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Stars and brown dwarfs in the sigma Orionis cluster. IV. IDS/INT and OSIRIS/GTC spectroscopy and Gaia DR2 astrometry

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Pith's one-line read New spectroscopy and Gaia astrometry remove 35 previously accepted stars from the young σ Orionis cluster's member list, shrinking the cluster by about 10%.

desk verdict A solid, necessary cleanup of the sigma Orionis membership list; the 35-interloper claim is robust and the paper deserves proper peer review. read the letter →

arxiv 1908.10340 v1 pith:YYLIOT6X submitted 2019-08-27 astro-ph.SR

J. A. Caballero , A. de Burgos , F. J. Alonso-Floriano , A. Cabrera-Lavers , D. Garcia-Alvarez , D. Montes This is my paper · ORCID
classification astro-ph.SR
keywords sigmaOrionisclusteropenclustersstellarmembershipGaiaDR2astrometrypre-main-sequencestarslithiumabundancebrowndwarfsinitialmassfunction
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper aims to clean up the member list of the young $\sigma$ Orionis cluster by combining new low-resolution optical spectroscopy of 168 targets with Gaia DR2 astrometry and literature youth indicators (lithium, $H\alpha$ emission, mid-infrared excess, X-rays). Its central result is that 51 of the observed targets are foreground or background stars, and 35 of those had previously been counted as cluster members in highly cited studies of disc frequency, the initial mass function, and chemical abundances. If the rejection holds, the known $\sigma$ Orionis population shrinks by roughly 10%, the halo appears more rarefied, and some derived cluster properties—such as the disc fraction near intermediate masses—would need revision. A sympathetic reader would care because $\sigma$ Orionis is one of the anchor populations for studying how stars and brown dwarfs form together in a 3-million-year-old open cluster.

What carries the argument

The load-bearing device is the membership screen. It first applies astrometric cuts taken from Caballero (2018): a total proper motion greater than 10 mas/yr or a parallax outside 1.5–3.5 mas (distance outside roughly 290–670 pc, judged against the cluster distance of $387.5 \pm 1.3$ pc) marks a star as a non-member. It then checks surviving stars for youth indicators—Li I $\lambda6707.80$ Å absorption, $H\alpha$ emission, mid-infrared excess, and X-ray emission—and rescues individual cases with poor astrometric quality, such as Mayrit 1073209. The Mayrit catalogue of Caballero (2008b) supplies the naming grid that ties the 35 rejected stars to the earlier widely used samples.

What would settle it

Measure high-resolution radial velocities and deep lithium spectra for the 35 rejected stars: genuine $\sigma$ Orionis members should share the cluster systemic velocity of $+31.10 \pm 0.16$ km/s and, for K and M types, show Li I in absorption. If a substantial fraction of the 35 show that velocity and lithium, or if Gaia DR3 astrometry puts them back inside the $\mu < 10$ mas/yr and parallax 1.5–3.5 mas window, the claim that they are interlopers collapses. The five high-RUWE (re-normalised unit-weight error) sources without clean parallaxes are the first place to look, since unresolved binarity could be corrupting their astrometry.

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Extended reading notes

Core claim

The paper's central claim is that combining Gaia DR2 proper motions and parallaxes with spectroscopic youth diagnostics removes 51 of the 165 analysed stars and brown dwarfs from $\sigma$ Orionis membership, and that 35 of these removed objects had been classified as cluster members in earlier, highly cited works. Those 35 make up about 10% of the cluster's known stellar population and a larger fraction in the ${\sim}0.5$ to $1.0\,M_\odot$ range, so previous results built on the old member lists—disc frequencies of roughly 10 to 35%, a continuous mass function from about $20\,M_\odot$ down to about $0.005\,M_\odot$, the radial concentration of the halo, and the mean [Fe/H]—would shift if the new membership is adopted. The paper also reports 14 strong accretors with $EW(H\alpha) < -50$ Å, 11 close binaries resolved by Gaia, and first-time spectral types or line measurements for dozens of objects.

Load-bearing premise

The argument depends on the assumption that the astrometric cuts—total proper motion above 10 mas/yr or a Gaia DR2 parallax outside 1.5–3.5 mas—really do separate interlopers from members, so that poor astrometry or unresolved binarity does not push genuine members across the boundary. The paper itself shows the boundary is leaky: it had to rescue Mayrit 1073209 and keep two close binaries (Mayrit 359179AB and Mayrit 873229AB) whose parallaxes are corrupted by binarity, so the exact count of 35 would change if more such cases exist.

