arXiv subjects

Nina Hernitschek

Publications and source records attributed to Nina Hernitschek.

16 recordsLinked to original sources

arXiv2024

Large Interferometer For Exoplanets (LIFE). XIV. Finding terrestrial protoplanets in the galactic neighborhood

The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization to distances from the solar system far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based mid-infrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. Here, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres, in young stellar associations in particular. Using the LIFE mission instrument simulator (LIFEsim) we assess how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplanets. We focus on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature using a blackbody assumption, and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Our simulations suggest that LIFE will be able to detect (S/N $\geq$ 7) hot protoplanets in young stellar associations up to distances of $\approx$100 pc from the solar system for reasonable integration times (up to $\sim$hours). Detection of an Earth-sized protoplanet orbiting a solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions $<$6 $μ$m is important for decreasing the detection threshold and discriminating emission temperatures.

astro-ph.EP↗

arXiv2023

nbi: the Astronomer's Package for Neural Posterior Estimation

Despite the promise of Neural Posterior Estimation (NPE) methods in astronomy, the adaptation of NPE into the routine inference workflow has been slow. We identify three critical issues: the need for custom featurizer networks tailored to the observed data, the inference inexactness, and the under-specification of physical forward models. To address the first two issues, we introduce a new framework and open-source software nbi (Neural Bayesian Inference), which supports both amortized and sequential NPE. First, nbi provides built-in "featurizer" networks with demonstrated efficacy on sequential data, such as light curve and spectra, thus obviating the need for this customization on the user end. Second, we introduce a modified algorithm SNPE-IS, which facilities asymptotically exact inference by using the surrogate posterior under NPE only as a proposal distribution for importance sampling. These features allow nbi to be applied off-the-shelf to astronomical inference problems involving light curves and spectra. We discuss how nbi may serve as an effective alternative to existing methods such as Nested Sampling. Our package is at https://github.com/kmzzhang/nbi.

astro-ph.IM↗

arXiv2022

Twinkle -- a small satellite spectroscopy mission for the next phase of exoplanet science

With a focus on off-the-shelf components, Twinkle is the first in a series of cost competitive small satellites managed and financed by Blue Skies Space Ltd. The satellite is based on a high-heritage Airbus platform that will carry a 0.45 m telescope and a spectrometer which will provide simultaneous wavelength coverage from 0.5-4.5 $\rm{μm}$. The spacecraft prime is Airbus Stevenage while the telescope is being developed by Airbus Toulouse and the spectrometer by ABB Canada. Scheduled to begin scientific operations in 2025, Twinkle will sit in a thermally-stable, sun-synchronous, low-Earth orbit. The mission has a designed operation lifetime of at least seven years and, during the first three years of operation, will conduct two large-scale survey programmes: one focused on Solar System objects and the other dedicated to extrasolar targets. Here we present an overview of the architecture of the mission, refinements in the design approach, and some of the key science themes of the extrasolar survey.

astro-ph.IM↗

arXiv2022

Rubin Observatory LSST Transients and Variable Stars Roadmap

The Vera C. Rubin Legacy Survey of Space and Time holds the potential to revolutionize time domain astrophysics, reaching completely unexplored areas of the Universe and mapping variability time scales from minutes to a decade. To prepare to maximize the potential of the Rubin LSST data for the exploration of the transient and variable Universe, one of the four pillars of Rubin LSST science, the Transient and Variable Stars Science Collaboration, one of the eight Rubin LSST Science Collaborations, has identified research areas of interest and requirements, and paths to enable them. While our roadmap is ever-evolving, this document represents a snapshot of our plans and preparatory work in the final years and months leading up to the survey's first light.

astro-ph.IM↗

arXiv2021

The Impact of Observing Strategy on Reliable Classification of Standard Candle Stars: Detection of Amplitude, Period, and Phase Modulation (Blazhko Effect) of RR Lyrae Stars with LSST

