The Astrophysical Journal Letters, 969:L8 (7pp), 2024 July 1

https://doi.org/10.3847/2041-8213/ad527b© 2024. The Author(s). Published by the American Astronomical Society.

A JWST Project on 47 Tucanae: NIRSpec Spectroscopy of Multiple Populations among
M Dwarfs*
A. F. Marino1 , A. P. Milone1,2 , A. Renzini1 , E. Dondoglio1 , E. Bortolan2, M. G. Carlos3 , G. Cordoni4 , A. Dotter5
S.1 Jang6 , E. P. Lagioia7 , M. V. Legnardi2 , F. Muratore2 , A. Mohandasan2 , M. Tailo1 , and T. Ziliotto2,

Istituto Nazionale di Astroﬁsica—Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova, IT-35122, Italy; anna.marino@inaf.it
2
Dipartimento di Fisica e Astronomia “Galileo Galilei”—Univ. di Padova, Vicolo dell’Osservatorio 3, Padova, IT-35122, Italy
3
Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden
4
Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
5
Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA
6
Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University, Seoul 03722, Republic of Korea
7
South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, Peopleʼs Republic of China
Received 2024 April 12; revised 2024 May 29; accepted 2024 May 30; published 2024 June 24

Abstract
We present the ﬁrst spectroscopic estimates of the chemical abundance of M dwarf stars in a globular cluster (GC),
namely 47 Tucanae. By exploiting NIRSpec on board the James Webb Space Telescope, we gathered lowresolution spectra for 28 stars with masses in the range ∼0.4–0.5 Me. The spectra are strongly affected by the H2O
water vapor bands, which can be used as indicators of oxygen abundance. The spectral analysis reveals that thetarget stars feature a different O abundance, with a difference of ∼0.40 dex between the ﬁrst and the most polluted
second population. The observed range is similar to that observed among red giant stars. This result reinforces
previous ﬁndings based on the analysis of photometric diagrams, including the “chromosome maps,” providing aﬁrst, and more direct, evidence of light element variations in the M dwarfs’ mass regime. The observation that themultiple populations, with their variations in light elements, exhibit the same patterns from the lower main
sequence all the way to the red giant branch further strengthens the notion that multiple stellar populations in GCs
formed in a series of bursts of star formation.
Uniﬁed Astronomy Thesaurus concepts: Globular star clusters (656)and C-depleted to various degrees, and enhanced in the
products of the H-burning at high temperatures, such as He, N,
Na, and Al (e.g., Gratton et al. 2004; Marino et al. 2019).
The multiple population phenomenon has been interpreted interms of successive stellar generations, i.e., multiple bursts ofstar formation taking place within the forming GC while thechemical composition of its interstellar medium was (rapidly)
evolving as a consequence of pollution from evolved 1P stars.
In this scenario, the observed predominance of the 2P
component, contributing more than half the mass of today’sGCs (Milone et al. 2017a), is a challenge, as only a small
fraction of the initial 1P mass is delivered with the composition
required to form 2P stars. This is known as the “mass budget
problem.” A way out from this is to postulate that the GC
progenitors were substantially more massive and lost at least
80%–90% of their 1P mass into the Halo before delivering the
naked present-day GCs (Decressin et al. 2007; D’Ercole et al.
2008; Ventura & D’Antona 2009; de Mink et al. 2009;
Denissenkov & Hartwick 2014). One way of achieving this has
been recently proposed in which the host (dwarf) galaxy
harbors extended star formation around the nascent GC, hence
the original 1P stars are distributed over a much larger volume
than the GC itself (Renzini et al. 2022). Subsequent evolution
and tidal interaction will then result in removing most of theoriginal 1P stars, as indicated by N-body simulations (Lacchin
et al. 2024).
An alternative scenario assumes that stellar populations arecoeval and the chemical variations are due to mass accreted
onto existing low-mass stars, rather than being the product ofdifferent episodes of star formation (Bastian et al. 2013). A
fascinating development of this scenario has been proposed by

