Experience

Publications

• gamma Doradus Stars: Defining a new class of pulsating variables

  Publications of the Astronomical Society of the Pacific

  In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf gamma 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. This paper has been cited 103 times as of October, 2013.

• gamma Doradus Stars: Defining a new class of pulsating variables

  Publications of the Astronomical Society of the Pacific

  In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf gamma 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. This paper has been cited 103 times as of October, 2013.

• HR 8799: A link between gamma Doradus variables and lambda Bootis stars

  Astronomical Journal

  On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered gamma Doradus variable star class, is a lambda Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields T-eff = 7430 K, log g = 4.35, microturbulent velocity xi(t) = 2.7 km s(-1), and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a "Vega-like" star in that it shows excess flux at 60 mu m, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the lambda Bootis phenomenon and the gamma Doradus pulsation phenomenon. This paper has been cited 41 times (as of September 2012).

• gamma Doradus Stars: Defining a new class of pulsating variables

  Publications of the Astronomical Society of the Pacific

  In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf gamma 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. This paper has been cited 103 times as of October, 2013.

• HR 8799: A link between gamma Doradus variables and lambda Bootis stars

  Astronomical Journal

  On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered gamma Doradus variable star class, is a lambda Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields T-eff = 7430 K, log g = 4.35, microturbulent velocity xi(t) = 2.7 km s(-1), and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a "Vega-like" star in that it shows excess flux at 60 mu m, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the lambda Bootis phenomenon and the gamma Doradus pulsation phenomenon. This paper has been cited 41 times (as of September 2012).

• Driving the gravity-mode pulsations in gamma Doradus variables

  Astrophysical Journal Letters

  The gamma Doradus stars are a newly discovered class of gravity-mode pulsators that lie just at or beyond the red edge of the delta 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 similar to 200,000 and similar to 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 gamma Doradus star has been observed that also pulsates in delta Scuti-type p-modes, and others have been identified as chemically peculiar. Since our calculated driving region is relatively deep, gamma Doradus pulsations are not necessarily incompatible with surface abundance peculiarities or with delta Scuti p-mode pulsations driven by the H and He ionization K-effect. Such stars will provide useful observational constraints on the proposed gamma Doradus pulsation mechanism. This paper has been cited 80 times (as of October 2013).

• gamma Doradus Stars: Defining a new class of pulsating variables

  Publications of the Astronomical Society of the Pacific

  In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf gamma 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. This paper has been cited 103 times as of October, 2013.

• HR 8799: A link between gamma Doradus variables and lambda Bootis stars

  Astronomical Journal

  On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered gamma Doradus variable star class, is a lambda Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields T-eff = 7430 K, log g = 4.35, microturbulent velocity xi(t) = 2.7 km s(-1), and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a "Vega-like" star in that it shows excess flux at 60 mu m, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the lambda Bootis phenomenon and the gamma Doradus pulsation phenomenon. This paper has been cited 41 times (as of September 2012).

• Driving the gravity-mode pulsations in gamma Doradus variables

  Astrophysical Journal Letters

  The gamma Doradus stars are a newly discovered class of gravity-mode pulsators that lie just at or beyond the red edge of the delta 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 similar to 200,000 and similar to 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 gamma Doradus star has been observed that also pulsates in delta Scuti-type p-modes, and others have been identified as chemically peculiar. Since our calculated driving region is relatively deep, gamma Doradus pulsations are not necessarily incompatible with surface abundance peculiarities or with delta Scuti p-mode pulsations driven by the H and He ionization K-effect. Such stars will provide useful observational constraints on the proposed gamma Doradus pulsation mechanism. This paper has been cited 80 times (as of October 2013).

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

  Nano Letters

  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.

• gamma Doradus Stars: Defining a new class of pulsating variables

  Publications of the Astronomical Society of the Pacific

  In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf gamma 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. This paper has been cited 103 times as of October, 2013.

• HR 8799: A link between gamma Doradus variables and lambda Bootis stars

  Astronomical Journal

  On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered gamma Doradus variable star class, is a lambda Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields T-eff = 7430 K, log g = 4.35, microturbulent velocity xi(t) = 2.7 km s(-1), and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a "Vega-like" star in that it shows excess flux at 60 mu m, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the lambda Bootis phenomenon and the gamma Doradus pulsation phenomenon. This paper has been cited 41 times (as of September 2012).

• Driving the gravity-mode pulsations in gamma Doradus variables

  Astrophysical Journal Letters

  The gamma Doradus stars are a newly discovered class of gravity-mode pulsators that lie just at or beyond the red edge of the delta 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 similar to 200,000 and similar to 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 gamma Doradus star has been observed that also pulsates in delta Scuti-type p-modes, and others have been identified as chemically peculiar. Since our calculated driving region is relatively deep, gamma Doradus pulsations are not necessarily incompatible with surface abundance peculiarities or with delta Scuti p-mode pulsations driven by the H and He ionization K-effect. Such stars will provide useful observational constraints on the proposed gamma Doradus pulsation mechanism. This paper has been cited 80 times (as of October 2013).

