6560 and 7000 K at the red and blue edge, respectively, approximately 2 mag more luminous. This theoretical strip, transformed to the observers color-magnitude diagram, overlays the region where most of the 30 bona fide γ Dor stars are found.

Spectroscopy of Early F Stars: γ Doradus Candidates and Possible Metallic Shell Stars

We obtained high-resolution spectroscopic observations of 34 γ Doradus candidates. From the red-wavelength spectra, we determined spectral classes, radial velocities, and projected rotational velocities. The spectra of seven late A or early F stars show metallic lines that have composite profiles consisting of a narrow component near the center of a broad line, indicating that they may be shell stars or binaries. Several stars, including HD 152896, HD 173977, HD 175337, and HD 195068/9, show large line profile asymmetries. Two stars, HD 11443 (=α Trianguli) and HD 149420, are ellipsoidal variables and not γ Doradus stars. The percentage of binary systems in our sample may be as high as 74%.

10 New γ Doradus and δ Scuti Stars

We present high-resolution spectroscopy and precision photometry of five new γ Doradus and five new δ Scuti variables. The five new γ Doradus variables substantially increase the number of confirmed stars of this class. All 10 stars fall in the spectral class range F0–F2, but they are cleanly separated into two groups by their luminosity and photometric periods. However, the period gap between the γ Doradus and δ Scuti stars is becoming very narrow since we confirm that HD 155154 is a γ Doradus star with the shortest periods reported to date (the shortest of its four periods is ~0.312 days). We do not find any evidence in our sample for stars exhibiting both δ Scuti– and γ Doradus–type pulsations.

Plasmonic Probe of the Semiconductor to Metal Phase Transition in Vanadium Dioxide

An array of 180 nm diameter gold nanoparticles (NPs) embedded in a thin vanadium dioxide film was used as a nanoscale probe of the thermochromic semiconductor-to-metal transition (SMT) in the VO2. The observed 30% reduction in plasmon dephasing time resulted from the interaction between the localized surface plasmon resonance of the NPs with the 1.4 eV electronic transitions in VO2. The NPs act as nanoantennas probing the SMT; homogeneous broadening of the gold plasmon resonance is observed at the temperatures where electron correlations are strongest in VO2.

A Spectroscopic Analysis of the γ Doradus Star HD 207223 = HR 8330

We present a detailed analysis of 76 high-resolution, high signal-to-noise spectra of the γ Doradus star HD 207223 = HR 8330. Periodogram analysis of the moments of the line profiles reveals a single period of 2.67 ± 0.01 days, a result consistent with that found in photometric studies. Mode identification was accomplished with both the moment method and from direct line-profile fitting. Both methods clearly point towards a sectoral mode of degree l ≥ 2. The inclusion of rotational effects, by taking the Coriolis force into account, reveals a single consistent solution: an l = m = +2 retrograde sectoral mode that has a period of 1.4 days in the corotating frame. The star is seen at an inclination of 30°. From this, we derive a rotational period of 4.9 days.

Plasmon-enhanced low-intensity laser switching of gold::vanadium dioxide nanocomposites

Transient absorption of gold nanoparticle (NP) arrays covered by a 60 nm thick film of VO2 was measured using a mechanically shuttered 785 nm pump laser and a 1550 nm cw probe. Even though the Au NPs constitute only 4% by volume of the nanocomposite, they increase the effective absorption coefficient by a factor of 1.5 and reduce the threshold laser power required to induce the semiconductor-to-metal transition (SMT) by as much as 37%. It is argued that the NPs function as thermal initiators for the SMT and as “nanoradiators” to increase the scattering and absorption of light into interband transitions of the VO2.

Ultraviolet and Optical Line Profile Variations in the Spectrum of epsilon Persei

The rapid variable star, v Per (B0.5 IVIII), displays the largest amplitude pro—le —uctuations known among the growing number of massive, spectrum-variable stars. Here we present an analysis of a contin- uous 5 day run of IUE UV spectroscopy, and we show for the —rst time that the systematic, blue-to-red moving patterns observed in high-quality optical spectra are also present in the UV photospheric lines. We present cross-correlation functions of the individual spectra with that of a narrow-lined standard that produce a high signal-to-noise ratio representation of the blue-to-red moving bump patterns found in individual lines. We then use time series analysis methods to determine the periodic components of the pro—le variations (after reregistering the spectra to correct for binary motion). There are at least six periods present (ranging from 8.46 to 2.27 hr), and most of these signals are also found in optical line variations observed in 1986 (although the relative amplitudes have changed signi—cantly). Furthermore, analysis of a shorter time series of IUE spectra from 1984 shows that similar periods were present then. We also present Ha and He I j6678 pro—les obtained with the Georgia State University Multi-Telescope Telescope, which were made simultaneously with IUE, and we show that the pro—le variations are essen- tially identical in the UV and optical ranges. We rule out rotational modulation and circumstellar gas obscuration as possible causes, and we suggest instead that the variations are the result of photospheric nonradial pulsations of relatively low degree (l \ 3¨5). There were signi—cant changes (D10%) in the equivalent widths of the UV stellar wind lines during the IUE run, and we suggest that wind strength- ening events are related to episodes of large-amplitude, constructive interference between the NRP modes. Thus, intermode beating may play an important role in promoting wind loss from massive stars. Subject headings: binaries: spectroscopicstars: early-typestars: individual (v Persei) ¨ stars: oscillationsstars: winds, out—owsultraviolet: stars