Low-excitation Lines Research Articles

MICONIC: JWST/MIRI MRS observations of the nuclear and circumnuclear regions of Mrk 231

We present JWST/MIRI MRS spatially resolved ∼5 − 28 μm observations of the central ∼4 − 8 kpc of the ultraluminous infrared galaxy and broad absorption line quasar Mrk 231. These are part of the Mid-Infrared Characterization of Nearby Iconic galaxy Centers (MICONIC) program of the MIRI European Consortium guaranteed time observations. No high excitation lines (i.e., [Mg V] at 5.61 μm or [Ne V] at 14.32 μm) typically associated with the presence of an active galactic nucleus (AGN) are detected in the nuclear region of Mrk 231. This is likely due to the intrinsically X-ray weak nature of its quasar. Some intermediate ionization potential lines, for instance, [Ar III] at 8.99 μm and [S IV] at 10.51 μm, are not detected either, even though they are clearly observed in a star-forming region ∼920 pc south-east of the AGN. Thus, the strong nuclear mid-infrared (mid-IR) continuum is also in part hampering the detection of faint lines in the nuclear region. The nuclear [Ne III]/[Ne II] line ratio is consistent with values observed in star-forming galaxies. Moreover, we resolve for the first time the nuclear starburst in the mid-IR low-excitation line emission (size of ∼400 pc, FWHM). Several pieces of evidence also indicate that it is partly obscured even at these wavelengths. At the AGN position, the ionized and warm molecular gas emission lines have modest widths (W80 ∼ 300 km s−1). There are, however, weak blueshifted wings reaching velocities v02 ≃ − 400 km s−1 in [Ne II]. The nuclear starburst is at the center of a large (∼8 kpc), massive rotating disk with widely-spread, low velocity outflows. Given the high star formation rate of Mrk 231, we speculate that part of the nuclear outflows and the large-scale non-circular motions observed in the mid-IR are driven by its powerful nuclear starburst.

H2CO and CS in diffuse clouds: Excitation and abundance

Context. Diffuse interstellar clouds present an active chemistry despite their relatively low density and the ubiquitous presence of far-UV radiation. Aims. To provide constraints on the chemical processes responsible for the observed columns of organic species, we used the NOEMA interferometer to observe the sight line toward NRAO150 (B0355+508) in the 2 mm spectral window. Methods. We targeted the low excitation lines of ortho H2 CO (21,1–11,0) and para H2 CO (20,2–10,1) as well as the nearby transitions of CS (3–2) and c-C3H2 (31,2–22,1), (41,4–30,3), and (22,0–11,1). We combined these data with previous observations of the same sight line to determine the excitation conditions, column densities, and abundances relative to H2 in the different velocity components. We performed non-LTE radiative transfer calculations including collision cross sections with ortho and para H2 and with electrons. New collision cross sections with electrons were computed for ortho and para formaldehyde. Results. All targeted lines were detected. The c-C3H2 line profiles are very similar to those of HCO+ and CCH, while the CS absorption features are narrower and mostly concentrated in two main velocity components at VLSR = −17.2 and −10.4 km s−1. H2CO absorption lines present an intermediate pattern with absorption in all velocity components but larger opacities in the two main velocity components. The ortho-to-para ratios of H2CO and c-C3H2 are consistent with the statistical value of three. While the excitation temperature of all c-C3H2 velocity components is consistent with the Cosmic Microwave Background (CMB), the two strong components detected in CS show a clear excess over the CMB indicating that CS resides at higher densities than other species along this particular sightline, n(H2) ~ 2500 cm−3 while n(H2) < 500 cm−3 for the other velocity components. We detected faint absorption from o-H213CO and C34S allowing us to derive isotopic ratios: o-H2CO/o-H213CO = 61 ± 12 and C32S/C34S = 24 ± 6. The excitation of the 11,0−11,1 line of formaldehyde at 4.8 GHz is sensitive to the electron fraction and its excitation temperature is predicted to be lower than the CMB at low and moderate electron fractions (x(e) < 6 × 10−5), and to rise above the CMB at high electron fractions (x(e) > 10−4).

