arXiv subjects

William D. Vacca

Publications and source records attributed to William D. Vacca.

At least 19 recordsLinked to original sources

arXiv2026

No Evidence for Nearby Circumstellar Material in the Type Ia Supernova 2025rbs

We present a high-resolution spectral time series of the Type Ia supernova (SN) 2025rbs discovered in the nearby galaxy NGC 7331. The Automated Planet Finder (APF) at Lick Observatory and the MAROON-X/IGRINS-2 at Gemini North were used to obtain echelle spectra between -5 and 15 days with respect to the epoch of maximum light. Several unsaturated NaID absorption components along the line of sight are identified, but there is no evidence of time variance in any of them. We measure the equivalent width of the observed diffuse interstellar band around 5780 A and constrain the extinction along the line of sight to SN 2025rbs as $A_V = 0.64\,\pm\,0.32$ mag, corresponding to a moderate reddening of $E(B-V) = 0.21\,\pm\,0.10$ mag (assuming $R_\mathrm{V}$ = 3.1). The observed Ca II H & K interstellar absorption roughly traces NaID in velocity space, suggesting a common origin. Quantitative comparisons between the column densities of Na and Ca gas in these host clouds ($N_{NaID}$ / $N_{Ca II}$ of order unity) argue against their origin in the Galactic halo gas and instead support absorption due to the interstellar gas of NGC 7331. Time invariance of all the observed absorption features suggests a lack of nearby circumstellar material ($\lesssim$ 10$^{16}$ cm) around the progenitor system of SN 2025rbs. This supports a progenitor scenario for SN 2025rbs with minimal ambient circumstellar gas, consistent with a double-degenerate CO white dwarf binary system.

astro-ph.HE↗

arXiv2026

Radiation-Driven Evolution and Gas Kinematics of the Bright-Rimmed Cloud SFO 25

Bright-rimmed clouds (BRCs) are valuable laboratories for investigating how ionizing radiation from massive stars reshapes molecular clouds and influences star formation. We present a kinematic and dynamical study of the BRC SFO 25 using archival JCMT-HARP observations of the $^{12}$CO, $^{13}$CO, and C$^{18}$O ($J = 3-2$) transitions at an angular resolution of about 14-15 arcsec (0.051-0.054 pc). Gaia parallaxes of young stellar objects associated with the cloud yield a revised distance placing SFO 25 behind the ionizing O7V star HD 47839, implying that ultraviolet radiation can directly affect both the head and tail. The molecular gas exhibits a pronounced head-tail velocity gradient, with the tail systematically redshifted relative to the head, inconsistent with the expectations of classical radiation-driven implosion (RDI) and suggestive of an evolved phase dominated by radiative dispersal and photoevaporation. The head is moderately denser than the tail, while a compact C$^{18}$O clump associated with IRAS 06382+1017 reaches densities of $10^4~\mathrm{cm}^{-3}$. Virial and energy analyses indicate that the head, tail, and dense clump are gravitationally unbound, with kinetic energy exceeding both the gravitational binding energy and the external ionized gas pressure. Class II young stellar object candidates identified in the tail demonstrate that star formation is not confined to the dense head. Although RDI may have triggered earlier star formation in the head, it cannot readily explain the activity observed in the tail, pointing to a more complex evolutionary history than predicted by the classical RDI scenario.

astro-ph.GA↗

arXiv2026

Near-Infrared Observations of BD+44 493 with IGRINS-2

We present high-resolution (R~45,000), high signal-to-noise (S/N>200) H and K-band spectra of BD+44 493, obtained with the newly commissioned IGRINS-2 instrument on the Gemini North Telescope. BD+44 493 is a well-known carbon-enhanced (CEMP; [C/Fe]=+1.40) ultra metal-poor (UMP; [Fe/H]~-4.0) star that has been extensively studied with high-resolution spectroscopy from the ultraviolet to the infrared. From the IGRINS-2 data, we derive new estimates of the abundances of C, Mg, Si, and Ca by measuring the equivalent widths of several atomic absorption features and comparing the values and line profiles with synthetic models. We measure, for the first time, a weak atomic carbon absorption feature (C I) in a UMP star in the H-band. Our estimated abundance agrees well with other values derived from CH molecules in the optical. The abundance values for the other elements are also in good agreement with previous estimates derived from ultraviolet, optical, and near-infrared observations, confirming the speculations that BD+44 493 is a bona fide second-generation star. With these data, we demonstrate that IGRINS-2 is a powerful new resource for studying and characterizing low-metallicity stars in the Milky Way and beyond.