1. Introduction
As vividly illustrated by data from the Hubble Space
Telescope (HST), globular clusters (GCs) harbor multiple
sequences along the color–magnitude diagram (CMD), frombelow the main-sequence turnoff all the way to the red giant
branch (RGB) and the asymptotic giant branch (e.g., Piotto
et al. 2015; Milone et al. 2015, 2017a). Multiband HST
imaging from the UV to optical has been instrumental inproducing a detailed taxonomy of the multiple population
phenomenon in GCs, thanks to different OH, NH, and CN
molecular blanketing due to different abundances of theseelements in the various populations in each cluster. Direct
spectroscopy of photometrically selected stars belonging todifferent populations has further validated the interpretation of
the photometric classiﬁcation (Marino et al. 2019).
This exquisite data set has allowed us to constrain thedetailed properties of the multiple population phenomenon. In
brief, we know that at least two populations (ﬁrst, 1P, andsecond, 2P, population) can be distinguished in all the analyzed
GCs, with several of them showing multiple 2P components.
Thus, 1P stars have chemical abundances similar to Halo ﬁeldstars of the same metallicity ([Fe/H]), 2P stars are instead O*
The JWST data presented in this article were obtained from the Mikulski
Archive for Space Telescopes (MAST) at the Space Telescope Science
Institute. The speciﬁc observations analyzed can be accessed via doi:10.17909/
6t82-4360.

Original content from this work may be used under the termsof the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the titleof the work, journal citation and DOI.

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Marino et al.

Gieles et al. (2018), who suggested that 2P stars form byaccretion of the material from extremely massive stars, which
can form in the dense environment of a proto-GC. In this
scenario, there is no need for proto-GCs to be signiﬁcantly
more massive than today.
Constraining the original mass of GCs is crucial to assess
what the contribution of GC progenitors was to the assembly of
the Milky Way halo. Moreover, depending on their mass, thesestellar systems might have substantially contributed to thecosmic reionization (Katz & Ricotti 2013; Renzini 2017). All
this, however, remains to be quantitatively assessed, with highredshift observations of nascent GCs starting to provide direct
critical evidence on the formation process (Vanzella et al. 2016,2020; Adamo et al. 2024).
Due to the observational limits imposed by the instrumentation, our knowledge about the chemical abundance pattern ofmultiple populations was, for a long time, conﬁned to the mostluminous phases of the CMD, e.g., the red giant stars. As mostof the stars are in fainter evolutionary stages, in the very low
mass regimes, our current knowledge of the chemicalcomposition of stars in GCs represents just the tip of theiceberg. Indeed, a way to disentangle between the available
scenarios is offered by the comparison of their properties instars with different masses. Any accretion phenomenon should
result in fact in a less efﬁcient capability of low-mass, fully
convective, stars to retain material (Bondi & Hoyle 1944),
which translates into less pronounced chemical variations withrespect to those present in higher-mass stars. Though GC stars
have low masses, a sizeable range can still be observed goingfrom very low-mass stars approaching the H-burning limit up
to 0.9 Me.
The very ﬁrst investigations of multiple populations amongstars with masses down to ∼0.1 Me have been conducted withnear-IR data from HST WFC3 for some GCs, including
NGC 2808 (Milone et al. 2012), NGC 6121 (M 4; Milone et al.
2014) and others (Milone et al. 2017b, 2019), and a survey forseveral clusters was recently presented by Dondoglio et al.
(2022). These analyses, entirely based on photometric diagrams
combined with synthetic spectra, indicate that internalvariations in chemical abundances persist all the way to very
low-mass stars, and these variations are quantitatively similar
to those observed among the giants in the same cluster. In
practice, chemical differentiation is possible thanks toblanketing in the F160W band due to water vapor, thus tracing
differences in oxygen abundance. Thus, multiple lower main
sequences have been identiﬁed, which appear to come in thesame proportion of the multiple sequences already identiﬁed
among, e.g., the RGB stars of the same clusters. In Dondoglio
et al. (2022) the present-day mass function in the exploredstellar mass range was estimated for the various 1P and 2P
populations, and found to be consistently the same (see also
Milone et al. 2019; Scalco et al. 2024).
Now, thanks to the new facilities on board the James Webb
Space Telescope (JWST), the observation of GC stars down tovery faint magnitudes has become considerably more efﬁcientthan with previous telescopes. The ﬁrst evidence of multiple
stellar populations in very low-mass stars below the mainsequence knee has been reported for NGC 104 (47 Tucanae;
Milone et al. 2023), M 92 (Ziliotto et al. 2023), and NGC 6440
(Cadelano et al. 2023), using H2O blanketing (e.g., in the
F150W, F200W, or F322W2 bands). Most recently, the CMDof 47 Tucanae from very deep JWST/NIRCam data hasreached, for the ﬁrst time, the H-burning limit and the moreluminous brown dwarfs (Marino et al. 2024).
In this work, we take advantage of the capabilities of the
JWST multiobject spectrograph NIRSpec to characterize themultiple stellar populations among the M dwarfs of this cluster.
Until now, the spectroscopic observation of such faint stars wasprohibitive, but JWST with NIRSpec allows us to make a big
step forward in this context. We in fact have reached thefaintest stars ever observed in a GC and use these data to
document the multiple stellar population phenomenon in M
dwarfs from direct spectroscopic measurements, thus providing
a consistent characterization combining photometric andspectroscopic data.
The layout of this work is as follows: Section 2 presents theobservations, Section 3 the analysis, and ﬁnally, Section 4
discusses the results.