Unveiling compact planetary nebulae: Broad-band survey analysis and LAMOST confirmation

Planetary nebulae (PNe) are pivotal for advancing our knowledge of stellar evolution and galactic chemical enrichment. Recent progress in surveys and data analysis has revolutionized PN research, leading to the discovery of new objects and deeper insights into their properties. We have devised a novel photometric selection method, integrating GAIA and Pan-STARRS photometry, to identify compact PN candidates. This approach utilizes color–color diagrams, specifically (G−g) versus (GBP−GRP) and (G−r) versus (GBP−GRP), as primary criteria for candidate selection. The subsequent verification step involves confirming these candidates through LAMOST spectroscopic data. By cross-referencing a comprehensive dataset of PNe, GAIA, Pan-STARRS, and LAMOST DR7 spectra, we explore the potential of our approach and the crucial role played by these surveys in the field of PN research. The LAMOST spectra provide compelling evidence supporting our selection criteria, especially for compact PNe characterized by strong emission lines and low continuum. This characteristic spectral profile in LAMOST data underscores its effectiveness in confirming compact PNe, enabling clear differentiation based on distinctive spectral features. Applying these criteria to a catalog of emission line objects, we have selected a PN candidate. Detailed analysis of its LAMOST spectrum unveiled classical Balmer emission lines and high-ionization lines (Heii, [Arv], [Ariii], and [Neiii]), characteristic of high-ionization PNe, with an absence of low-excitation lines. Utilizing the 1D-photoionization code cloudy, our modeling revealed crucial parameters, including an ionizing source with an effective temperature of 180×103 K, luminosity around 3,400 L⊙, and gas abundances encompassing various elements. Comparing the PN’s evolution track, the progenitor star was estimated to have a mass of 2M⊙. Our findings show the greatest promise for cleanly separating compact PNe from other objects and provide a robust framework for further exploration of these surveys in the context of planetary nebulae.

The Late-Epoch, Matter-Bounded Spectrum of the Nova V1405 Cassiopeiae

Optical and near-infrared spectra of the classical nova V1405 Cassiopeiae (V1405 Cas) from more than two years after outburst show a still bright nova of intermediate excitation with coronal lines, some high-ionization but non-coronal forbidden lines, and very strong emission features of He ii. However, the common nebular lines of [N ii], [O ii], and [O iii] are absent and the lines of He i are very weak. Analysis with the photoionization code CLOUDY indicates a emission line region that is sharply matter-bounded, truncating before the gas excitation drops sufficiently to support the parent ions of lower excitation lines.

The Behavior of the Bp Stars in the Far UV—Paper VII: HD 22316

The analysis of seven spectra of the B9 Cr Hg Si star HD 22316 recorded by the SWP camera on board the International Ultraviolet Explorer does not reveal any variations of the far ultraviolet spectral energy distribution shortwards of 1800 Å in spectra collected over 2 yr with identical exposure times. The low excitation line at 1363.45 Å and the resonance line of Cl i at 1379.53 Å are most likely present. Several lines of Al ii, Si ii, P ii but none of S i could be found at their expected wavelengths. The synthesis of several Al ii lines consistently yields a clear underabundance of aluminum of about 0.40 times the solar aluminum abundance.

A Rotating Accretion Disk around MWC 297, a Young B1.5Ve Star

High-resolution spectra with iSHELL on IRTF in the K and M bands of the young, heavily accreting B1.5Ve star MWC 297 show numerous double-peaked CO lines. These CO lines originate in an inclined gaseous disk in Keplerian rotation. MWC 297 is the only early B star known to show a Keplerian disk in CO. Analysis of the spectra shows that 12CO 1–0 is optically thick for the low excitation lines. Even the 13CO 1–0 and 12CO 2–1 have somewhat optically thick lines at low J levels. We find that the CO emission in the disk can be fitted with CO being in a narrow ring at a radius of 12 au, with a temperature of 1500 K and a CO column density of 1.6 × 1018 cm−2. This model underestimates the line strength of high-J lines, indicating that they are excited by fluorescence. The CO overtone lines have a similar temperature. The 13CO lines are much brighter than expected from interstellar isotope ratios. The 13CO lines are wider than the 12CO ones, suggesting different excitation conditions. The same is true for 12CO 2–1. We see strong absorption in 12CO and 13CO 1–0 at low J levels, which is due to two cold foreground clouds. These clouds, one with a temperature of 8.3 K and a column density of 6.7 1017 cm−2 and the other one colder and with lower column density, can fully account for the observed extinction toward MWC 297.

The FUSE Spectrum of HD 215441 (Babcock’s Star)

The analysis of the far-ultraviolet spectrum of the B9 Si HD 215441 recorded by the LWRS camera on board FUSE reveals a large deficiency of the flux when compared to the non-magnetic late B-type star HD 182308 of similar fundamental parameters. Most features can be identified using spectrum synthesis. The FUSE spectrum of HD 215441 contains mostly blends of low-excitation lines of Ti ii, Mn ii, Cr ii, Fe ii, a few lines of Ti iii, Cr iii and Fe iii and interesting lines of C iii, P ii, S ii and Cl i.

