---
title: 'MIRION Catalog: Mid-IR Compact Nebulae'
url: https://www.emergentmind.com/topics/mirion-catalog
type: topic
---

# MIRION Catalog: Mid-IR Compact Nebulae

Searching arXiv for the specified MIRION catalog paper and closely related yellowball work to ground the article.
The **MIRION Catalog**—**Mid-InfraRed Interstellar Objects and Nebulae**—is a curated, multi-table catalog built from **6176** compact mid-infrared sources originally identified as “yellowballs” by citizen scientists in the Milky Way Project. In the historical usage retained by the catalog, yellowballs are compact, roughly circular patches in Spitzer three-color images that appear yellow because **8 μm** polycyclic aromatic hydrocarbon emission and **24 μm** warm-dust emission overlap within a small area. Physically, most MIRION sources are young, compact photodissociation regions surrounding intermediate- and high-mass young stellar objects and compact clusters, although the catalog explicitly treats them as a heterogeneous class of mid-IR bright, compact nebulae associated predominantly with star-forming regions. By compiling photometry, velocities, distances, clump properties, and catalog cross-identifications into five linked tables, MIRION provides an extensive database for studying the transition from isolated low-mass to clustered high-mass star formation and increases the number of candidate intermediate-mass star-forming regions by nearly two orders of magnitude [2512.06213].

## 1. Origin, nomenclature, and astrophysical meaning

MIRION sources originate in the **Milky Way Project (MWP)**, where participants identified compact objects now historically known as **yellowballs**. The designation comes from the appearance of these sources in Spitzer three-color images: they look yellow because green **8 μm** emission and red **24 μm** emission are spatially coincident over a compact area. In physical terms, the **8 μm** band traces UV-excited polycyclic aromatic hydrocarbons in the photodissociation region, while the **24 μm** band traces warm dust close to embedded sources [2512.06213].

Most yellowballs are interpreted as compact photodissociation regions generated by intermediate- and high-mass young stellar objects and compact clusters. Many appear to be precursors to H II regions, including **UC H II** and **compact H II** regions, and the population includes a mix of **intermediate-mass star-forming regions (IMSFRs; producing stars up to 3–8–10 \(M_\odot\))** and massive star-forming regions. Because the associated clumps span a wide dynamic range in mass and luminosity, MIRION sources trace the transition between isolated low-mass and clustered high-mass star formation. A plausible implication is that the catalog is especially useful for population-level studies of the poorly sampled regime between classical isolated low-mass cores and established massive H II-region environments.

The sky coverage reflects the underlying infrared survey footprint from which the original identifications were drawn. Sources were identified in Spitzer surveys covering the **inner Galactic plane** \((|\ell| \lesssim 65^\circ;\ \mathrm{GLIMPSE}+\mathrm{MIPSGAL})\), the **Cygnus-X complex** \((\sim 24\ \mathrm{deg}^2\ \mathrm{centered\ on}\ \ell \approx 79.3^\circ,\ b \approx 1^\circ)\), and a portion of the **outer Galaxy** \((\ell \approx 102^\circ\text{–}109^\circ,\ b \approx 0^\circ\text{–}3^\circ;\ \mathrm{SMOG})\). All photometry was re-measured in this work at **8, 12, 24, and 70 μm** across these regions [2512.06213].

## 2. Catalog architecture and linked data products

MIRION is organized as **five linked tables**, each indexed by a unique **Source ID**, so that joined queries can be performed across photometry, kinematics, distances, physical clump properties, and external catalog associations. This structure is central to the catalog’s design because it allows filtering by distance, environment, evolutionary indicators, or association with other star-formation tracers.

| Table | Core contents | Purpose |
|---|---|---|
| MIRION Catalog — Photometry | Positions, source size, hit rate, \(F_8\), \(F_{12}\), \(F_{24}\), \(F_{70}\), errors, measurement counts, quality flags | Mid- and far-IR fluxes and photometric QA |
| MIRION Catalog — Velocities | Adopted \(V_{\rm LSR}\), uncertainty, spectral type, survey-specific velocities, dense-gas tracer information | Molecular-cloud kinematics |
| MIRION Catalog — Distances | Adopted distance, uncertainty, external distances, \(P_{\rm far}\), posterior distance solutions, spiral-arm assignments | Bayesian distance inference |
| MIRION Catalog — Herschel-Matched Sources | Diameter, mass, luminosity, \(L/M\), temperature, \(L_{\rm bol}/L_{\rm smm}\), \(T_{\rm bol}\), \(\Sigma\) | Physical properties of associated clumps |
| MIRION Catalog — Catalog Cross-matches | Matched identifiers from Hi-GAL, ATLASGAL, CORNISH, RMS, and WISE H II catalogs | Environmental and source-type context |