2. The Data Set
We have acquired JWST data for 47 Tucanae from parallelobservations, including both spectra from NIRSpec andphotometric data from NIRCam (GO2560, PI: Marino).
NIRSpec data were gathered by using the G235M/F170LP
disperser-ﬁlter combination, which provides a 1.66–3.07 μm
range in wavelength and a resolution of R ∼ 1000. To
simultaneously observe many targets, we employed themultiobject spectroscopy mode using the micro-shutter
assembly (MSA) conﬁguration. In parallel, NIRCam wasobserved in the two ﬁlters F115W and F322W2, those thatmost efﬁciently ensure the separation of multiple sequencesamong M dwarfs (Milone et al. 2023; Marino et al. 2024),
again using H2O blanketing but this time in the F322W2
passband.
Observations were gathered in 2022 July in the ﬁrst JWST
cycle, but they experienced technical issues. Speciﬁcally, windtilt events affected the quality of the data (see Milone et al.
2023 for more details), and nodding issues resulted in missing
spectra in one of the two nod positions. In the end, while
photometric images were analyzed anyway in Milone et al.
(2023), NIRSpec data were not suited for a straightforward
analysis, with only a few stars having spectra of sufﬁcientquality.
New observations were then completed in 2023 September.
We employed the same strategy as the ﬁrst observations andgathered spectroscopic and photometric data in parallel. This
time we did not experience any failure and obtained newimages and spectra. While for the photometric analysis and thepresentation of the ﬁrst spectra we refer to Marino et al. (2024),
here we analyze the full spectroscopic data set.
NIRSpec spectra for a sample of 29 M dwarfs were obtainedin the magnitude interval 19.5  mF160W  20.3, corresponding
to stellar masses between 0.35 and 0.5 Me. Our exposures wereorganized into 16 groups, and the NRSIRS2 was employed as areadout pattern, with a total on-target time of 47,280 s for eachof the observed stars.
The exposures were subdivided into two shutter nodding
positions, and a dither pattern of 100 shutters has been applied,which corresponds to 20″ in the dispersion direction. Each
exposure was integrated for 1182 s. Of our 28 M dwarf targets,
17 are observed in both dither positions, resulting in a total of
40 exposures, while the remaining 12 are only in one dither
position, providing half of the total exposure. The 1D extracted
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Figure 1. NIRSpec spectra for the two stars #32620 and #25862 marked with large starred symbols in the ChM from Marino et al. (2024; right panel). Based on thelocation on the ChM, the two stars have different values both in ΔF606W,F814W and ΔC F606W,F814W,F322W2, with #32620 being a 1P star, and #25862 a 2P one. The
similar values of the atmospheric parameters for the plotted stars have been reported in the inset of the left panel. The shaded regions indicate the spectral range
spanned by the H2O bands.