Editorial extensions

If this is right

  • Disc frequencies for intermediate-mass T Tauri stars in $\sigma$ Orionis would rise, from about 35% toward almost 40%, because the discarded stars had been counted as part of the disc-less population.
  • The cluster mass function loses about 27 stars near ${\sim}0.5\,M_\odot$, sharpening the contrast between the steep high-mass Salpeter slope and the nearly flat low-mass slope.
  • The halo becomes more rarefied, making $\sigma$ Orionis more compact than previously thought and easing the separation of its young population from neighbouring Orion OB1b groups.
  • The mean metallicity [Fe/H] derived from the earlier sample may shift, although it would probably remain near solar.
  • If the two radial-velocity discordant candidates Mayrit 1275190 and Mayrit 1285339 are also foreground stars, the number of previously believed members discarded would rise to 37.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If this membership list is adopted, earlier studies that used the Mayrit catalogue for multiplicity statistics, variability surveys, or disc demographics would need to be re-evaluated with the 35 removed objects excluded, not just the papers named in the implications section.
  • The rescue of one poor-astrometry field star and two close binaries hints that some of the 35 rejected stars could re-enter the member list if Gaia DR3 or high-resolution imaging resolves their companions; the count is better read as a lower bound on contamination until then.
  • The same screen—low-resolution spectra of lithium and $H\alpha$ plus Gaia astrometry plus youth flags—could be applied to other young, heavily contaminated clusters, and would probably revise their member lists in similar proportions.
  • The five high-RUWE (re-normalised unit-weight error) objects without clean parallaxes (Mayrit 203283, 1082115, 1073209, 521199, 157155) are the paper's implicit follow-up list; if they are unresolved binaries, their youth features would be consistent with membership despite noisy astrometry.
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Editorial analysis

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Desk editor's note, referee report, and a circularity audit.

Referee Report

0 major / 6 minor

Summary. This paper reports IDS/INT and OSIRIS/GTC low-resolution spectroscopy for 168 targets toward the σ Orionis cluster, together with Gaia DR2 astrometry and compiled youth indicators (Li I, Hα, mid-infrared excess, X-ray emission). The central result is a membership re-classification that removes 51 foreground/background stars, 35 of which had previously been considered cluster members in the surveys of Hernández et al. (2007) and Caballero (2008b); the authors argue that this affects published disc fractions, the initial mass function, the radial distribution, and metallicity studies. The paper also presents 14 strong accretors, 11 resolved close binaries, and radial-velocity discordances for three stars.

Significance. The claimed correction to the σ Orionis census is significant: if correct, it removes roughly 10% of the known cluster population and changes the interpretation of several widely used studies. The exclusion is anchored in conservative, pre-existing Gaia DR2 astrometric boundaries, and the late-type rejections are corroborated by the absence of Li I, mid-infrared excess, and X-ray emission in the same objects. The spectral-type and equivalent-width measurements are cross-validated with three methods and carry stated uncertainties. The paper's own rescue of Mayrit 1073209 and of two close binaries demonstrates awareness of Gaia DR2 pathologies; because the Table 3 stars violate the membership boundaries by large margins and lack youth features, the central claim is robust to those pathologies.

minor comments (6)
  1. [Abstract] The abstract contains a stray ')}' after 'Intermediate Dispersion Spectrograph (IDS)'; please remove it.
  2. [§4, Summary] The sentence '14 strong accretors with EW(α) < –50 Å' should read 'EW(Hα)', since the text concerns the Balmer line.
  3. [§3.1, Table 3] In the 'Reason for discarding' column, 'No µ' and 'No ̟' could be misread as missing data; please define them in the table note as 'outside the cluster boundaries in total proper motion' and 'outside the parallax interval 1.5–3.5 mas', respectively.
  4. [§3.1] Because unresolved binarity corrupted the Gaia DR2 astrometry of Mayrit 1073209, Mayrit 359179AB, and Mayrit 873229AB, a brief statement of the RUWE values, or a Gaia EDR3 cross-check, for the 35 stars in Table 3 would further harden the non-member list; this is a validation request rather than a blocking issue, because most Table 3 objects deviate by many sigma.
  5. [§3.3] The RUWE values quoted in the text for six tight binaries are not tabulated; adding them to Table 5 would make the binary discussion easier to audit.
  6. [Fig. 3 legend] The legend groups Haro 5-17 and Haro 5-46 with 'non-member stars with Li i'; since these are discarded by position rather than by astrometry, please distinguish them from the 51 astrometrically rejected non-members.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity; the non-member claim is based on external Gaia DR2 astrometry and independent youth-feature checks.

