arXiv subjects

Nikhil Patten

Publications and source records attributed to Nikhil Patten.

3 recordsLinked to original sources

arXiv2026

Using GALEX UV Excess to Search for Metal-poor Halo Stars

Metal-poor solar-type stars display a significant reduction in metal-line blanketing at short wavelengths, leading to an excess of near-ultraviolet (NUV) flux compared to their metal-rich counterparts. We utilize GALEX NUV and $\it{Gaia}$ DR3 photometry along with ground-based spectroscopy to establish a correlation between NUV excess and [Fe/H]. We construct a sample of 492 solar-type (F5-G9) halo stars with NUV excess and measured metallicitices. We perform our own observations with the KOSMOS spectrograph at Apache Point Observatory's 3.5m telescope to measure the abundances of 13 halo stars, 11 of which did not have previous metallicity measurements. Our targeted 13 halo stars span $-$2.92 $<$ [Fe/H] $<$ $-$1.97 and are all $\alpha$ enhanced with [$\alpha$/Fe] = 0.05-0.73. For our full sample of 492 objects, we find an anti-correlation between NUV excess and [Fe/H] that is statistically significant at the 8$\sigma$ level. GALEX NUV excess can be used to distinguish very metal-poor (VMP) stars ([Fe/H] $<$ $-$2) from their metal-rich counterparts. However, there is significant dispersion in the relation due to NUV chromospheric variability caused by rotational effects and magnetic cycle activity. The NUV chromospheric variability inhibits our ability to reliably distinguish extremely metal-poor (EMP) stars ([Fe/H] $<$ $-$3) from VMP stars based on photometry alone. UV spectra of EMP halo stars are needed to better calibrate their atmospheric properties and variability.

astro-ph.SR↗

arXiv2025

Fundamental Parameters for Central Stars of 103 Infrared Bowshock Nebulae

Stellar bowshock nebulae are arcuate shock fronts formed by the interaction of radiation-driven stellar winds and the relative motion of the ambient interstellar material. Stellar bowshock nebulae provide a promising means to measure wind-driven mass loss, independent of other established methods. In this work, we characterize the stellar sources at the center of bowshock nebulae drawn from all-sky catalogs of 24 $\mu$m-selected nebulae. We obtain new, low-resolution blue optical spectra for \numstars~stars and measure stellar parameters temperature \teff, surface gravity \logg, and projected rotational broadening \vsini. We perform additional photometric analysis to measure stellar radius \rstar, luminosity \lstar, and visual-band extinction $A_V$. All but one of our targets are O and early B stars, with temperatures ranging from $T$=16.5--46.8~k\kelvin, gravities $\log g=$2.57--4.60, and \vsini~from $<$100--400~\kms. With the exception of rapid rotator $\zeta$ Oph, bowshock stars do not rotate at or near critical velocities. At least 60 of 103 (60\%) OB bowshock stars are binaries, consistent with the multiplicity fraction of other OB samples. The sample shows a runaway fraction of 23\%, with \numrunaway~stars having $v_{\text{2D}}\geq25$~\kms. Of the 19 runaways, at least 15 ($\geq$79\%) are binaries, favoring dynamical ejection over the binary supernova channel for producing runaways. We provide a comprehensive census of stellar parameters for bowshock stars, useful as a foundation for determining the mass-loss rates for OB-type stars -- one of the single most critical factors in stellar evolution governing the production of neutron stars and black holes.

astro-ph.SR↗

arXiv2025

The Milky Way Project MOBStIRS: Parametrizing Infrared Stellar-Wind Bow Shock Morphologies with Citizen Science

Mass-loss influences stellar evolution, especially for massive stars with strong winds. Stellar wind bow shock nebulae driven by Galactic OB stars can be used to measure mass-loss rates ($\dot{M}$). The standoff distance ($R_{0}$) between the star and the bow shock is set by momentum flux balance between the stellar wind and the surrounding interstellar medium (ISM). We created the Milky Way Project: MOBStIRS (Mass-loss rates for OB Stars driving IR bow Shocks) using the online Zooniverse citizen science platform. We enlisted several hundred students to measure $R_0$ and two other projected shape parameters for 764 cataloged IR bow shocks. MOBStIRS incorporated 1528 JPEG cutout images produced from Spitzer GLIMPSE and MIPSGAL survey data. Measurements were aggregated to compute shape parameters for each bow shock image deemed high-quality by participants. The average statistical uncertainty on $R_0$ is $12.5\%$ but varies from ${<}5\%$ to ${\sim}40\%$ among individual bow shocks, contributing significantly to the total error budget of $\dot{M}$. The derived nebular morphologies agree well with (magneto)hydrodynamic simulations of bow shocks driven by the winds of OB stars moving at $V_a = 10-40~km~s^{-1}$ with respect to the ambient interstellar medium (ISM). A systematic correction to $R_0$ to account for viewing angle appears unnecessary for computing $\dot{M}$. Slightly more than half of MOBStIRS bow shocks are asymmetric, which could indicate anisotropic stellar winds, ISM clumping on sub-pc scales, time-dependent instabilities, and/or misalignments between the local ISM magnetic field and the star-bow shock axis.

astro-ph.GA↗