spectra processed with the JWST Science Calibration Pipeline(Bushouse et al. 2023) have been used for the analysis.
The location of the NIRSpec and NIRCam parallel ﬁelds areshown in Marino et al. (2024; see their Figure 1). Speciﬁcally,
the NIRSpec ﬁeld has pointed to the same ﬁeld previously
covered with HST, for which the ﬁlters F110W and F160W
from GO11453 are available. The spectroscopic targets havebeen selected to sample the entire range in color as observed
from the mF160W versus mF110W − mF160W CMD that we
obtained from HST archive data. This CMD-based selection
has ensured a proper sampling of all the stellar populations
hosted in the cluster.
The signal-to-noise ratio of our spectra ranges from ∼60 to∼35 per pixel, mostly depending on whether the target has
been observed in both or in only one of the nod positions,
respectively. An example of our spectra is shown in Figure 1,
for the two stars #32620 and #25862, with very similar
atmospheric parameters, logg and Teff. These spectra will befurther discussed below, but we can already note that #32620
has much stronger H2O molecular bands than the other. Notethat this spectrum has a gap, due to the physical edges of thetwo detectors in the MSA, that depends on the shutter position
and the disperser. Some of the gathered spectra were notanalyzed because they are affected by wavelength cutoffs incrucial spectral regions for our analysis. In the spectrum of#32620, the gap is in a region not affected by the H2O
molecules, which does not prevent the analysis of this star.

by comparing the observed spectra to synthetic spectra in therange from 1.5 to 2.2 μm, where the H2O absorption is deepest.
3.1. Synthetic Spectra
The theoretical spectra have been computed by using ATLAS
and SYNTHE programs8 and α-enhanced model atmosphereswith Teff ranging from 3500 to 4000 K, and log g from 4.5 to
5.0, while metallicity has been set to [Fe/H] = −0.75 dex (e.g.,
Carretta et al. 2009a; Marino et al. 2023). The abundances of
C, N, and O have been varied to mimic the chemical diversity
between stars belonging to different stellar populations in
GCs.9 Spectral synthesis includes all the lines and molecules
listed in the Kurucz website, including the H2O molecular
bands (Partridge & Schwenke 1997) that dominate the nearinfrared (NIR) spectra of M dwarfs.
Figure 2 displays examples of the synthetic spectra withchanging one-at-a-time oxygen, effective temperatures (Teff),
surface gravities (log g), and carbon. An inspection of these
theoretical spectra immediately indicates that oxygen abundances heavily impact on the shape of the spectra, with
O-enhanced stars highly absorbed all over the observed range
in the wavelength (top left panel). Besides O, the next element
affecting the spectra is carbon (bottom right panel), which
subtracts oxygen to H2O by locking it into CO. However,increasing [C/Fe] by 1 dex, from −1.00 to +0.00, a range
often observed among different stellar populations in GCs, only
marginally diminishes the overall water absorption. The impactof the other elements that are observed to vary among higher
mass stars in GCs, e.g., N and Mg, is negligible in the NIR
wavelength range explored here.
Together with O abundances, the effective temperature is theparameter that more heavily affects the spectra. Decreasing Teffby 100 K produces a similar effect as increasing O by 0.10 dex.
Instead, the effect of surface gravity is negligible.
This discussion highlights the sensitivity to O of our
NIRSpec spectra of M dwarfs, which is one of the mainchemical players in the multiple stellar population

3. Spectral Analysis
A visual comparison of the two spectra shown in Figure 1(see also Figure 8 of Marino et al. 2024 for another pair ofstars) clearly reveals different absorption for stars belonging todistinct stellar populations. The position of these two stars on
the cluster chromosome map (ChM; see Milone et al. 2017a)
shown in the right panel of Figure 1, suggests that #32620 is a
1P star, whereas #25862 is one of the most extremely polluted
2P stars, which we expect to be associated with the lowest
oxygen abundance.
In the following, we describe our employed technique toestimate the difference in the chemical abundances of theobserved M dwarfs. The spectral analysis has been conducted8

http://kurucz.harvard.edu
Other elements, such as Na and Al, are not expected to produce changes in
the overall NIRSpec spectra.
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Marino et al.