The **Photometry** table includes **Galactic longitude GLON**, **latitude GLAT**, **MWP radius MWPR**, their uncertainties, and the Milky Way Project **hit rate HRATE**. It also records flux densities and errors in **Jy** for **\(F_8\)**, **\(F_{12}\)**, **\(F_{24}\)**, and **\(F_{70}\)**; the number of measurements per band (**\(N8\), \(N12\), \(N24\), \(N70\)**); and several flags: **\(f_{\rm SAT}\)** for per-band saturation, **\(f_{\rm MULTI}\)** for multiple compact sources in the **8 μm** aperture, **\(f_{\rm NOSRC}\)** for bands with no obvious source, **\(f_{\rm PCONF}\)** for poor confidence in photometry, and **\(f_{\rm CEXT}\)** for highly circular extended objects likely to be planetary nebulae or compact bubbles [2512.06213].

The **Velocities** table stores an adopted **\(V_{\rm LSR}\)** with uncertainty **\(e_{V_{\rm LSR}}\)** and a spectral classification **SType**. Its categories are **SP** for single peak, **MP:n** for multi-peak spectra, **LSN** for low signal-to-noise, and **OUT** for sources outside CO coverage. Survey-specific velocities are preserved for **GRS**, **SEDIGISM**, **FCRAO OGS/CGPS**, **ThrUMMS**, a low-resolution **1.2 m CO archive**, and dense-gas tracers, along with external comparisons from **Elia (2021)** and **Mége et al. (2021)** [2512.06213].

The **Distances** table provides the adopted distance **DIST**, its uncertainty, external distance estimates, and the outputs of the **Reid et al. (2019) Parallax-Based Distance Calculator v2**: **\(P_{\rm far}\)**, two posterior distance solutions, their integrated probabilities, and spiral-arm labels. The **Herschel-Matched Sources** table holds physical parameters derived from Hi-GAL compact-source crossmatches, after rescaling distance-dependent quantities to the adopted MIRION distances. Finally, the **Cross-matches** table records positional associations within **24″** to **Hi-GAL**, **ATLASGAL**, **CORNISH**, **RMS**, and the **WISE H II** catalog [2512.06213].

## 3. Photometric construction and quality control

The catalog’s photometric measurements were made interactively and aperture-based, with explicit attention to the complex backgrounds of the Galactic plane. For each source and band, users selected **at least three vertices** to define a polygonal mask around the object in an image regridded to the **8 μm** pixel scale for consistency. The local background was then estimated by **multiquadratic radial basis function interpolation** over the masked region. A “source-only” image was produced by subtracting the background model, and the flux density was summed over the mask [2512.06213].

The procedure was designed to quantify user-selection uncertainty rather than suppress it. Each source in each band received **at least five independent measurements**, and the cataloged **\(e_{F_\lambda}\)** values are fractional errors defined as the standard deviation divided by the mean flux. In practice, the measurement statistics reported for sources with fractional uncertainty below **0.5** are **4669** at **8 μm**, **4176** at **12 μm**, **4545** at **24 μm**, and **4757** at **70 μm**. This explicit multiplicity of measurements is one of MIRION’s distinguishing methodological features because it treats aperture selection uncertainty as part of the measurement model rather than as an unreported subjective choice [2512.06213].

The input imaging products were drawn from multiple facilities: **Spitzer/IRAC** for **8 μm**, **WISE W3 Atlas coadds** for **12 μm**, **Spitzer/MIPS** for **24 μm**, and **Herschel/PACS Highly Processed Data Products** for **70 μm**. For the clump physical properties, multi-band Herschel data from **Hi-GAL** were used by **Elia et al.**, whereas MIRION’s own **70 μm** photometry was measured directly by the catalog team. A practical implication is that the catalog separates direct MIRION flux measurement from inherited SED-based clump characterization, which is important when combining photometric and physical-property analyses [2512.06213].