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

  Nano Letters

  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.

• Plasmonic enhancement of the vanadium dioxide phase transition induced by low-power laser irradiation

  Applied Physics A - Materials Science & Processing

  Nanocomposites consisting of gold nanoparticle (NP) arrays and vanadium dioxide (VO2) thin films are noteworthy for the tunability of both their thermal and optical properties. The localized surface plasmon resonance (LSPR) of the Au can be tuned when its dielectric environment is modulated by the semiconducting-to-metal phase transition (SMT) of the VO2; the LSPR itself can be altered by changing the shape of the NPs and the pitch of the NP array. In principle, then it should be possible to choose a combination of VO2 film and Au LSPR properties that maximizes the overall optical response of the nanocomposite. To demonstrate this effect, transient transmission measurements were conducted on lithographically fabricated arrays of Au NPs of diameter 140 nm, array spacing 350 nm, and covered with a 60 nm thick films of VO2 via pulsed laser deposition. Both Au::VO2 nanocomposites and bare VO2 film were irradiated with a shuttered 785 nm pump laser, and their optical response was probed at 1550 nm by a fixed-frequency diode laser. The Au::VO2 nanocomposite exhibited an increased effective absorption coefficient 1.5 times that of the plain film and required 37 % less laser power to induce the SMT. The time-dependent temperature rise in the film as a function of laser intensity was calculated from these measurements and compared with both analytic and finite-element models. Our results suggest that Au::VO2 nanocomposites may be useful in applications such as thermal-management coatings for energy efficient "smart" windows.

• gamma Doradus Stars: Defining a new class of pulsating variables

  Publications of the Astronomical Society of the Pacific

  In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf gamma 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. This paper has been cited 103 times as of October, 2013.

• HR 8799: A link between gamma Doradus variables and lambda Bootis stars

  Astronomical Journal

  On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered gamma Doradus variable star class, is a lambda Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields T-eff = 7430 K, log g = 4.35, microturbulent velocity xi(t) = 2.7 km s(-1), and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a "Vega-like" star in that it shows excess flux at 60 mu m, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the lambda Bootis phenomenon and the gamma Doradus pulsation phenomenon. This paper has been cited 41 times (as of September 2012).

• Driving the gravity-mode pulsations in gamma Doradus variables

  Astrophysical Journal Letters

  The gamma Doradus stars are a newly discovered class of gravity-mode pulsators that lie just at or beyond the red edge of the delta 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 similar to 200,000 and similar to 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 gamma Doradus star has been observed that also pulsates in delta Scuti-type p-modes, and others have been identified as chemically peculiar. Since our calculated driving region is relatively deep, gamma Doradus pulsations are not necessarily incompatible with surface abundance peculiarities or with delta Scuti p-mode pulsations driven by the H and He ionization K-effect. Such stars will provide useful observational constraints on the proposed gamma Doradus pulsation mechanism. This paper has been cited 80 times (as of October 2013).

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

  Nano Letters

  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.

• Plasmonic enhancement of the vanadium dioxide phase transition induced by low-power laser irradiation

  Applied Physics A - Materials Science & Processing

  Nanocomposites consisting of gold nanoparticle (NP) arrays and vanadium dioxide (VO2) thin films are noteworthy for the tunability of both their thermal and optical properties. The localized surface plasmon resonance (LSPR) of the Au can be tuned when its dielectric environment is modulated by the semiconducting-to-metal phase transition (SMT) of the VO2; the LSPR itself can be altered by changing the shape of the NPs and the pitch of the NP array. In principle, then it should be possible to choose a combination of VO2 film and Au LSPR properties that maximizes the overall optical response of the nanocomposite. To demonstrate this effect, transient transmission measurements were conducted on lithographically fabricated arrays of Au NPs of diameter 140 nm, array spacing 350 nm, and covered with a 60 nm thick films of VO2 via pulsed laser deposition. Both Au::VO2 nanocomposites and bare VO2 film were irradiated with a shuttered 785 nm pump laser, and their optical response was probed at 1550 nm by a fixed-frequency diode laser. The Au::VO2 nanocomposite exhibited an increased effective absorption coefficient 1.5 times that of the plain film and required 37 % less laser power to induce the SMT. The time-dependent temperature rise in the film as a function of laser intensity was calculated from these measurements and compared with both analytic and finite-element models. Our results suggest that Au::VO2 nanocomposites may be useful in applications such as thermal-management coatings for energy efficient "smart" windows.

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

  Applied Physics Letters

  Transient absorption of gold nanoparticle (NP) arrays covered by a 60 nm thick film of VO(2) 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 VO(2).