Figure 2. Synthetic spectra computed at different O abundances, keeping all the other chemical abundances and stellar parameters ﬁxed, are shown in the top left
panel. As indicated in the inset, the simulations have been computed for [O/Fe] ranging from −0.3 to +0.5 dex, with a step of 0.10 dex. The other panels illustrate theimpact of changing temperatures (top right) up to 300 K (in steps of 100 K), log g by 1 dex (bottom left), and C abundances (bottom right) by 1 dex.
Table 1
Coordinates, Adopted Stellar Temperatures and Surface Gravities, Mass, Location on the Chromosome Map, and Inferred [O/Fe] Values for the NIRSpec Targets
ID
3001
3819
8749
11213
12058
13694
14443
15654
17065
17470
19400
20051
20391
20477
22777
23062
25260
25862
32620
34657
36425
37942
40504
40579
41668
41764
42484
48348

R.A.
(J2000)Decl.
(J2000)

Teff
(K)

log g
(cgs)

Mass
(Me)

ΔF606W,F814W(mag)

ΔC F606W,F814W,F322W2(mag)[O/Fe](dex)

5.63973525
5.64094229
5.67429569
5.65667045
5.58636195
5.70320857
5.74027949
5.55751693
5.57984182
5.59731765
5.57128890
5.72710583
5.55149616
5.56336608
5.68245973
5.70849703
5.59224113
5.75511193
5.62479859
5.64627470
5.64309275
5.66283930
5.61419155
5.65405635
5.67021510
5.64695293
5.63151002
5.65298871−72.09532277−72.09409725−72.08765474−72.08465712−72.08445120−72.08262859−72.08185326−72.07992265−72.07817684−72.07867065−72.07541568−72.07569270−72.07522199−72.07397661−72.07146076−72.07175055−72.06959101−72.06753918−72.06054481−72.05729149−72.05518242−72.05331521−72.04907397−72.04884485−72.04763272−72.04810886−72.04646898−72.03644822

3602
3659
3579
3759
3904
3745
3567
3569
3643
3785
3736
3788
3667
3792
3668
3758
3618
3554
3557
3755
3902
3568
3663
3713
3575
3637
3821
3573

4.89
4.86
4.90
4.81
4.75
4.81
4.91
4.91
4.87
4.79
4.82
4.79
4.85
4.79
4.85
4.81
4.88
4.91
4.91
4.81
4.75
4.91
4.85
4.83
4.90
4.87
4.78
4.90

0.38
0.41
0.36
0.45
0.49
0.44
0.36
0.36
0.40
0.46
0.44
0.46
0.41
0.46
0.41
0.45
0.38
0.35
0.35
0.45
0.49
0.36
0.41
0.43
0.36
0.39
0.47
0.36

−0.0637−0.1669−0.1084−0.0391−0.0952−0.1133−0.0899−0.1843−0.1270−0.1172−0.1455−0.0454−0.1917−0.1991−0.0327−0.1803−0.0818−0.1934−0.0423−0.0749−0.1091−0.0637−0.1150−0.2234−0.0998−0.0282−0.0711−0.1199

0.03680.18680.10050.01230.10700.11520.12140.15960.12680.10840.10420.0576
0.19840.23300.02850.2276
0.04650.21590.0311
L
L
L
0.1152
L
L
L
L
L

0.29
0.11
0.29
0.26
0.30
0.15
0.25
−0.02
0.09
0.15
0.05
0.60
−0.13
−0.03
0.30
0.05
0.29
−0.25
0.42
0.30
0.32
0.06
0.15
−0.05
0.05
0.25
0.36
0.16

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Figure 3. Examples of two spectral syntheses. The displayed observed spectra (gray crosses) are relative to stars #25862 and #34657, which have different locations
on the chromosome map. Overall, the analyzed spectral regions of star #34657, photometrically classiﬁed as a 1P star, are deeper than those of #25862, which is
associated with the most polluted 2P population. This is in spite of the lower temperature of the latter, which acts in the direction of deeper H2O features (see Figure 2).
The best-ﬁt synthetic spectrum is shown in red. Both the observed and the simulated spectra have been normalized at the same pseudo-continuum, and the differenceof the simulated−observed (Δ(Flux)) is shown in the bottom panels.

phenomenon. Once the Teff is ﬁxed, as discussed in the next
section, we can proceed to estimate the O abundances from thespectra of these M dwarfs.