The Vera C. Rubin Observatory will carry out its Legacy Survey of Space and Time (LSST) with a single-exposure depth of $r{\sim}24.7$ and an anticipated baseline of 10 years, allowing to access the Milky Way's old halo not only deeper, but also with a longer baseline and better cadence than e.g. PS1 3$π$ (Chambers et al. 2016). This will make LSST ideal to study populations of variable stars such as RR Lyrae stars (RRL). Here, we address the question of observing strategy optimization of LSST, as survey footprint definition, single visit exposure time as well as the cadence of repeat visits in different filters are yet to be finalized. We present metrics used to assess the impact of different observing strategies on the reliable detectability and classification of standard-candle variable stars, including detection of amplitude period, phase modulation effects of RR Lyrae stars, the so-called Blazhko effect (Blazhko 1907, Kollath et al. 2011), by evaluating metrics for simulated potential survey designs. So far, due to depth and cadence of typical all-sky surveys, it was nearly impossible to study this effect on a larger sample. All-sky surveys with relatively few observations over a moderately long baseline allow only for fitting phase-folded RRL light curves, thus integrating over the complete survey length and hiding any information regarding possible period or phase modulation during the survey. On the other hand, surveys with a cadence to detect slightly changing light curves usually have a relatively small footprint. LSST's survey strategy, however, will allow for studying variable stars in a way that makes population studies possible.

astro-ph.SR↗

arXiv2020

Long-Period High-Amplitude Red Variables in the KELT Survey

We present a sample of 4,132 Mira-like variables (red variables with long periods and high amplitudes) in the Kilodegree Extremely Little Telescope (KELT) survey. Of these, 814 are new detections. We used 2MASS colors to identify candidate asymptotic giant branch (AGB) stars. We tested for photometric variability among the sample and used Lomb-Scargle to determine the periodicity of the variable sample. We selected variables with high amplitudes and strong periodic behavior using a Random Forest classifier. Of the sample of 4,132 Mira-like variables, we estimate that 70% are Miras, and 30% are semi-regular (SR) variables. We also adopt the method of using (W_{RP} - W_{K_s}) vs. (J - K_s) colors (Lebzelter et al. 2018) in distinguishing between O-rich and C-rich Miras and find it to be an improvement over 2MASS colors.

astro-ph.SR↗

arXiv2019

Constraints on the Galactic Inner Halo Assembly History from the Age Gradient of Blue Horizontal-branch Stars

We present an analysis of the relative age distribution of the Milky Way halo, based on samples of blue horizontal-branch (BHB) stars obtained from the Panoramic Survey Telescope and Rapid Response System and \textit{Galaxy Evolution Explorer} photometry, as well a Sloan Digital Sky Survey spectroscopic sample. A machine-learning approach to the selection of BHB stars is developed, using support vector classification, with which we produce chronographic age maps of the Milky Way halo out to 40\,kpc from the Galactic center. We identify a characteristic break in the relative age profiles of our BHB samples, corresponding to a Galactocentric radius of $R_{\rm{GC}} \sim 14$\,kpc. Within the break radius, we find an age gradient of $-63.4 \pm 8.2$ Myr kpc$^{-1}$, which is significantly steeper than obtained by previous studies that did not discern between the inner- and outer-halo regions. The gradient in the relative age profile and the break radius signatures persist after correcting for the influence of metallicity on our spectroscopic calibration sample. We conclude that neither are due to the previously recognized metallicity gradient in the halo, as one passes from the inner-halo to the outer-halo region. Our results are consistent with a dissipational formation of the inner-halo population, involving a few relatively massive progenitor satellites, such as those proposed to account for the assembly of \textit{Gaia}-Enceladus, which then merged with the inner halo of the Milky Way.

astro-ph.GA↗

arXiv2019

The Complete Calibration of the Color-Redshift Relation (C3R2) Survey: Analysis and Data Release 2

The Complete Calibration of the Color-Redshift Relation (C3R2) survey is a multi-institution, multi-instrument survey that aims to map the empirical relation of galaxy color to redshift to i~24.5 (AB), thereby providing a firm foundation for weak lensing cosmology with the Stage IV dark energy missions Euclid and WFIRST. Here we present 3171 new spectroscopic redshifts obtained in the 2016B and 2017A semesters with a combination of DEIMOS, LRIS, and MOSFIRE on the Keck telescopes. The observations come from all of the Keck partners: Caltech, NASA, the University of Hawaii, and the University of California. Combined with the 1283 redshifts published in DR1, the C3R2 survey has now obtained and published 4454 high quality galaxy redshifts. We discuss updates to the survey design and provide a catalog of photometric and spectroscopic data. Initial tests of the calibration method performance are given, indicating that the sample, once completed and combined with extensive data collected by other spectroscopic surveys, should allow us to meet the cosmology requirements for Euclid, and make significant headway toward solving the problem for WFIRST. We use the full spectroscopic sample to demonstrate that galaxy brightness is weakly correlated with redshift once a galaxy is localized in the Euclid or WFIRST color space, with potentially important implications for the spectroscopy needed to calibrate redshifts for faint WFIRST and LSST sources.