First observations of warm and cold methanol in Class 0/I proto-brown dwarfs

ABSTRACT We present results from the first molecular line survey to search for the fundamental complex organic molecule, methanol (CH3OH), in 14 Class 0/I proto-brown dwarfs (proto-BDs). IRAM 30-m observations over the frequency range of 92–116 and 213–280 GHz have revealed emission in 14 CH3OH transition lines, at upper state energy level, Eupper ∼7–49 K, and critical densities, ncrit of 105–109 cm−3. The most commonly detected lines are at Eupper < 20 K, while 11 proto-BDs also show emission in the higher excitation lines at Eupper ∼21–49 K and ncrit ∼ 105 to 108 cm−3. In comparison with the brown dwarf formation models, the high excitation lines likely probe the warm (∼25–50 K) corino region at ∼10–50 au in the proto-BDs, while the low-excitation lines trace the cold (<20 K) gas at ∼50–150 au. The column density for the cold component is an order of magnitude higher than the warm component. The CH3OH ortho-to-para ratios range between ∼0.3 and 2.3. The volume-averaged CH3OH column densities show a rise with decreasing bolometric luminosity among the proto-BDs, with the median column density higher by a factor of ∼3 compared to low-mass protostars. Emission in high-excitation (Eupper > 25 K) CH3OH lines together with the model predictions suggest that a warm corino is present in ∼78 per cent of the proto-BDs in our sample. The remaining shows evidence of only the cold component, possibly due to the absence of a strong, high-velocity jet that can stir up the warm gas around it.

Sub-kiloparsec empirical relations and excitation conditions of HCN and HCO+ J = 3–2 in nearby star-forming galaxies

We present new HCN and HCO+ (J = 3–2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of ∼290–440 pc and resolve the inner Rgal ≲ 0.6–1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO+, and CO and analyse the behaviour of a representative set of line ratios: HCN(3–2)/HCN(1–0), HCN(3–2)/HCO+(3–2), HCN(1–0)/CO(2–1), and HCN(3–2)/CO(2–1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO+ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density (Σstar), the intensity-weighted CO(2–1) (⟨ICO⟩), and the star formation rate surface density (ΣSFR). We find no apparent correlation with ΣSFR, but positive correlations with the other two parameters, which are stronger in the case of ⟨ICO⟩. The HCN/CO–⟨ICO⟩ relations show ≲0.3 dex galaxy-to-galaxy offsets, with HCN(3–2)/CO(2–1)–⟨ICO⟩ being ∼2 times steeper than HCN(1–0)/CO(2–1). In contrast, the HCN(3–2)/HCN(1–0)–⟨ICO⟩ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g., density or temperature).

Early Infrared and Visible Spectra from the 2022 Outburst of the Recurrent Novae U Scorpii

Spectroscopy from 0.46–2.5 μm is presented for the 2022 outburst of the recurrent nova U Scorpii from 2.54, 3.60, and 8.59 days after maximum light. During this period the spectra evolved from comparatively low excitation to the verge of displaying coronal lines. Between days 2.54 and 3.60, the nova showed a marked decrease in the low excitation lines of N i, O i, and Mg ii , an increase in He i, the emergence of N iii, and changes to the very broad line profiles. By day 8.59, the He i lines had been largely replaced by He ii emission and most broad component were weak or undetectable. No optical coronal lines were observed during or immediately following the detection of infrared coronal lines by Banerjee et al.

The Behavior of HgMn Stars in the Far-UV—Paper 24: HD 159975 (μ Oph)

The analysis of two high resolution spectra of the Hg Mn star HD 159975 recorded by the SWP camera on board the International Ultraviolet Explorer does not reveal variations of the far-ultraviolet flux over a 18 yr time interval. From its effective temperature and surface gravity, HD 155975 appears to have evolved away from the Main Sequence in a theoretical Hertzsprung Russell diagram. It is one of the rare subgiant HgMn stars. Several Si ii resonance or low excitation lines are strong absorbers in the mean high resolution SWP spectrum of HD 159975. The Al ii resonance line at 1670.79 Å is a strong absorber too. There is no evidence of real variations of the low excitation Si ii lines.