Color measurements are expressed either as logarithmic flux ratios or as magnitude-like color indices. For bands \(a\) and \(b\), the color is

$$
[a-b] = -2.5 \log_{10}\left(\frac{F_a}{F_b}\right).
$$

The propagated color uncertainty from independent flux errors is

$$
\sigma_{[a-b]} = \frac{2.5}{\ln 10}\,\sqrt{\left(\frac{\sigma_{F_a}}{F_a}\right)^2 + \left(\frac{\sigma_{F_b}}{F_b}\right)^2}.
$$

MIRION commonly reports colors as logarithmic flux ratios such as \(\log_{10}(F_{12}/F_8)\) in order to emphasize physical trends in compactness and polycyclic aromatic hydrocarbon excitation [2512.06213].

## 4. Velocities, Bayesian distances, and environmental associations

The kinematic component of MIRION is built from molecular-line data and dense-gas or star-formation tracers. The adopted velocity for each source was determined from **48″ spectral-cube cutouts** centered on the MIRION position, a size chosen to encompass typical MIRION scales while minimizing unrelated emission. The default rule was to adopt the Gaussian-fit velocity of the most intense CO peak above a noise floor defined as \(\langle I\rangle + 3\sigma_I\) over approximately **30** channels near a spectrum end. Minor peaks had to be **< 0.2 \(I_{\max}\)** to count as part of a single major peak; spectra with a secondary peak **> 0.8 \(I_{\max}\)** were flagged **MP:n**. If a dense-gas-tracer peak overlapped the FWHM of a CO secondary peak, the dense-gas-tracer-associated velocity was preferentially adopted when the dense-gas-tracer peak had **SNR \(\ge 1.75\)** [2512.06213].

The velocity inputs came from several surveys. High-resolution CO data were drawn from **GRS**, **SEDIGISM**, **ThrUMMS**, and **FCRAO OGS/CGPS**; low-resolution CO came from a **1.2 m archive**; and dense-gas/star-formation tracers included **THOR OH masers**, **SPLASH OH masers**, and **HOPS H\(_2\)O masers** and **NH\(_3\)(1,1)**. In survey-overlap regions, **GRS** was preferred over **SEDIGISM** because of finer spectral resolution; ambiguities were resolved by comparing spectral flags and, if necessary, the integrated distance probability in the distance solution [2512.06213].

The catalog reports the origins of adopted velocities as follows: **GRS (1830; 29.5%)**, **SEDIGISM (2612; 42.3%)**, **FCRAO/CGPS (240; 3.9%)**, **ThrUMMS (597; 9.7%)**, **low-res CO (570; 9.2%)**, and **Mége et al. (327; 5.3%)**. The corresponding spectral-type distribution is **SP (4085; 66.1%)**, **LSN (849; 13.8%)**, **OUT (416; 6.7%)**, and **MP (826; 13.4%; 651 have one additional peak)**. Dense-gas-tracer velocities overrode CO in **107** cases, or approximately **2%** [2512.06213].

Distances were then computed using the **Reid et al. (2019) Parallax-Based Distance Calculator v2**, which combines \((\ell,b)\), the adopted **\(V_{\rm LSR}\)**, spiral-arm modeling, and a prior **\(P_{\rm far}\)** to produce posterior distance estimates and arm assignments. Near-far ambiguity was handled probabilistically through **\(P_{\rm far}\)**. When a matched Hi-GAL source had **\(d_{\rm near}=d_{\rm adopted}\)**, MIRION set **\(P_{\rm far}=0.0\)**; when **MDIST** or **SDIST_GROUP** was available from **Mége et al. (2021)**, the non-kinematic Mége distance was adopted directly; for **NO_KDA**, **KDA_NO**, **TGT_POINT**, or missing status, **\(P_{\rm far}=0.5\)** was used [2512.06213].

Distances were determined for approximately **94%** of MIRION sources. Compared with the subset criteria from **Mége et al. (2011)**, the average distance difference is approximately **0.3 kpc**, and **61%** agree within **0.7 kpc**. Velocity comparisons agree within **5 km s\(^{-1}\)** for approximately **80%**. This suggests that the catalog’s kinematic backbone is sufficiently homogeneous for large-scale statistical work, while still requiring caution in complex or low-S/N sightlines [2512.06213].

