Lena Murchikova's Home Page Lena (Elena M.) Murchikova's Home Page

I am a Corning Glass Works Foundation Fellows at the Institute for Advanced Study in Princeton

office: Bloomberg Hall 249
email: lena AT ias DOT edu

Mailing address
School of Natural Sciences
Institute for Advanced Study
Princeton, NJ 08540

Education
Specialist, Theoretical Physics, Moscow State University
PhD, Astrophysics, California Institute of Technology (2018)

Research Interests
I am a physicist working on a range of theoretical and observational problems in astrophysics. My research interests span black holes, in particular our own Galactic Center black hole Sagittarius A*, neutron stars, black hole accretion theory, exoplanets and star formation. For my research related to Sagittarius A*, I work a lot with interferometric data from the Atacama Large Millimeter/submillimeter Array (ALMA).
In my past life I also wrote papers in string theory and particle physics. And in nanophotonics as an underaduate.

Recent Works
Full list of publications is available on ArXiv and ADS
(published as Lena Murchikova, Elena Murchikova and E.M. Murchikova)

Reconstructing EUV spectrum of star forming regions from millimeter recombination lines of HI, HeI, and HeII
Lena Murchikova, Eric J. Murphy, Dariusz C. Lis, Lee Armus, Nadia Zakamska et al, Astrophysical Journal in press (2020)

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross-section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The mm/submm recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultra-compact HII regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using mm recombination lines of HI, HeI and HeII. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: L_\nu ~ \nu^{-\gamma} ~\nu^{-4.5 +/- 0.4}. We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in HII regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star forming regions providing critical insight into photon production rates at < 912 Angstrom and can serve as calibration to starburst synthesis models, improving our understanding of star formation in distant universe and the properties of ionizing flux during reionization.

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Peas in a Pod? Radius correlations in Kepler multi-planet systems
Lena Murchikova & Scott Tremaine, The Astronomical Journal (2020)

In this work we address the claim of Weiss et al. (2018) that the radii of adjacent planets in Kepler multi-planet systems are correlated. We explore two simple toy models---in the first the radii of the planets are chosen at random from a single universal distribution, and in the second we postulate several types of system with distinct radius distributions. We show that an apparent correlation between the radii of adjacent planets similar to the one reported by Weiss et al. (2018) can arise in both models. In addition the second model fits the radius and signal-to-noise distribution of the observed planets.

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A Cool Accretion Disk around the Galactic Centre Black Hole
Elena M. Murchikova, Anna Pancoast, E. Sterl Phinney, Roger D. Blandford, Nature (2019)

A supermassive black hole SgrA* with the mass ~4x10^6 Msun resides at the centre of our galaxy. Building up such a massive black hole within the ~10^10 year lifetime of our galaxy would require a mean accretion rate of ~4x10^-4 Msun/yr. At present, X-ray observations constrain the rate of hot gas accretion at the Bondi radius (10^5 R_Sch = 0.04 pc at 8kpc) to \dot{M}_Bondi ~ 3x10^-6 Msun/yr, and polarization measurements constrain it near the event horizon to \dot{M}_horizon ~ 10^{-8} Msun/yr. A range of models was developed to describe the accretion gas onto an underfed black hole. However, the exact physics still remains to be understood. One challenge with the radiation inefficient accretion flows is that even if one understands the dynamics there is no accepted prescription for associating emissivity (and absorption) with the flow. The other issue is the lack of model-independent probes of accretion flow at intermediate radii (between few and ~ 10^5 R_Sch), i.e. the constraints that do not assume a model of accretion flow as an input parameter. We report detection and imaging of the 10^4 K ionized gas disk within 2x10^4 R_Sch in a mm hydrogen recombination line H30alpha: n = 31 -> 30 at 231.9 GHz using the ALMA. The emission was detected with a double-peaked line profile spanning full width of 2,200 km/s with the approaching and the receding components straddling Sgr A*, each offset from it by 0.11arcsec= 0.004pc. The red-shifted side is displaced to the north-east, while the blue-shifted side is displaced to the south-west. The limit on the total mass of ionized gas estimated from the emission is 10^-4 - 10^-5 Sun at a mean hydrogen density 10^5-10^6 cm^-3, depending upon whether or not we assume the presence of a uniform density disk or an ensemble of orbiting clouds, and the amplification factor of the mm radiation due to the strong background source which is Sgr A* continuum.

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