The 2020 Eclipse of R Aquarii in the Near-infrared: Mid-eclipse Observations of Disk Distress

The Mira in the bright, dusty, symbiotic binary R Aquarii undergoes eclipses of multiyear duration every ∼44 yr by a large, opaque accretion disk. The 2020 eclipse was confirmed by I-band photometry. High-resolution M- and K-band spectra were observed near the midpoint of the eclipse, in 2020 August and September. The 4.5–5.5 μm spectrum during eclipse is a complex blend of disk and Mira features. Continuum emission from the disk region contributes to both the 2.3 μm and 4.6 μm region. The lowest energy vibration-rotation CO lines contain multiple absorption features from ∼780 K gas flowing across the disk away from the Mira. CO fundamental and overtone emission lines are also present. The eccentricity of the orbit results in significant orbital variation in the size of the Roche lobes. At periastron the Roche radius of the secondary is ∼4.0 au, smaller than both the 5 au geometric radius for the disk and estimates for the disk size from models. Fundamental band CO 2–1 emission originates from a thin, eccentric ring with inner radius ∼4.75 au and outer radius ∼6.9 au. The CO emission is identified with regions where the disk has been disrupted near the time of periastron. CO 3–2 fundamental band lines and low-excitation lines in the CO 2–0 and 3–1 overtone bands originate in a Mira-facing spot, 6.3 au from the accretion disk center, near the inner Lagrange point.

The Line Variability of 78 Vir (A2p SrCrEu) in the Far-ultraviolet

Abstract Line variations are studied in the far-ultraviolet spectra of 78 Vir collected with International Ultraviolet Explorer at phases of far-UV maximum and at phases of far-UV minimum. The far-UV spectrum of 78 Vir is populated with low-excitation transitions of light elements and iron-peak elements. The low excitation C i and N i lines are stronger at phases of far-UV minimum. Carbon appears to be underabundant and aluminum very underabundant in the atmosphere of 78 Vir. The lines of Si i and Si ii are stronger at phases of far-UV minimum. Many lines which strengthen at phases of far-UV minimum are due to low-excitation lines of Fe ii and Cr ii which suggests that both iron and chromium are unevenly distributed over the surface of 78 Vir.

The Distribution of Europium over the Surface of 21 Com

Abstract Comparison of optical high resolution high signal-to-noise ELODIE spectra of 21 Com taken in 2004 April and separated by about half of the 2 days rotational period, reveals variations of the Eu ii resonance and low excitation lines between rotational phases 0.94 and 0.48. All Eu ii lines are consistently stronger at phase 0.94. The synthesis of these Eu ii lines yields disk-averaged estimates of the abundances of europium: about 50 times solar at phase 0.94 and 25 times solar at phase 0.48.

Starburst Energy Feedback Seen through HCO+/HOC+ Emission in NGC 253 from ALCHEMI

Abstract Molecular abundances are sensitive to the UV photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be stronger. We examine these astrochemical signatures through multiple transitions of HCO+ and its metastable isomer HOC+ in the center of the starburst galaxy NGC 253 using data from the Atacama Large Millimeter/submillimeter Array large program ALMA Comprehensive High-resolution Extragalactic Molecular inventory. The distribution of the HOC+(1−0) integrated intensity shows its association with “superbubbles,” cavities created either by supernovae or expanding H ii regions. The observed HCO+/HOC+ abundance ratios are ∼10–150, and the fractional abundance of HOC+ relative to H2 is ∼1.5 × 10−11–6 × 10−10, which implies that the HOC+ abundance in the center of NGC 253 is significantly higher than in quiescent spiral arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of G 0 ≳ 103 if the maximum visual extinction is ≳5, or a cosmic-ray ionization rate of ζ ≳ 10−14 s−1 (3–4 orders of magnitude higher than that within clouds in the Galactic spiral arms) to reproduce the observed results. From the difference in formation routes of HOC+, we propose that a low-excitation line of HOC+ traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV photons and cosmic rays, sources of energy feedback.

The Distribution of Silicon and Magnesium over the Surface of 21 Com

Abstract Inspection of 10 individual spectra taken over the 24 hr monitoring of 21 Com with International Ultraviolet Explorer on 1991 April 17 reveals variations of low excitation lines of Si ii between rotational phases ϕ = 0.18 and ϕ = 0.65 of the 2 days period, suggesting a inhomogeneous repartition of silicon over the surface of 21 Com. The silicon lines are weaker at rotational phase ϕ = 0.18. More recent optical high resolution high signal-to-noise ELODIE spectra taken in 2004 April confirm the variations of silicon lines from phase ϕ = 0.99 to ϕ = 0.47. The synthesis of the optical Si ii lines yields estimates of the abundances of silicon: 8 times solar at phase 0.99 and 10 times solar at phase 0.47. The large FUV variations of 21 Com appear then to be partly due to changes in the silicon line and continuous opacity as the star rotates. In contrast, the Mg ii lines do not show convincing variations between the two rotational phases observed in the optical.