Crossmatching further anchors MIRION in the broader Galactic star-formation literature. Within survey coverage, the match fractions are **3979 (65.26%)** to **Hi-GAL CSC-360**, **1623 (30.48%)** to **ATLASGAL Condensations**, **1563 (41.24%)** to **ATLASGAL CSC2014**, **613 (11.03%)** to **RMS**, **2315 (37.48%)** to the **WISE H II** catalog, and **576 (12.08%)** to **CORNISH**. The WISE total includes **K = 435 (7.04%)**, **C = 486 (7.87%)**, **G = 113 (1.83%)**, and **Q = 1276 (20.66%)**. Spurious-association tests using **100** simulated MIRION distributions yielded approximately **3.4% ± 0.2%** random matches to **Hi-GAL** and **< 1%** to the other catalogs. The mean **HRATE** is approximately **0.43–0.48** for matched catalogs, compared with **0.32** for unmatched MIRION sources, consistent with many unmatched sources being fainter and/or below detection thresholds of other surveys [2512.06213].

## 5. Physical parameter space and the bridge between intermediate- and high-mass star formation

For MIRION sources with **Hi-GAL** compact-source matches and adopted distances, the catalog provides a physically characterized sample of **3945** objects after removing entries missing any physical property. The associated clumps span a wide range in mass and luminosity: masses extend roughly from **10** to **\(10^6\,M_\odot\)**, with most lying in **\(0 < \log_{10}(M/M_\odot) < 4\)**, while bolometric luminosities span approximately **10–\(10^5\,L_\odot\)**, with most in **\(1 < \log_{10}(L/L_\odot) < 5\)**. Clump diameters are typical of **“clumps”**, around **0.2–3 pc** [2512.06213].

The median physical values reported for the matched sample are **\(\log_{10}(M/M_\odot)=2.34\)** and **\(\log_{10}(L/L_\odot)=3.14\)**. The table also includes **\(L/M\)**, **\(L_{\rm bol}/L_{\rm smm}\)**, **\(T_{\rm bol}\)**, graybody temperature **TEMP**, and surface density **\(\Sigma\)**, all showing broad ranges consistent with mixed evolutionary stages and mixed mass regimes. This broad parameter coverage is what allows MIRION to “bridge” intermediate- and high-mass star formation in an operational sense: it densely populates the **10–100 \(M_\odot\)** clump regime that had previously been sparsely sampled [2512.06213].

Distance-dependent quantities in the Herschel-matched table were explicitly rescaled to the MIRION distances. In particular, luminosity and mass were rescaled according to

$$
L_{\rm MIRION} = L_{\rm Hi\!-\!GAL}\left(\frac{d_{\rm MIRION}}{d_{\rm Hi\!-\!GAL}}\right)^2,
$$

and

$$
M_{\rm MIRION} = M_{\rm Hi\!-\!GAL}\left(\frac{d_{\rm MIRION}}{d_{\rm Hi\!-\!GAL}}\right)^2.
$$

Diameter scales linearly with distance, and the context formula for dust/gas mass from sub-millimeter continuum is

$$
M = \frac{S_\nu d^2}{\kappa_\nu B_\nu(T_d)}.
$$

Because MIRION adopts the Hi-GAL SED-derived physical quantities and then rescales them using the adopted MIRION distances, the catalog is internally structured so that distance revisions propagate directly into the inferred physical state of the clumps [2512.06213].

The catalog also tabulates the fractions of the full Hi-GAL-matched MIRION sample meeting widely used threshold conditions: **\(\log_{10}(L_{\rm bol}/M) > 1.35\)** for **22%**, **\(\log_{10}(L_{\rm bol}/L_{\rm smm}) \ge 2\)** for **35%**, **\(T_{\rm bol} < 40\ {\rm K}\)** for **20%**, and **\(\log_{10}(\Sigma) > 0\)** for **16%**. In the MIRION interpretation, these thresholds respectively mark likely high-mass star-forming regions, more evolved star-forming regions, younger protostellar clumps, and high surface density likely associated with massive star-forming regions [2512.06213].

## 6. Infrared color diagnostics, candidate selection, and limitations

One of MIRION’s principal scientific results is that infrared colors trace physical conditions and evolutionary state. For **3116** sources with good **8 μm** and **12 μm** photometry \((\mathrm{fractional\ error} < 0.5)\), the mean color is **\(\log_{10}(F_{12}/F_8) = -0.48 \pm 0.19\)**. Subsamples show **RMS** at **\(-0.37 \pm 0.24\)**, **WISE C/G/K** at **\(-0.42 \pm 0.21\)**, and **CORNISH** at **\(-0.39 \pm 0.21\)**, all significantly below the typical **WISE H II** mean color of **\(-0.09\)**. **WISE Q** sources, at **\(-0.51 \pm 0.15\)**, and the **“No Association”** population, at **\(-0.49 \pm 0.18\)**, are even more negative, indicating particularly compact photodissociation regions and/or softer UV fields, consistent with IMSFRs [2512.06213].