astro-ph.SR↗

arXiv2025

The Discovery of 25 um Interstellar Methanol

We present the first astrophysical detection of methanol (CH3OH) in the torsional band near 25 um. Using high resolution mid-infrared (MIR) spectroscopy, we identified over seventy gas-phase CH3OH absorption lines between 20 and 28 um towards the massive protostar NGC 7538 IRS 1 with SOFIA/EXES. We derive a temperature of 180 K and a total column density of 2 x 10^17 cm-2, comparable to sub-mm measurements. Complementary analysis of acetylene (C2H2) absorption lines is also included. Both CH3OH and C2H2 reveal an unresolved second velocity component. These MIR absorption lines likely probe the molecular material in two edge-on disks, supporting the scenario that NGC 7538 IRS 1 consists of multiple protostars. We provide an updated line list for the torsional band of CH3OH, which was generated from lab work and model calculations. This discovery and the updated line list will enable the search for CH3OH in JWST/MIRI spectra.

astro-ph.GA↗

arXiv2024

GHOST Reduced Data Products for the Gemini Observatory Community and Beyond

The Gemini High-resolution Optical SpecTrograph (GHOST) at Gemini South started regular queue operations in early 2024, bringing a long-sought open-access capability to the astronomy community. This research note briefly describes an effort to provide easy-to-access reduced spectra for GHOST programs from all Gemini partner countries and encourage prompt data exploration and analysis. Since March 2024, over 4500 spectra have been reduced and made available to principal investigators (PIs). The aim is to increase demand for GHOST and expedite the publication of scientific results.

astro-ph.IM↗

arXiv2023

DRAGONS -- A Quick Overview

DRAGONS (Data Reduction for Astronomy from Gemini Observatory North and South) is a platform for the reduction and processing of astronomical data. The Python-based, open-source package includes infrastructure for automation and algorithms for the processing of imaging and spectroscopic data, up to the analysis-ready stage. DRAGONS currently focuses on the reduction of Gemini data, although it allows for support of data from other instruments and telescopes through third-party extensions. Its latest release (v3.1) enables automated reduction of all currently-active Gemini imaging facility instruments, as well as optical longslit spectroscopic data, acquired with GMOS.

astro-ph.IM↗

arXiv2023

Characterization and Absolute Calibration of the Far Infrared Field Integral Line Spectrometer for SOFIA

We present the characterization and definitive flux calibration of the Far-Infrared Field Integral Line Spectrometer (FIFI-LS) instrument on-board SOFIA. The work is based on measurements made in the laboratory with an internal calibrator and on observations of planets, moons, and asteroids as absolute flux calibrators made during the entire lifetime of the instrument. We describe the techniques used to derive flat-fields, water vapor column estimates, detector linearity, spectral and spatial resolutions, and absolute flux calibration. Two sets of responses are presented, before and after the entrance filter window was changed in 2018 to improve the sensitivity at 52um, a wavelength range previously not covered by PACS on Herschel. The relative spectral response of each detector and the illumination pattern of the arrays of the FIFI-LS arrays are derived using the internal calibrator before each observational series. The linearity of the array response is estimated by considering observations of bright sources. We find that the deviation from linearity of the FIFI-LS arrays affects the flux estimations less than 1%. The flux calibration accuracy is estimated to be 15% or better across the entire wavelength range of the instrument. The limited availability of sky calibrators during each observational series is the major limiting factor of the flux calibration accuracy.

astro-ph.IM↗

arXiv2022

SOFIA observations of 30 Doradus: II -- Magnetic fields and large scale gas kinematics

The heart of the Large Magellanic Cloud, 30 Doradus, is a complex region with a clear core-halo structure. Feedback from the stellar cluster R$\,$136 has been shown to be the main source of energy creating multiple pc-scale expanding-shells in the outer region, and carving a nebula core in the proximity of the ionization source. We present the morphology and strength of the magnetic fields (B-fields) of 30 Doradus inferred from the far-infrared polarimetric observations by SOFIA/HAWC+ at 89, 154, and 214$\,μ$m. The B-field morphology is complex, showing bending structures around R$\,$136. In addition, we use high spectral and angular resolution [\textsc{CII}] observations from SOFIA/GREAT and CO(2-1) from APEX. The kinematic structure of the region correlates with the B-field morphology and shows evidence of multiple expanding shells. Our B-field strength maps, estimated using the Davis-Chandrasekhar-Fermi method and structure-function, show variations across the cloud within a maximum of 600, 450, and 350$\,μ$G at 89, 154, and 214$\,μ$m, respectively. We estimated that the majority of the 30 Doradus clouds are sub-critical and sub-Alfvénic. The probability distribution function of the gas density shows that the turbulence is mainly compressively driven, while the plasma beta parameter indicates supersonic turbulence. We show that the B-field is sufficient to hold the cloud structure integrity under feedback from R$\,$136. We suggest that supersonic compressive turbulence enables the local gravitational collapse and triggers a new generation of stars to form. The velocity gradient technique (VGT) using [\textsc{CII}] and CO(2-1) is likely to confirm these results.

