Ve sion Ap il 24, 2024
P ep in ypese using L
A
T
EX s yle openjou nal . 09/06/15
VARIABILITY IN SSTC2D J163134.1-240100, A BROWN DWARF WITH QUASI-SPHERICAL MASS LOSS
Aleks Scholz
SUPA, School o Physics & As onomy, Uni e si y o S And ews, No h Haugh, S And ews, KY16 9SS, Uni ed Kingdom, email:
[email p o ec ed]
Ko aljka Muzic
Ins i u o de As o ísica e Ciências do Espaço, Faculdade de Ciências, Uni e sidade de Lisboa, Ed. C8, Campo G ande, 1749-016 Lisbon,
Po ugal
Vic o Almend os-Abad
Is i u o Nazionale di As o isica (INAF) – Osse a o io As onomico di Pale mo, Piazza del Pa lamen o 1, 90134 Pale mo, I aly
An onella Na a
School o Cosmic Physics, Dublin Ins i u e o Ad anced S udies, 31 Fi zwilliam Place, Dublin 2, I eland
Da y Ruiz-Rod iguez
Na ional Radio As onomy Obse a o y, 520 Edgemon Rd., Cha lo es ille, VA 22903, USA
Lucas A. Cieza
Nucleo de As onomia, Facul ad de Ingenie ia, Uni e sidad Diego Po ales, A . Eje ci o 441, San iago, Chile
C is ina Rod iguez-Lopez
Ins i u o de As o ísica de Andalucía (IAA-CSIC), Glo ie a de la As onomía s/n, 18008 G anada, Spain
Ve sion Ap il 24, 2024
Abs ac
We epo on a sea ch o a iabili y in he young b own dwa SST1624 (∼M7 spec al ype,
M∼0.05 M⊙), p e iously ound o ea u e an expanding gaseous shell and o unde go quasi-sphe ical
mass loss. We ind no a iabili y on imescales o 1-6 hou s. Speci ically, on hese imescales, we ule
ou he p esence o a pe iod wi h ampli ude >1%. A pho ome ic pe iod in ha ange would ha e
been e idence o ei he pulsa ion powe ed by Deu e ium bu ning o o a ion nea b eakup. Howe e ,
we see a 3% dec ease in he K-band magni ude be ween wo consecu i e obse ing nigh s (a 10 σ
esul ). The e is also clea e idence o a ia ions in he WISE ligh cu es a 3.6 and 4.5 µm on
imescales o days, wi h a en a i e pe iod o abou 6 d (wi h a plausible ange be ween 3 and 7 d).
The bes explana ion o he a ia ions o e days is o a ional modula ion due o spo s. These esul s
dis a ou cen i ugal winds d i en by as o a ion as mechanism o he mass loss, which, in u n,
makes he al e na i e scena io – a he mal pulse due o Deu e ium bu ning – mo e plausible.
1. INTRODUCTION
Pho ome ic moni o ing has p o en o be a use ul
obse a ional ool in he explo a ion o b own dwa s.
When hey a e young, subs ella objec s o en show a i-
abili y due o magne ic ac i i y, acc e ion, and disks,
analogous o he mo e massi e T Tau i s a s (Scholz e
al. 2009; Moo e e al. 2019). Fo e ol ed b own dwa s,
pho ome ic a iabili y can signi y he p esence and e o-
lu ion o clouds in he a mosphe e (e.g. Me che e al.
2015). Va iabili y s udies ha e been able o measu e sub-
s ella o a ion pe iods as a unc ion o age (Bou ie e
al. 2014), as well as cons ain a mosphe ic p ope ies and
ea ly e olu iona y p ocesses in b own dwa s.
Thanks o nume ous moni o ing s udies in he pas
wen y yea s, we know ha young b own dwa s show o-
a ion pe iods anging om a ew hou s o se e al days,
whe eas old e ol ed b own dwa s gene ally ha e pe i-
ods sho e han one day (Moo e e al. 2019; Vos e al.
2022). A young ages, he sho es measu ed pe iods
a e a ound he physical limi – he pe iod co espond-
a Xi :2307.14087 2 [as o-ph.SR] 23 Ap 2024
2 Scholz e al.
ing o b eakup eloci y, whe e cen i ugal and g a i a-
ional o ces a e in balance a he equa o . Ro a ion
nea o a ound b eakup in young b own dwa s has been
epo ed by a ious au ho s, including Zapa e o Oso io
e al. (2003); Caballe o e al. (2004); Scholz & Eislö el
(2005); Rod íguez-Ledesma e al. (2009). Such as o-
a ion will ha e signi ican e ec s on in e nal s uc u e
and ea ly e olu ion, which needs o be accoun ed o in
models (Yoshida 2023).