A second diagnostic uses the color-color plane **\(\log_{10}(F_{24}/F_8)\)** versus **\(\log_{10}(F_{70}/F_{24})\)**. For **2819** sources with good photometry and without **NOSRC** or **PCONF** flags, MIRION sources associated with high-mass tracers—**RMS**, **CORNISH**, and **WISE K/C/G**—cluster in the H II-region color space defined by **Anderson et al. (2012)**. Sources without high-mass tracers—especially **WISE Q** and unmatched MIRION sources—tend toward lower **\(\log_{10}(F_{24}/F_8)\)**, consistent with softer UV fields in which polycyclic aromatic hydrocarbons are excited but not destroyed, while the average dust temperatures inferred from **\(\log_{10}(F_{70}/F_{24})\)** remain similar [2512.06213].

The luminosity-mass diagrams strengthen that interpretation. **RMS** and **WISE K** sources lie largely above the **90th percentile lower limit for H II clumps** from **Elia (2017)**, whereas **WISE Q** and **MIRION-only** samples lie above pre-stellar upper limits but below the H II locus. Many of these have masses of **10–100 \(M_\odot\)** and would track toward **\(L \approx 10^3\text{–}10^4\,L_\odot\)** in simple single-star clump models, consistent with IMSFRs. When uncertainties are included, **57–66%** of the **WISE Q + MIRION-only** clumps have **\(M < 350\,M_\odot\)**, which the catalog interprets as strengthening their identification as IMSFR candidates [2512.06213].

The catalog therefore supports several operational sample definitions. High-mass star-forming region candidates can be selected using crossmatches to **RMS** types **“H II region”** or **“H II/YSO”**, **CORNISH** types **UC H II/H II (Diffuse/Dark)/MYSO/IR Quiet**, or **WISE** types **K/C/G**, together with thresholds such as **\(\log_{10}(L_{\rm bol}/M) > 1.35\)** or **\(\log_{10}(\Sigma) > 0\)** and colors typical of the Anderson H II box. IMSFR candidates are preferentially found among **WISE Q** and **“No Association”** MIRION sources, especially when they show compact PAH colors **\(\log_{10}(F_{12}/F_8) \lesssim -0.45\)**, lower **\(\log_{10}(F_{24}/F_8)\)**, moderate clump masses **10–100 \(M_\odot\)**, and an absence of radio or H II-region tracers. Very young protostellar clumps can be isolated with **\(T_{\rm bol} < 40\ {\rm K}\)** and lower **\(L_{\rm bol}/L_{\rm smm}\)** [2512.06213].

The catalog documentation also emphasizes several limitations. MIRION sources are heterogeneous and include a small fraction of non-star-forming objects, for example possible planetary nebulae flagged **\(f_{\rm CEXT}\)**. The parent sample is tied to **Spitzer** coverage, so faint or extended objects may be underrepresented. Complex mid-infrared backgrounds, saturation, and subjective apertures are mitigated by multiple measurements and flags, but residual photometric systematics remain. In the inner Galaxy, the near-far ambiguity is handled probabilistically rather than deterministically, and distances near the Galactic Center have historically been uncertain. Crossmatch associations use a **24″** radius and are therefore probabilistic rather than guaranteed for every source. A small systematic positional offset was corrected for sources **3035–3101** \((\ell \approx 101.23^\circ\text{–}103.65^\circ;\ \mathrm{offsets}\ 0.35′\text{–}0.1′)\) [2512.06213].

MIRION is not static. The **PERYSCOPE** project—**People Enabling Research: a Yellowball Survey of the Colors Of Protostellar Environments**—provides a classroom-oriented workflow in which volunteers, typically introductory astronomy students, measure photometry for approximately **40** MIRION sources in **Google Colab** using a simplified version of the MIRION photometry tool. After quality checking against current catalog values and mask overlap, these measurements are periodically incorporated into the online catalog to refine mean fluxes and reduce fractional errors as the number of measurements grows. The latest online version is hosted at **https://github.com/astrodevine/MIRION** [2512.06213].

Source: https://www.emergentmind.com/topics/mirion-catalog