astro-ph.GA↗

arXiv2022

Multi-wavelength Observations of MWC 297: Constraints on Disk Inclination and Mass Outflow

MWC 297 is a young, early-type star driving an ionized outflow and surrounded by warm, entrained dust. Previous analyses of near- and mid-IR interferometric images suggest that the emission at these wavelengths arises from a compact accretion disk with a moderate ($i < 40$ degrees) inclination. We have obtained 5-40 micron images of MWC 297 with FORCAST on SOFIA, as well as near-infrared spectra acquired with SpeX on the IRTF and radio data obtained with the VLA and BIMA, and supplemented these with archival data from Herschel/PACS and SPIRE. The FORCAST images, combined with the VLA data, indicate that the outflow lobes are aligned nearly north-south and are well separated. Simple geometrical modeling of the FORCAST images suggests that the disk driving the outflow has an inclination of $55\pm 5$ degrees, in disagreement with the results of the interferometric analyses. Analysis of the SpeX data, with a wind model, suggests the the mass loss rate is on the order of $6.0 \pm ^{3.7}_{1.7} \times 10^{-7} M_\odot ~\mathrm{yr}^{-1}$ and the extinction to the source is $A_V \sim 8.1 \pm^{2.5}_{1.5}$ mag. We have combined our data with values from the literature to generate the spectral energy distribution of the source from $0.35~ μ$m to $6$ cm and estimate the total luminosity. We find the total luminosity to be about $7900 ~ L_\odot$, if we include emission from an extended region around the star, only slightly below that expected for a B1.5V star. The reddening must be produced by dust along the line of sight, but distant from the star.

astro-ph.SR↗

arXiv2022

SpeX near-infrared spectroscopic extinction curves in the Milky Way

Interstellar dust extinction curves provide valuable information about dust properties, including the composition and size of the dust grains, and are essential to correct observations for the effects of interstellar dust. In this work, we measure a representative sample of near-infrared (NIR; 0.8-5.5 $μ$m) spectroscopic extinction curves for the first time, enabling us to investigate the extinction at wavelengths where it is usually only measured in broad photometric bands. We use IRTF/SpeX spectra of a sample of reddened and comparison stars to measure 15 extinction curves with the pair method. Our sample spans A(V) values from 0.78 to 5.65 and R(V) values from 2.43 to 5.33. We confirm that the NIR extinction curves are well fit by a power law, with indices and amplitudes differing from sight line to sight line. Our average diffuse NIR extinction curve can be represented by a single power law with index $α= 1.7$, but because of the sight line-to-sight line variations, the shape of any average curve will depend on the parental sample. We find that most of the variation in our sample can be linked to the ratio of total-to-selective extinction R(V), a rough measurement of the average dust grain size. Two sight lines in our sample clearly show the ice extinction feature at 3 $μ$m, which can be fitted by a modified Drude profile. We find tentative ice detections with slightly over 3$σ$ significance in two other sight lines. In our average diffuse extinction curve, we measure a 3$σ$ upper limit of A(ice)/A(V) = 0.0021 for this ice feature.

astro-ph.GA↗

arXiv2021

SOFIA observations of 30 Doradus: I -- Far-Infrared dust polarization and implications for grain alignment and disruption by radiative torques

Located in the Large Magellanic cloud and mostly irradiated by a massive-star cluster R$\,$136, 30 Doradus is an ideal target to test the leading theory of the grain alignment and rotational disruption by RAdiative Torques (RATs). Here, we use publicly available polarized thermal dust emission observations of 30 Doradus at 89, 154, and 214$\,μ$m using SOFIA/HAWC+. We analyse the variation of the dust polarization degree ($p$) with the total emission intensity ($I$), the dust temperature ($T_{\rm d}$), and the gas column density ($N_{\rm H}$) constructed from ${\it Herschel}$ data. The 30 Doradus complex is divided into two main regions relative to R$\,$136, namely North and South. In the North, we find that the polarization degree first decreases and then increases before decreasing again when the dust temperature increases toward the irradiating cluster R$\,$136. The first depolarization likely arises from the decrease of grain alignment efficiency toward the dense medium due to the attenuation of the interstellar radiation field and the increase of the gas density. The second trend (the increase of $p$ with $T_{\rm d}$) is consistent with the RAT alignment theory. The final trend (the decrease of $p$ with $T_{\rm d}$) is consistent with the RAT alignment theory only when the grain rotational disruption by RATs is taken into account. In the South, we find that the polarization degree is nearly independent of the dust temperature, while the grain alignment efficiency is higher around the peak of the gas column density and decreases toward the radiation source. The latter feature is also consistent with the prediction of the rotational disruption by RATs.