full rationale

The central claim—that 35 of the 51 newly identified non-members were previously considered σ Orionis members—rests on Gaia DR2 parallaxes and proper motions, an external catalogue not produced by the authors, combined with new IDS/INT and OSIRIS/GTC spectroscopy. The astrometric membership boundaries are adopted from Caballero (2018) and the cluster distance from Schaefer et al. (2016), but those are calibration inputs, not outputs of this paper; the specific list of 35 discarded former members in Table 3 is new and is not contained in the cited prior work. The paper also states that 'none of the astrometrically discarded stars displayed lithium in absorption, MIR excess, or X-ray emission', providing an independent youth-based check for the late-type stars that dominate Table 3. No equation in the paper defines membership in terms of the claimed result, no fitted parameter is relabelled as a prediction, and no load-bearing argument reduces to a self-citation. The possible concern that Gaia DR2 astrometry of a few stars could be corrupted by unresolved binarity is a data-quality risk rather than circularity, and the paper itself rescues Mayrit 1073209, Mayrit 359179AB, and Mayrit 873229AB on exactly that basis. The derivation is self-contained against external benchmarks, so the circularity score is 0.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The paper introduces no new physical entities and fits no new physical constants. The central membership result rests on externally supplied astrometric data and on hand-chosen thresholds from prior work; these are listed above. Youth features are cross-checks, not fitted parameters.

free parameters (2)
  • Astrometric membership boundaries = mu > 10 mas/yr; 1.5 < parallax < 3.5 mas
    Adopted from Caballero (2018) as conservative cuts; not fitted in this paper, but the membership rejection and the 35 non-member count depend on these thresholds.
  • EW(Li I) detection threshold = 0.1 A
    Hand-chosen lower limit for claiming a lithium detection in low-resolution spectra; affects the youth-feature statistics.
assumptions (5)
  • domain assumption Gaia DR2 astrometry is reliable for the surveyed targets, except where flagged by poor quality indicators or binarity.
    The membership classification in Sect. 3.1 uses Gaia DR2 parallaxes and proper motions as primary discriminants; the paper rescues Mayrit 1073209 based on quality flags, showing that astrometric quality is a necessary condition.
  • domain assumption The sigma Orionis cluster has distance about 387.5 pc and kinematics as measured by Schaefer et al. (2016) and Simon-Diaz et al. (2015).
    The 1.5 to 3.5 mas parallax window and the proper-motion cuts are calibrated to these assumed cluster properties (Sect. 3.1).
  • domain assumption Li I absorption and H-alpha emission are valid youth indicators for late-type stars in a about 3 Myr cluster.
    Used to confirm membership and identify accretors (Sect. 2.4); based on literature (Bertout 1989; Barrado y Navascues and Martin 2003).
  • domain assumption The MILES synthetic library and the observed K7 to M6.5 standard-star grid are adequate for spectral typing at the resolution used.
    Spectral types in Sect. 2.2.3 are assigned by comparison with these templates; large MILES/PyHammer discrepancies are resolved by adopting the visual classification.
  • domain assumption The Barrado y Navascues and Martin (2003) empirical EW(H-alpha) criterion separates accreting classical T Tauri stars from chromospherically active stars.
    Defines the 14 strong accretors in Sect. 3.2.