astro-ph.GA↗

arXiv2018

Precision distances to dwarf galaxies and globular clusters from Pan-STARRS1 3$π$ RR Lyrae

We present new spatial models and distance estimates for globular clusters (GC) and dwarf spheroidals (dSphs) orbiting our Galaxy based on RR Lyrae (RRab) stars in the Pan-STARRS1 (PS1) 3$π$ survey. Using the PS1 sample of RRab stars from Sesar et al. (2017) in 16 globular clusters and 5 dwarf galaxies, we fit structural models in $(l,b,D)$ space; for 13 globular clusters and 6 dwarf galaxies, we give only their mean heliocentric distance $D$. We verify the accuracy of the period-luminosity (PL) relations used in Sesar et al. (2017) to constrain the distance to those stars, and compare them to period-luminosity-metallicity (PLZ) relations using metallicities from Carretta et al. (2009). We compare our Sesar et al. (2017) distances to the parallax-based \textit{Gaia} DR2 distance estimates from Bailer-Jones et al. (2018), and find our distances to be consistent and considerably more precise.

astro-ph.GA↗

arXiv2018

The Profile of the Galactic Halo from Pan-STARRS1 3$π$ RR Lyrae

We characterize the spatial density of the Pan-STARRS1 (PS1) sample of RR Lyrae stars, to study the properties of the old Galactic stellar halo as traced by RRab stars. This sample of 44,403 sources spans Galactocentric radii of $0.55 \; \mathrm{kpc} \leq R_{\mathrm{gc}} \leq 141 \; \mathrm{kpc}$ with a distance precision of 3\% and thus is able to trace the halo out to larger distances than most previous studies. After excising stars that are attributed to dense regions such as stellar streams, the Galactic disc and bulge as well as halo globular clusters, the sample contains ${\sim}11,000$ sources within $20 \; \mathrm{kpc} \leq R_{\mathrm{gc}} \leq 131 \; \mathrm{kpc}$. We then apply forward modeling using ellipsoidal stellar density models $ρ(l,b,R_{\mathrm{gc}})$ both with a constant and a radius-dependent halo flattening $q(R_{\mathrm{gc}})$. Assuming constant flattening $q$, the distribution of the sources is reasonably well fit from $20 \; \mathrm{kpc}$ to $131 \; \mathrm{kpc}$ by a single power law with $n=4.40^{+0.05}_{-0.04}$ and $q=0.918^{+0.016}_{-0.014}$. The distance distribution is fit comparably well by an Einasto profile with $n=9.53^{+0.27}_{-0.28}$, an effective radius $r_{\mathrm{eff}}=1.07 \pm 0.10 \; \mathrm{kpc}$ and a halo flattening of $q=0.923 \pm 0.007$. If we allow for a radius-dependent flattening $q(R_{\mathrm{gc}})$, we find evidence for a distinct flattening of $q{\sim}0.8$ of the inner halo at ${\sim} 25 \; \mathrm{kpc}$. Additionally, we find that the south Galactic hemisphere is more flattened than the north Galactic hemisphere. The results of our work are largely consistent with many earlier results, e.g. \cite{Watkins2009}, \cite{Iorio2017}. We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after all known overdensities have been masked.

astro-ph.GA↗

arXiv2018

Connecting the Milky Way potential profile to the orbital timescales and spatial structure of the Sagittarius Stream