astro-ph.GA↗

arXiv2020

A Survey of 3 to 5.4 Micron Emission From Planetary Nebulae using SOFIA/FLITECAM

Here we present the results of an airborne 3-5.4$μ$m spectroscopic study of three young, Carbon-rich planetary nebulae IC 5117, PNG 093.9-00.1, and BD $+$30 3639. These observations were made using the grism spectroscopy mode of the FLITECAM instrument during airborne science operations onboard NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA). The goal of this study is to characterize the 3.3 $μ$m and 5.25 $μ$m PAH dust emission in planetary nebulae and study the evolution of PAH features within evolved stars before their incorporation into new stellar systems in star-forming regions. Targets were selected from IRAS, KAO and ISO source lists, and were previously observed with FLITECAM on the 3-meter Shane telescope at Lick Observatory to allow direct comparison between the ground and airborne observations. We measure PAH emission equivalent width and central wavelength, classify the shape of the PAH emission, and determine the PAH/Aliphatic ratio for each target. The 3.3 $μ$m PAH emission feature is observed in all three objects. PNG 093.9-00.1 exhibits NGC 7027-like aliphatic emission in the 3.4 to 3.6 $μ$m region while IC 5117 and BD +30 3639 exhibit less aliphatic structure. All three PNs additionally exhibit PAH emission at 5.25 $μ$m.

astro-ph.SR↗

arXiv2020

The CO-dark molecular gas mass in 30 Doradus

Determining the efficiency with which gas is converted into stars in galaxies requires an accurate determination of the total reservoir of molecular gas mass. However, despite being the most abundant molecule in the Universe, H$_2$ is challenging to detect through direct observations and indirect methods have to be used to estimate the total molecular gas reservoir. These are often based on scaling relations from tracers such as CO or dust, and are generally calibrated in the Milky Way. Yet, evidence that these scaling relations are environmentally dependent is growing. In particular, the commonly used CO-to-H$_2$ conversion factor (X$_{\rm CO}$) is expected to be higher in metal-poor and/or strongly UV-irradiated environments. We use new SOFIA/FIFI-LS observations of far-infrared fine structure lines from the ionised and neutral gas and the Meudon photodissociation region model to constrain the physical properties and the structure of the gas in the massive star-forming region of 30 Doradus in the Large Magellanic Cloud, and determine the spatially resolved distribution of the total reservoir of molecular gas in the proximity of the young massive cluster R136. We compare this value with the molecular gas mass inferred from ground-based CO observations and dust-based estimates to quantify the impact of this extreme environment on commonly used tracers of the molecular gas. We find that the strong radiation field combined with the half-solar metallicity of the surrounding gas are responsible for a large reservoir of "CO-dark" molecular gas, leaving a large fraction of the total H$_2$ gas (> 75%) undetected when adopting a standard X$_{\rm CO}$ factor in this massive star-forming region.

astro-ph.GA↗

arXiv2018

SOFIA - HIRMES: Looking forward to the HIgh-Resolution Mid-infrarEd Spectrometer

The HIgh-Resolution Mid-infrarEd Spectrometer (HIRMES) is the 3rd Generation Instrument for the Stratospheric Observatory For Infrared Astronomy (SOFIA), currently in development at the NASA Goddard Space Flight Center (GSFC), and due for commissioning in 2019. By combining direct-detection Transition Edge Sensor (TES) bolometer arrays, grating-dispersive spectroscopy, and a host of Fabry-Perot tunable filters, HIRMES will provide the ability for High Resolution (R~100,000), Mid-Resolution (R~10,000), and Low-Resolution (R~600) slit-spectroscopy, and 2D Spectral Imaging (R~2000 at selected wavelengths) over the 25 - 122 μm mid-far infrared waveband. The driving science application is the evolution of proto-planetary systems via measurements of water-vapor, water-ice, deuterated hydrogen (HD), and neutral oxygen lines. However, HIRMES has been designed to be as flexible as possible to cover a wide range of science cases that fall within its phase-space, all whilst reaching sensitivities and observing powers not yet seen thus far on SOFIA, providing unique observing capabilities which will remain unmatched for decades.