1.9 μm, which span the spectral range from 1.75 to 2.05 μm(see Figure 1). For the second band visible in our data, e.g., theH2O band around 2.7 μm, a pseudo-continuum is much harderto deﬁne, and we prefer not to use this part of the spectrum.
Figure 3 shows some examples of the comparison between
predicted and observed spectra normalized at the same pseudocontinuum. Even if the synthetic spectra do not reproduce all
the observed features, probably due to the used line lists
missing some features, still the overall spectral pattern issatisfactorily reproduced. Speciﬁcally, the right panel illustrates
a match for a 1P star, with the best-ﬁt synthetic spectrum
having [O/Fe] = +0.32 dex, while for the spectrum on theright, which belongs to a 2P star, the abundance that best
matches the observations is [O/Fe] = −0.25 dex. The spectrumof the O-enhanced star associated with the 1P displays much
deeper H2O band features (note that this pattern is clear even if
this spectrum belongs to a star with a temperature higher thanthe one of the displayed 2P star, which reduces the difference
due to oxygen abundances, as discussed in the previous
section). The oxygen abundances inferred for the 28 analyzed
M dwarfs are listed in Table 1.
As clearly shown by the synthetic spectra discussed in
Section 3.1, our estimates are mostly internally affected byuncertainties in temperatures that have been ﬁxed byisochrones. As discussed, the photometric errors suggest thatthe internal uncertainties in temperatures are as small as 50 K,
which corresponds to an error in [O/Fe] of ∼0.05 dex. We notehowever that our temperatures could be affected by systematics
that can be much larger than the internal errors. The overall
quality of the spectra, including the limited S/N and the ﬁtting
procedure, introduce an additional random error of ∼0.15 dex.
Although, in general, systematic errors are expected to affectabundances uniformly, not changing the internal range in thechemical content, we discuss here possible systematics that
could instead depend on the actual abundances of each star.
Indeed, changes in the light element abundances, especially inhelium, do impact on the adopted boundary conditions used in

3.2. Observed Spectra
The observed spectra have been compared to synthetic ones
with the Teff/log g values obtained from the position of eachstar on the CMD. Speciﬁcally, the mF814W versus mF606W–
mF814W CMD has been ﬁtted with α-enhanced isochrones at a
metallicity [Fe/H] = −0.7 dex from the Dartmouth database(Dotter et al. 2008). The distance modulus has been ﬁxed to13.23 mag, and reddening and age to E(B − V ) = 0.02 mag and12.5 Gyr, respectively. Then, we assume as Teff/log g for eachstar the values of the isochrone at the same mag level. A list of
the adopted parameters, together with the estimated stellar
mass, is presented in Table 1. While with this approach ourestimates can be affected by systematics, we are interested here
in the relative star-to-star abundance difference to infer theoverall O range in these low-mass stars.
We notice here that the lower mass in our target sample is0.35 Me, with a total of 12 stars with mass smaller than
0.40 Me, the approximate mass below which stars are fully
convective. Hence, these stars are potentially the mostimportant benchmarks for constraining the formation scenario
of multiple stellar populations.
To compare with synthetic models, we need to normalize theobserved spectra. Although the true-continuum level cannot beseen in the observed spectrum, we can exploit a pseudocontinuum. We take advantage of the two spectral regions
around 1.7 and 2.3 μm, not heavily absorbed by H2O
molecules. These two regions deﬁne the edges of the spectral
range used for the synthesis. The same pseudo-continuum used
for the observed spectra has been assumed for the synthetic
spectra. Then, we perform a χ2 minimization to ﬁnd the best-ﬁt
model. In this way, the region exploited to estimate O
abundances includes the H2O set of bands centered at
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Table 2Average [O/Fe] Abundances, Associated Error, and rms of the Inferred for the
Four Stellar Populations Selected on the M Dwarf Chromosome Map

1P
2Pa
2Pb
2Pc

[O/Fe]

