---
title: 'Yasone-1: Hypercompact Stellar Cluster Candidate'
url: https://www.emergentmind.com/topics/yasone-1
type: topic
---

# Yasone-1: Hypercompact Stellar Cluster Candidate

Searching arXiv for the Yasone catalogue paper and closely related compact Milky Way satellite literature.
arXiv search query: "2509.15345 Yasone catalogue Yasone-1 hypercompact stellar cluster Milky Way satellite candidate"
Yasone-1 is a compact stellar overdensity in the “Yasone catalogue,” presented as one of three newly discovered low-latitude Milky Way satellites and, more specifically in the paper’s discussion, as a halo hypercompact stellar cluster / Milky Way satellite candidate and hypercompact cluster candidate (HCC). Its defining feature is a combination of extreme compactness, very low luminosity, and an old, metal-poor stellar population. The available photometric and astrometric evidence indicates that it is more consistent with a real compact stellar system than with random field contamination, but the current data do not permit a secure discrimination between an extremely faint dwarf satellite and a star cluster / hypercompact cluster. The discovery and characterization were reported in “The Yasone catalogue: three new Milky Way satellites and 17 further hypercompact candidates” [2509.15345].

## 1. Discovery context and designation

Yasone-1 was introduced as part of a broader search for compact stellar overdensities in the Milky Way. Within that framework, it belongs to the subset of three newly discovered halo candidates in the Yasone catalogue, alongside Yasone-2 and Yasone-3. The abstract describes these systems as new low-latitude Milky Way satellites, while the results and discussion use the more cautious label hypercompact cluster candidate. This dual usage reflects the paper’s central interpretive point: Yasone-1 is clearly a compact stellar-system candidate, but its final astrophysical classification remains open [2509.15345].

The object is located at equatorial coordinates R.A. \(265.52019^\circ\), Dec. \(+13.17146^\circ\) (J2000). It is described as a halo object at low latitude, with the halo candidates broadly situated at roughly \(|b| \sim 20^\circ\). The paper does not tabulate an exact Galactic coordinate for Yasone-1 in the provided excerpt, and it does not claim a definitive final classification.

A useful way to situate Yasone-1 is to note that the paper treats it simultaneously as a compact halo overdensity, a possible Milky Way satellite, and an HCC-like system. This suggests that Yasone-1 occupies an observational regime where the conventional categories of star cluster and ultra-faint dwarf satellite are not cleanly separated.

## 2. Detection workflow and data sources

Yasone-1 was found through a multi-stage search based on Pan-STARRS1 Data Release 2 as the initial wide-area photometric database, combined with Gaia Data Release 3 astrometry for kinematic consistency. The search first selected halo-like sources with \(|b| > 20^\circ\), then identified statistically significant two-dimensional overdensities using a KDTree-based clustering approach with a \(15''\) linking scale, and finally filtered candidates by color–magnitude-diagram consistency with MIST isochrones [2509.15345].

Yasone-1 survived these selection stages and was subsequently followed up with deeper \(gri\) imaging from the GTC/OSIRIS instrument. That follow-up was described as crucial because Pan-STARRS and Gaia alone traced only the bright upper red-giant branch. The deeper GTC data revealed a richer, fainter stellar sequence and enabled a stronger characterization of the object.

The isochrone-fitting procedure used MIST grids spanning ages \(5 \le \log(\mathrm{age/yr}) \le 10.3\) and metallicities \(-4 \le [\mathrm{Fe/H}] \le +0.5\). The free parameters adjusted were extinction \(A_V\) and heliocentric distance. The best-fit isochrone was defined as the one maximizing the number of stars aligned with the main-sequence locus in the CMD, and stars farther than \(1\sigma\) from the locus in both color spaces were discarded. This workflow anchors the paper’s characterization of Yasone-1 as a photometrically coherent stellar system rather than a purely statistical overdensity.

## 3. Photometric and astrometric characterization

Deep GTC imaging resolves Yasone-1 as a very compact overdensity. Within \(40''\), the paper reports 29 \(g\)-band and 34 \(r\)-band point-like sources, corresponding to 42 unique stars in total. The system is stated to occupy an angular size of about \(40''\), and the catalog lists an angular half-light radius of \(24''\). Importantly, the half-light radius is defined here as the radius enclosing half the stellar members, not a surface-brightness half-light radius [2509.15345].

The CMDs in both \(g-r\) and \(r-i\) exhibit an old, metal-poor stellar sequence consistent with a main sequence plus a small red-giant-branch-like extension. A background annulus of equal area does not show the same coherent sequence. On that basis, the paper argues that Yasone-1 is more consistent with a real compact stellar system than with random field contamination.

Gaia astrometry provides an additional layer of coherence. The catalog lists 42 stellar members and a proper-motion dispersion of \(0.10\ \mathrm{mas\ yr^{-1}}\). For the brightest Gaia members, the authors derive a mean proper motion of
\[
\mu_{\alpha}\cos\delta = -3.63\ \mathrm{mas\ yr^{-1}}, \qquad
\mu_{\delta} = -3.70\ \mathrm{mas\ yr^{-1}}.
\]
At the inferred distance, this corresponds to a tangential speed of about \(295\ \mathrm{km\ s^{-1}}\). The tentative tangential-velocity dispersion is reported as \(5.89^{+1.47}_{-0.98}\ \mathrm{km\ s^{-1}}\), but the paper explicitly treats this as an upper limit because Gaia astrometric uncertainties may inflate the dispersion.