astro-ph.IM↗

arXiv2017

The Inner 25 AU Debris Distribution in the epsilon Eri System

Debris disk morphology is wavelength dependent due to the wide range of particle sizes and size-dependent dynamics influenced by various forces. Resolved images of nearby debris disks reveal complex disk structures that are difficult to distinguish from their spectral energy distributions. Therefore, multi-wavelength resolved images of nearby debris systems provide an essential foundation to understand the intricate interplay between collisional, gravitational, and radiative forces that govern debris disk structures. We present the SOFIA 35 um resolved disk image of epsilon Eri, the closest debris disk around a star similar to the early Sun. Combining with the Spitzer resolved image at 24 um and 15-38 um excess spectrum, we examine two proposed origins of the inner debris in epsilon Eri: (1) in-situ planetesimal belt(s) and (2) dragged-in grains from the cold outer belt. We find that the presence of in-situ dust-producing planetesmial belt(s) is the most likely source of the excess emission in the inner 25 au region. Although a small amount of dragged-in grains from the cold belt could contribute to the excess emission in the inner region, the resolution of the SOFIA data is high enough to rule out the possibility that the entire inner warm excess results from dragged-in grains, but not enough to distinguish one broad inner disk from two narrow belts.

astro-ph.SR↗

arXiv2015

Observations of Type Ia Supernova 2014J with FLITECAM/SOFIA

We present medium resolution near-infrared (NIR) spectra, covering 1.1 to 3.4 microns, of the normal Type Ia supernova (SN Ia) SN 2014J in M82 obtained with the FLITECAM instrument aboard SOFIA approximately 17-25 days after maximum B light. Our 2.8-3.4 micron spectra may be the first ~3 micron spectra of a SN Ia ever published. The spectra spanning the 1.5-2.7 micron range are characterized by a strong emission feature at ~1.77 microns with a full width at half maximum of ~11,000-13,000 km/s. We compare the observed FLITECAM spectra to the recent non-LTE delayed detonation models of Dessart et al. (2014) and find that the models agree with the spectra remarkably well in the 1.5-2.7 micron wavelength range. Based on this comparison we identify the ~1.77 micron emission peak as a blend of permitted lines of Co II. Other features seen in the 2.0 - 2.5 micron spectra are also identified as emission from permitted transitions of Co II. However, the models are not as successful at reproducing the spectra in the 1.1 - 1.4 micron range or between 2.8 and 3.4 microns. These observations demonstrate the promise of SOFIA by allowing access to wavelength regions inaccessible from the ground, and serve to draw attention to the usefulness of the regions between the standard ground-based NIR passbands for constraining SN models.

astro-ph.SR↗

arXiv2014

The SOFIA Observatory at the Start of Routine Science Operations : Mission capabilities and performance

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities are viewed as a first comprehensive assessment of the Observatory's performance and are used to guide future development activities, as well as to identify additional Observatory upgrades. Pointing stability was evaluated, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an active mass damper system installed on the telescope. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have also been performed. Additional tests targeted basic Observatory capabilities and requirements, including pointing accuracy, chopper evaluation and imager sensitivity. This paper reports on the data collected during these flights and presents current SOFIA Observatory performance and characterization.

astro-ph.IM↗

arXiv2013

Overview of the SOFIA Data Cycle System: An integrated set of tools and services for the SOFIA General Investigator

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne astronomical observatory comprised of a 2.5 meter infrared telescope mounted in the aft section of a Boeing 747SP aircraft that flies at operational altitudes between 37,000 and 45,00 feet, above 99% of atmospheric water vapor. During routine operations, a host of instruments will be available to the astronomical community including cameras and spectrographs in the near- to far-IR; a sub-mm heterodyne receiver; and an high-speed occultation imager. One of the challenges for SOFIA (and all observatories in general) is providing a uniform set of tools that enable the non-expert General Investigator (GI) to propose, plan, and obtain observations using a variety of very different instruments in an easy and seamless manner. The SOFIA Data Cycle System (DCS) is an integrated set of services and user tools for the SOFIA Science and Mission Operations GI Program designed to address this challenge. Program activities supported by the DCS include: proposal preparation and submission by the GI; proposal evaluation by the telescope allocation committee and observatory staff; Astronomical Observation Request (AOR) preparation and submission by the GI; observation and mission planning by observatory staff; data processing and archiving; data product distribution. In this poster paper we present an overview of the DCS concepts, architecture, and user tools that are (or soon will be) available in routine SOFIA operations. In addition, we present experience from the SOFIA Basic Science program, and planned upgrades.

astro-ph.IM↗