±

rms

#

+0.30
+0.18
+0.04
−0.08

0.04
0.03
0.09
0.06

0.13
0.09
0.09
0.13

10
11
2
5

Note. The number of stars (#) for each population is also indicatedand cyan, respectively). We can then inspect the O abundancesof the selected groups.
From the O abundance distributions as a function of ΔF606W,
F814W and ΔC F606W,F814W,F322W2 plotted in Figure 4, we notice aclear trend along the ChM sequence. Speciﬁcally, there is acorrelation between [O/Fe] and ΔF606W,F814W, and an anticorrelation with ΔC F606W,F814W,F322W2. The average abundancesfor each population, with relative errors determined as therms N - 1 (with N the number of stars in each group), arelisted in Table 2. From these values, we have that the differencein the average oxygen abundance between the 1P and the
population 2Pa is Δ([O/Fe]1P–[O/Fe]2Pa) = 0.12 ± 0.05 dex,
which means at a level of ∼2.5σ. Moving upwards in the ChM,
the mean difference with the 2Pb population increases toΔ([O/Fe]1P − [O/Fe]2Pb) = + 0.26 ± 0.10 dex, but, given that
only two stars are analyzed in the 2Pb, the signiﬁcance remains
marginally less than 3σ. The oxygen abundance is observed tomore signiﬁcantly decrease in the 2Pc populations (with ﬁve[O/Fe] estimates available), with Δ([O/Fe]1P − [O/Fe]2Pe) =0.38 ± 0.07 dex, a >5σ difference. Note that also the oxygen
difference between 2Pa and 2Pc is at a level of more than 3σ,e.g., Δ([O/Fe]2Pa − [O/Fe]2Pe) = 0.26 ± 0.07 dex. We note thatthe rms of each group is between ∼0.10 and ∼0.15 dex (seeTable 2), which is smaller than the internal error that is estimatedto be associated with the individual measurements (inSection 3.2), which might have been overestimated.
Compared to the literature values available for more massive
stars, our [O/Fe] values are systematically underestimated (up to
0.15 dex), a pattern that can be expected given the diversity inthe stellar parameters and the different used spectral features(Collet et al. 2007; Amarsi et al. 2016). On the other hand, thegoal of this analysis is to establish the range of the oxygen
abundances spanned by very low-mass stars, instead of the realabundances. In this context, we conclude that a total range ofΔ[O/Fe] = 0.38 dex, found in this study, is fully consistent withthe range exhibited by the red giants, as from high-resolution
spectra (e.g., Carretta et al. 2009b; Dobrovolskas et al. 2014).
The presence of the same abundance variations in stars withdifferent masses, from ∼0.8 down to ∼0.4 Me, suggests that, in
old Milky Way GCs, the multiple stellar populations phenomenon does not depend on the stellar mass. This fact representsa challenge for any accretion scenario (e.g., Bastian et al. 2013;
Gieles et al. 2018) where the chemical pollution degree is
expected to depend on the stellar mass. We notice that even if
only a subsample of our target stars might reside below the mass
limit of a fully convective regime, our results are a direct
spectroscopic conﬁrmation of that previously photometricallyobserved for stars fainter than the main-sequence knee, from
∼0.5 Me down to ∼0.1 Me. Similar abundance patterns among
today's GC main sequence, red giants, and low-mass stars down
to the H-burning limit, support the scenario that the multiple
stellar populations formed in different bursts of star formation.

Figure 4. ChM diagram of M dwarfs in 47 Tucanae from JWST-HST
photometry (gray dots). Spectroscopic targets observed with NIRSpec at JWST
are represented with ﬁlled colored circles. Targets with no available mF322W2
from NIRCam at JWST are plotted with ΔC F606W,F814W,F322W2 = 0.00 as
empty circles. Different colors have been used for stars associated withdifferent stellar populations on the ChM, speciﬁcally: ﬁrst population (1P) stars
are colored in red, second population ones have been colored in orange (2Pa),
cyan (2Pb), and blue (2Pe). The [O/Fe] abundance vs. the ΔF606W,F814W andΔC F606W,F814W,F322W2 values are shown in the upper and right panels,
respectively. For each population, we plot the average values and the associatederror.