Recent maps of the halo using RR Lyrae from Pan-STARRS1 have clearly depicted the spatial structure of the Sagittarius stream. These maps show the leading and trailing stream apocenters differ in galactocentric radius by a factor of two, and also resolve substructure in the stream at these apocenters. Here we present dynamical models that reproduce these features of the stream in simple Galactic potentials. We find that debris at the apocenters must be dynamically young, in the sense of being stripped off in the last two pericentric passages, while the Sagittarius dwarf is currently experiencing a third passage. The ratio of apocenters is sensitive to both dynamical friction and the outer slope of the Galactic rotation curve. These dependences can be understood with simple regularities connecting the apocentric radii, circular velocities, and orbital period of the progenitor. The effect of dynamical friction on the stream can be constrained using substructure within the leading apocenter. Our ensembles of models are not intended as statistically proper fits to the stream. Nevertheless, out of the range of models we consider, we consistently find the mass within 100 kpc to be $\sim 7 \times 10^{11} \, M_{\odot}$, with a nearly flat rotation curve between 50 and 100 kpc. This points to a more extended Galactic halo than assumed in some current models. As in previous work, we find prolate or triaxial halos ease agreement with the track of the leading stream. We display the behavior of our models in various observational spaces and characterize the substructure expected within the stream. In particular, the young trailing stream visible near trailing apocenter should exhibit a tight trend of velocity with distance separate from the older debris, and we suggest that this will serve as an especially useful probe of the outer Galactic potential.

astro-ph.GA↗

arXiv2017

The Geometry of Sagittarius Stream from Pan-STARRS1 3$π$ RR Lyrae

We present a comprehensive and precise description of the Sagittarius (Sgr) stellar stream's 3D geometry as traced by its old stellar population. This analysis draws on the sample of ${\sim}44,000$ RR Lyrae (RRab) stars from the Pan-STARRS1 (PS1) 3$π$ survey (Hernitschek et al. 2016,Sesar et al. 2017b), which is ${\sim}80\%$ complete and ${\sim}90\%$ pure within 80~kpc, and extends to ${\gtrsim} 120$~kpc with a distance precision of ${\sim} 3\%$. A projection of RR Lyrae stars within $|\tilde{B}|_{\odot}<9^\circ$ of the Sgr stream's orbital plane reveals the morphology of both the leading and the trailing arms at very high contrast, across much of the sky. In particular, the map traces the stream near-contiguously through the distant apocenters. We fit a simple model for the mean distance and line-of-sight depth of the Sgr stream as a function of the orbital plane angle $\tildeΛ_{\odot}$, along with a power-law background-model for the field stars. This modeling results in estimates of the mean stream distance precise to ${\sim}1\%$ and it resolves the stream's line-of-sight depth. These improved geometric constraints can serve as new constraints for dynamical stream models.

astro-ph.GA↗

arXiv2017

The $>100$ kpc Distant Spur of the Sagittarius Stream and the Outer Virgo Overdensity, as Seen in PS1 RR Lyrae Stars

We report the detection of spatially distinct stellar density features near the apocenters of the Sagittarius (Sgr) stream's main leading and trailing arm. These features are clearly visible in a high-fidelity stellar halo map that is based on RR Lyrae from Pan-STARRS1: there is a plume of stars 10 kpc beyond the apocenter of the leading arm, and there is a "spur" extending to 130 kpc, almost 30 kpc beyond the previously detected apocenter of the trailing arm. Such apocenter substructure is qualitatively expected in any Sgr stream model, as stars stripped from the progenitor at different pericenter passages become spatially separated there. The morphology of these new Sgr stream substructures could provide much-needed new clues and constraints for modeling the Sgr system, including the level of dynamical friction that Sgr has experienced. We also report the discovery of a new, presumably unrelated halo substructure at 80 kpc from the Sun and $10^\circ$ from the Sgr orbital plane, which we dub the Outer Virgo Overdensity.

astro-ph.GA↗

arXiv2017

Machine-Learned Identification of RR Lyrae Stars from Sparse, Multi-band Data: the PS1 Sample