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Cite this review

Pith. "Pith review of Stars and brown dwarfs in the sigma Orionis cluster. IV. IDS/INT and OSIRIS/GTC spectroscopy and Gaia DR2 astrometry." pith.science (2026). https://pith.science/paper/YYLIOT6X

@misc{pith2026190810340,
  author       = {Pith},
  title        = {Pith review of: Stars and brown dwarfs in the sigma Orionis cluster. IV. IDS/INT and OSIRIS/GTC spectroscopy and Gaia DR2 astrometry},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/YYLIOT6X}},
  note         = {Machine review of arXiv:1908.10340}
}
read the original abstract

Context. Only a few open clusters are as important for the study of stellar and substellar objects, and their formation and evolution, as the young sigma Orionis cluster. However, a complete spectroscopic characterisation of its whole stellar population is still missing. Aims. We filled most of that gap with a large spectroscopic and astrometric survey of targets towards sigma Orionis. Eventually, it will be one of the open clusters with the lowest proportion of interlopers and the largest proportion of confirmed cluster members with known uncontrovertible youth features. Methods. We acquired 317 low-resolution optical spectra with the Intermediate Dispersion Spectrograph (IDS)} at the 2.5 m Isaac Newton Telescope (INT) and the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) at the 10.4 m Gran Telescopio Canarias (GTC). We measured equivalent widths of Li I, H-alpha, and other key lines from these spectra, and determined spectral types. We complemented this information with Gaia DR2 astrometric data and other features of youth (mid-infrared excess, X-ray emission) compiled with Virtual Observatory tools and from the literature. Results. Of the 168 observed targets, we determined for the first time spectral types of 39 stars and equivalent widths of Li I and H-alpha of 34 and 12 stars, respectively. We identified 11 close (rho <~ 3 arcsec) binaries resolved by Gaia, of which three are new, 14 strong accretors, of which four are new and another four have H-alpha emission shifted by over 120 km/s, two juvenile star candidates in the sparse population of the Ori OB1b association, and one spectroscopic binary candidate. Remarkably, we found 51 non-cluster-members, 35 of which were previously considered as sigma Orionis members and taken into account in high-impact works on, for example, disc frequency and initial mass function.

Figures

Figures reproduced from arXiv: 1908.10340 by the authors.

Figure 1
Figure 1. Literature and adopted spectral types. Red filled cir￾cles and blue crosses stand for cluster members and non￾members, respectively. Literature spectral types were retrieved from Greenstein & Keenan (1958), Guetter (1981), Nesterov et al. (1995), Wolk (1996), Houk & Swift (1999), Zapatero Osorio et al. (2002), Sacco et al. (2008), Caballero et al. (2008, 2012), Manara et al. (2013), and Hernández et al. (2014). The … view at source ↗
Figure 2
Figure 2. Histograms of Gaia DR2 total proper motions (top) and paral￾laxes (bottom). Vertical dashed lines indicate the conservative σ Orionis boundary limits explained in Sect. 3.1. The bin sizes of these histograms have been adjusted according to Freedman & Diaconis (1981). (1994); Montes et al. (2001b); Zapatero Osorio et al. (2002)). We highlight the fact that for open clusters as young as σ Ori￾onis (τ ∼ 3 Ma) this is t… view at source ↗
Figure 3
Figure 3. Measured Li i EWs as a function of adopted spectral type. Filled red circles show young targets observed with IDS R1200Y; open red circles show young targets observed with IDS R150V (controvertible); filled orange circles show young targets observed with OSIRIS R100B; blue crosses show non-member stars with Li i (Haro 5-17 and Haro 5- 46); solid magenta lines show upper and lower EW(Li i) envelopes of the Pleiades (… view at source ↗
Figures from the paper (4 more)
Figure 4
Figure 4. Figure 4: Histogram of the number of sources as a function of the mea￾sured EW(Hα). The bin size of this histogram has been adjusted ac￾cording to Freedman & Diaconis (1981). by the radial velocity analysis of Pérez-Blanco et al. (2018). Mayrit 1285339 and some G- and K-type sta…
Figure 5
Figure 5. Figure 5: Measured EW(Hα) of our sample of stars as a function of adopted spectral type. The black dashed line shows the curve fitting from K0 to L0 of the EWs used in Barrado y Navascués & Martín (2003) as an empirical criterion for identifying classical T Tauri stars. The blac…
Figure 6
Figure 6. Figure 6: Normalised spectra around Hα λ6562.80 Å of the 14 strong ac￾cretors in [PITH_FULL_IMAGE:figures/full_fig_p010_6.png]
Figure 7
Figure 7. Figure 7: Gaia DR2 radial velocity as a function of adopted spectral type. Red and blue circles highlight σ Orionis stars and non-cluster-member stars, respectively; coloured vertical error bars show 3σ uncertainties; and the black horizontal line shows the σ Ori systemic veloci…

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