Which molecule traces what: Chemical diagnostics of protostellar sources

Context. The physical and chemical conditions in Class 0/I protostars are fundamental in unlocking the protostellar accretion process and its impact on planet formation. Aims. The aim is to determine which physical components are traced by different molecules at subarcsecond scales (<100–400 au). Methods. We used a suite of Atacama Large Millimeter/submillimeter Array (ALMA) datasets in band 6 (1 mm), band 5 (1.8 mm), and band 3 (3 mm) at spatial resolutions 0.″5–3″ for 16 protostellar sources. For a subset of sources, Atacama Compact Array (ACA) data at band 6 with a spatial resolution of 6″ were added. The availability of low- and high-excitation lines and data on small and larger scales, is important to understand the full picture. Results. The protostellar envelope is well traced by C18O, DCO+, and N2D+, which stems from the freeze-out of CO governing the chemistry at envelope scales. Molecular outflows are seen in classical shock tracers such as SiO and SO, but ice-mantle products such as CH3OH and HNCO that are released with the shock are also observed. The molecular jet is a key component of the system. It is only present at the very early stages, and it is prominent not only in SiO and SO, but occasionally also in H2CO. The cavity walls show tracers of UV-irradiation such as C2H, c-C3H2 and CN. In addition to showing emission from complex organic molecules (COMs), the hot inner envelope also presents compact emission from small molecules such as H2S, SO, OCS, and H13CN, which most likely are related to ice sublimation and high-temperature chemistry. Conclusions. Subarcsecond millimeter-wave observations allow us to identify these (simple) molecules that best trace each of the physical components of a protostellar system. COMs are found both in the hot inner envelope (high-excitation lines) and in the outflows (lower-excitation lines) with comparable abundances. COMs can coexist with hydrocarbons in the same protostellar sources, but they trace different components. In the near future, mid-infrared observations with JWST–MIRI will provide complementary information about the hottest gas and the ice-mantle content, at unprecedented sensitivity and at resolutions comparable to ALMA for the same sources.

The Mean Underabundances of Carbon and Aluminum Over the Surface of 21 Com from Its Far-ultraviolet Spectra Recorded by IUE

Abstract The comparison of a coadded International Ultraviolet Explorer (IUE) far-ultraviolet spectrum of 21 Com (A3p SrCr) to that of the standard star α Psa (A3V) reveals large differences in the resonance and low-excitation lines of C i and Al ii. All the lines of these species are significantly weaker in the mean spectrum of 21 Com. The synthesis of these lines consistently yields sizable underabundances for 21 Com of about 0.5 times the solar carbon, and 0.20 times solar for aluminum. Inspection of 10 individual spectra taken over the 24 hr monitoring of 21 Com with IUE on 1991 April 17 does not reveal convincing variations of the lines analyzed here between rotational phases 0.18 and 0.65 of the 2 days period, suggesting a uniform repartition of these 2 elements over the hemisphere of 21 Com observed then.

Short-term Spectroscopic Variability of the Early Post-AGB Stage Star IRAS 22272+5435

Abstract High-resolution spectra gathered during an extensive monitoring of IRAS 22272+5435 are inspected to search for spectroscopic variability in the time span of less than 2 weeks. Significant variation are found both in the intensity and position of CN(1, 0) red system lines near the light maximum, when the star is the hottest. The intensity of variable CN lines correlates with their Doppler redshift—an increase in the radial velocity leads to a decrease of its equivalent width. The inward flow of increasing velocity from ≃14.4–21.6 km s−1 was confirmed in the atmosphere, propagating ∼25 R ☉ toward the mass center of the star in 11 days. The variability of CN(1, 0) lines are accompanied by changes in the red wings of strong low-excitation lines. The observed short-term variations are attributed to large-scale convective flows and shock waves in the extended atmosphere of a cool pulsating post-AGB star.

The Large Flux Variations of 17 Com A as Recorded by FUSE

Abstract Inspection of archival FUSE spectra of the Ap star 17 Com A (HD 108662) reveals large flux variations from 905 to 1187 Å over the 5.0808 days rotational period. These variations are of larger amplitude than those observed in the ultraviolet range with IUE from 1200 to 2000 Å. At 1161 Å, where the flux reaches a maximum, the variation of flux with rotational phase shows a double-wave structure similar to that of the Transiting Exoplanet Survey Satellite lightcurve. These large flux variations are most likely caused by the passage of spots or belts of overabundant elements in the line of sight as the star rotates The FUSE spectrum of 17 Com A contains mostly blends of low-excitation lines of Mn ii, Cr ii and Fe ii and interesting lines of C i, C iii, P ii, S ii and Cl i.