## 4. Stellar population and derived physical properties

The best-fit stellar population for Yasone-1 corresponds to an age of \(12^{+1}_{-2}\) Gyr and a metallicity of \([\mathrm{Fe/H}] = -1.5 \pm 0.25\), with line-of-sight extinction \(A_V = 1.0\) mag. The best-fit isochrone places the system at a heliocentric distance of \(12^{+3}_{-2}\) kpc. Using the angular half-light radius of \(24''\), the paper derives a physical half-light radius of \(r_h = 1.40^{+0.35}_{-0.23}\) pc [2509.15345].

Its luminosity is extremely low, with an absolute magnitude
\[
M_V = +2.36^{+0.40}_{-0.48}.
\]
The total observed stellar mass is estimated as
\[
18.2^{+10.2}_{-5.6}\ M_\odot,
\]
and the authors explicitly note that this is a lower limit because only stars detected down to the photometric limit are included.

The mass estimate is based on the summed \(g\)-band luminosity and a color-based mass-to-light ratio from Into & Portinari (2013),
\[
\log_{10}(M/L_g)=1.774\,(g-r)-0.783,
\]
with an adopted average \(M/L_g \approx 1.09\) for Yasone-1. Absolute magnitudes are computed from the distance modulus \(5\log_{10}(d/10\,\mathrm{pc})\), with solar magnitudes \(M_{\odot,g}=5.16\) and \(M_{\odot,V}=4.83\), using the \(g\)-to-\(V\) conversion from Tonry et al. (2012).

The GTC depth reaches about \(g \sim 26.5\), corresponding at 12 kpc to a main-sequence mass of roughly \(0.23\ M_\odot\). Assuming a Kroupa IMF, the authors infer that the observations directly capture about 73% of the total stellar mass. This is relevant because it constrains how incomplete the current stellar census is while also underscoring that the object remains extremely low-mass even after accounting for undetected low-luminosity members.

| Property | Value |
|---|---|
| Sky position | R.A. \(265.52019^\circ\), Dec. \(+13.17146^\circ\) |
| Distance | \(12^{+3}_{-2}\) kpc |
| Angular half-light radius | \(24''\) |
| Physical half-light radius | \(1.40^{+0.35}_{-0.23}\) pc |
| Absolute magnitude | \(M_V = +2.36^{+0.40}_{-0.48}\) |
| Total observed stellar mass | \(18.2^{+10.2}_{-5.6}\ M_\odot\) |
| Age | \(12^{+1}_{-2}\) Gyr |
| Metallicity | \([\mathrm{Fe/H}] = -1.5 \pm 0.25\) |
| Extinction | \(A_V = 1.0\) mag |
| Stellar members | 42 |

## 5. Classification and relation to other compact systems

The paper is explicit that Yasone-1 cannot yet be classified securely as either a very faint dwarf galaxy or a star cluster / hypercompact cluster. Its compactness and stellar population make it HCC-like, but the present data do not settle the question. In the size–luminosity plane, it lies in an extreme compact and faint regime and is compared to known globular clusters, dwarf galaxies, and ambiguous systems; the paper specifically notes similarity to Ursa Major III [2509.15345].

This ambiguity arises from the conjunction of several properties. Yasone-1 is old and metal-poor, structurally very compact, photometrically coherent, and extremely faint. Those attributes are all compatible with a genuine bound stellar system, yet they do not uniquely identify whether that system is dark-matter-dominated. A plausible implication is that Yasone-1 inhabits the boundary region where structural and CMD-based diagnostics alone become insufficient for taxonomy.

The paper therefore treats its current designation conservatively. Calling it a Milky Way satellite emphasizes discovery context and possible galactic significance; calling it an HCC emphasizes morphology and compactness; calling it a candidate preserves the uncertainty. That usage is methodologically important, because it avoids over-interpreting photometric evidence in a regime where spectroscopy is normally decisive.

## 6. Intermediate-mass-black-hole scenario and required follow-up

The Yasone catalogue paper discusses the possibility that Yasone clusters may host an intermediate-mass black hole (IMBH), but for Yasone-1 specifically it reports no direct evidence for an IMBH. One diagnostic mentioned in general is the ratio \(r_c/r_h\), with values \(>0.5\) sometimes suggested as evidence for IMBH-driven core inflation. However, no core radius is reported for Yasone-1, because the sparse membership and Poisson noise make robust profile fitting impossible [2509.15345].

The paper also notes that Yasone-2, not Yasone-1, is the candidate singled out as a potentially stronger IMBH host. For Yasone-1, the IMBH discussion therefore remains hypothetical rather than evidentiary.

Follow-up spectroscopy is identified as the key next step. The authors state that spectroscopy is needed to measure member radial velocities and chemical abundances, determine whether the system has a dark-matter-dominated velocity dispersion, establish a dynamical mass, and clarify whether the stellar population is more consistent with a star cluster or a dwarf galaxy. Better kinematic data would also be required to test any IMBH scenario, since a genuine IMBH host would ideally show dynamical signatures or an inflated core structure that cannot yet be measured robustly from the current small sample of resolved stars.

In that sense, Yasone-1 is best understood as a compact and unusually faint Milky Way stellar-system candidate whose reality is supported by deep imaging, CMD coherence, and Gaia astrometry, but whose astrophysical nature remains provisional pending spectroscopy.

Source: https://www.emergentmind.com/topics/yasone-1