the stellar model computations, and this impacts on thepredicted effective temperature scale (Pietrinferni et al. 2021;
VandenBerg 2023). The average internal variation in He
between 2P and 1P stars in 47 Tucanae is Δ(Y) = 0.011 ±0.005, with a maximum variation of Δ(Y) = 0.049 ± 0.005
between the most extreme (in He enhancement/O depletion)
2P and the 1P stars (Milone et al. 2018). We have veriﬁed, byusing He-enhanced isochrones, that an increase in temperaturearound 10 K is associated by assuming the average enhancement in He, while if we consider the most extremely Heenhanced population, the temperature rises from ∼30 for thecoolest stars up to ∼50 K for the warmest ones. As previously
discussed, rising temperatures systematically decrease theinferred O chemical content. However, the small variations atplay only marginally change the O abundances, by at most
∼0.05 dex.
4. Results and Discussion
The comparison of the spectra illustrated in previous sections
indicates the existence of a sizable spread in oxygen
abundances among the M dwarfs of 47 Tucanae. Figure 4
shows the location of all the analyzed targets on the
Chromosome Map (ChM) of the cluster, indicating that thewhole stellar population's pattern of 47 Tucanae has beensampled by our NIRSPec spectra. We divide the ChM into fourstellar populations, from the 1P (represented in red) up to theextreme population 2Pc (represented in blue), through twointermediate populations 2Pa and 2Pb (represented in orange
6

The Astrophysical Journal Letters, 969:L8 (7pp), 2024 July 1

Marino et al.

M. Tailo https://orcid.org/0000-0002-1128-098X
T. Ziliotto https://orcid.org/0000-0001-8538-2068

We conclude by emphasizing that, thanks to the multiobject
capabilities of NIRSpec combined with the high efﬁciency of
the JWST in the infrared region of the spectrum, we havemeasured the ﬁrst spectroscopic chemical abundances for M
dwarfs in a GC. These observations provide the spectroscopic
data of the faintest stars ever observed in these old stellar
systems. Our results conﬁrm the presence of a genuine oxygen
internal spread among M dwarfs of 47 Tucanae. For the ﬁrst
time, the existence of a spread in light elements chemical
abundances of M dwarfs has been constrained directly fromspectra, conﬁrming previous results based on photometric
diagrams (Milone et al. 2012, 2014; Dondoglio et al. 2022;
Milone et al. 2023; Marino et al. 2024).

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Acknowledgments
We thank the anonymous referee for suggestions that
improved the manuscript. This work has been funded by theEuropean Union – NextGenerationEU RRF M4C2 1.1 (PRIN2022 2022MMEB9W: “Understanding the formation of
globular clusters with their multiple stellar generations”, CUP
C53D23001200006) and from INAF Research GTO-Grant
Normal RSN2-1.05.12.05.10 (PI A. F. Marino) of the “Bando
INAF per il Finanziamento della Ricerca Fondamentale 2022”.
T.Z. acknowledges support from the European Union’s Horizon
2020 research and innovation program under the Marie
Skodowska-Curie grant Agreement No. 101034319 and fromthe European Union NextGenerationEU. S.J. acknowledges
support from the NRF of Korea (2022R1A2C3002992 and2022R1A6A1A03053472).
ORCID iDs
A. F. Marino https://orcid.org/0000-0002-1276-5487
A. P. Milone https://orcid.org/0000-0001-7506-930X
A. Renzini https://orcid.org/0000-0002-7093-7355
E. Dondoglio https://orcid.org/0000-0001-8415-8531
M. G. Carlos https://orcid.org/0000-0003-1757-6666
G. Cordoni https://orcid.org/0000-0002-7690-7683
A. Dotter https://orcid.org/0000-0002-4442-5700
S. Jang https://orcid.org/0000-0002-1562-7557
E. P. Lagioia https://orcid.org/0000-0003-1713-0082
M. V. Legnardi https://orcid.org/0000-0003-3153-1499
F. Muratore https://orcid.org/0009-0003-0121-7500
A. Mohandasan https://orcid.org/0000-0001-5182-0330

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