RR Lyrae stars may be the best practical tracers of Galactic halo (sub-)structure and kinematics. The PanSTARRS1 (PS1) $3π$ survey offers multi-band, multi-epoch, precise photometry across much of the sky, but a robust identification of RR Lyrae stars in this data set poses a challenge, given PS1's sparse, asynchronous multi-band light curves ($\lesssim 12$ epochs in each of five bands, taken over a 4.5-year period). We present a novel template fitting technique that uses well-defined and physically motivated multi-band light curves of RR Lyrae stars, and demonstrate that we get accurate period estimates, precise to 2~sec in $>80\%$ of cases. We augment these light curve fits with other {\em features} from photometric time-series and provide them to progressively more detailed machine-learned classification models. From these models we are able to select the widest ($3/4$ of the sky) and deepest (reaching 120 kpc) sample of RR Lyrae stars to date. The PS1 sample of $\sim 45,000$ RRab stars is pure (90\%), and complete (80\% at 80 kpc) at high galactic latitudes. It also provides distances precise to 3\%, measured with newly derived period-luminosity relations for optical/near-infrared PS1 bands. With the addition of proper motions from {\em Gaia} and radial velocity measurements from multi-object spectroscopic surveys, we expect the PS1 sample of RR Lyrae stars to become the premier source for studying the structure, kinematics, and the gravitational potential of the Galactic halo. The techniques presented in this study should translate well to other sparse, multi-band data sets, such as those produced by the Dark Energy Survey and the upcoming Large Synoptic Survey Telescope Galactic plane sub-survey.

astro-ph.GA↗

arXiv2015

Finding, characterizing and classifying variable sources in multi-epoch sky surveys: QSOs and RR Lyrae in PS1 3$π$ data

In area and depth, the Pan-STARRS1 (PS1) 3$π$ survey is unique among many-epoch, multi-band surveys and has enormous potential for all-sky identification of variable sources. PS1 has observed the sky typically seven times in each of its five bands ($grizy$) over 3.5 years, but unlike SDSS not simultaneously across the bands. Here we develop a new approach for quantifying statistical properties of non-simultaneous, sparse, multi-color lightcurves through light-curve structure functions, effectively turning PS1 into a $\sim 35$-epoch survey. We use this approach to estimate variability amplitudes and timescales $(ω_r, τ)$ for all point-sources brighter than $r_{\mathrm{P1}}=21.5$ mag in the survey. With PS1 data on SDSS Stripe 82 as ``ground truth", we use a Random Forest Classifier to identify QSOs and RR Lyrae based on their variability and their mean PS1 and WISE colors. We find that, aside from the Galactic plane, QSO and RR Lyrae samples of purity $\sim$75\% and completeness $\sim$92\% can be selected. On this basis we have identified a sample of $\sim 1,000,000$ QSO candidates, as well as an unprecedentedly large and deep sample of $\sim$150,000 RR Lyrae candidates with distances from $\sim$10 kpc to $\sim$120 kpc. Within the Draco dwarf spheroidal, we demonstrate a distance precision of 6\% for RR Lyrae candidates. We provide a catalog of all likely variable point sources and likely QSOs in PS1, a total of $25.8\times 10^6$ sources.

astro-ph.GA↗

arXiv2014

Estimating Black Hole Masses in Hundreds of Quasars

We explore the practical feasibility of AGN broad-band reverberation mapping and present first results. We lay out and apply a rigorous approach for stochastic reverberation mapping of unevenly sampled multi-broad-band flux measurements, assuming that the broad-line region (BLR) line flux is contributing up to 15 % in some bands, and is directly constrained by one spectroscopical epoch. The approach describes variations of the observed flux as the continuum, modeled as a stochastic Gaussian process, and emission line contribution, modeled as a scaled, smoothed and delayed version of the continuum. This approach is capable not only to interpolate in time between measurements, but also to determine confidence limits on continuum -- line emission delays. This approach is applied to SDSS observations in 'Stripe 82' (S82) providing flux measurements precise to 2 % at $\sim$ 60 epochs over $\sim$ 10 years. The strong annual variations in the epoch sampling prove a serious limitation in practice. Also, suitable redshift ranges must be identified, where strong broad emission line contribute to one filter, but not to another. Through generating and evaluating problem-specific mock data, we verify that S82-like data can constrain $τ_{\mathrm{delay}}$ for a simple transfer function model. In application to real data, we estimate $τ_{\mathrm{delay}}$ for 323 AGN with $0.225 < z < 0.846$, combining information for different objects through the ensemble-scaling relationships for BLR size and BH mass. Our analysis tentatively indicates a 1.7 times larger BLR size of H$α$ and MgII compared to Kaspi2000 and Vestergaard2002, but the seasonal data sampling casts doubt on the robustness of the inference.

astro-ph.GA↗