The b eakup eloci y is calcula ed using =
p2GM/3R, ollowing Po e (1996). The ac o 2/3ac-
coun s o he ac ha an objec o a ing a b eakup
will be obla e. Fo con enience, his co esponds o
P= 0.14205×R1.5/M0.5. Fo subs ella masses and ages
1-5 My , his yields b eakup pe iods >5h, as shown in
Figu e 1 o wo adii, ypical o young b own dwa s. As
he objec s age and con ac , he b eakup pe iod d ops
wi h ∼R1.5, bu he o a ion pe iod (assuming angula
momen um conse a ion) changes wi h ∼R, i.e. wi h
inc easing age he b eakup pe iod will d op u he be-
low he o a ion pe iod. As a esul , ield b own dwa s
which some imes ea u e e y sho o a ion pe iods o
∼1h (Tannock e al. 2021) a e s ill signi ican ly abo e
b eakup.
Ano he physical p ocess ha can be p obed by pho-
ome ic moni o ing is pulsa ions. B own dwa s ex-
hibi Deu e ium bu ning du ing he i s ew My s o
hei exis ence, p edic ed o lead o a luminosi y bu s
(Salpe e 1992) and o adial o non- adial pulsa ions.
Palla & Ba a e (2005) show in a s abili y analysis o
Deu e ium bu ning b own dwa s he p esence o un-
damen al modes wi h pe iods be ween 1 and 5 h. Fo
0.1 M⊙s a s, Rod íguez-López e al. (2012) ind pe i-
ods o modes exci ed by Deu e ium bu ning o 4.2-5.2 h.
A dedica ed obse a ional sea ch o pulsa ion pe iods
down o mmag p ecision, by Cody & Hillenb and (2014),
howe e , emained unsuccess ul. In p inciple, some o he
sho es pho ome ic pe iods measu ed so a o b own
dwa s (see abo e) could be caused by pulsa ions. Mos
au ho s p e e , howe e , o in e p e he pe iodic a i-
abili y in young b own dwa s as a esul o o a ion
combined wi h magne ic su ace spo s. I pulsa ions we e
con i med, i would open a new ou e o de e mine and
asce ain undamen al p ope ies in b own dwa s du ing
he phase o con ac ion.
In Figu e 1 we include he es ima ed pulsa ion pe iods
om Palla & Ba a e (2005) o ages o 1-2 My and sub-
s ella masses. As can be app ecia ed om his igu e,
he gap be ween pulsa ion pe iods and b eakup pe iods
widens wi h dec easing mass and inc easing adius. A
pe iod below 4 h o such a young e y low mass objec
would be e y di icul o in e p e as o a ion pe iod,
o e ing a window o sea ch o pulsa ions.
2. THE TARGET
SSTc2d J163134.1-240100 (he ea e SST1624) is a
young b own dwa in he Ophiuchus s a o ming com-
plex, iden i ied o iginally in he Spi ze ’Co es o Disks’
0.02 0.03 0.04 0.05 0.06 0.07 0.08
Mass(
M
)
2.0
4.0
8.0
16.0
32.0
Pe iod (h)
P
b eak
R
= 0.5
R
P
b eak
R
= 1.0
R
mass o a ge
Pulsa ion 1-3My
Fig. 1.— Es ima ed o a ion pe iod a b eakup eloci y (lines)
and pulsa ion pe iod (do s) o young b own dwa s, as a unc ion
o mass. The b eakup pe iods a e shown o wo di e en adii,
consis en wi h he expec ed alues o in la ed, young subs ella
objec s. Pulsa ion pe iods a e he alues gi en by Palla & Ba a e
(2005) o ages be ween 1 and 2 My ( hei Table 1). Fo masses o
0.06 and 0.08 M⊙ hese au ho s lis se e al pulsa ion pe iods, o
ages anging om below 1 o 3 My . The es ima ed mass o ou
a ge is ma ked by a dash-do ed line.
Fig. 2.— 2MASS JHK colou image o he ield a ound ou
a ge . Ma ked in g een is he 4.9′×4.9′SOFI ield. The a ge
is ma ked in o ange; he ou s a s used as compa ison s a s in
yellow. No h is up and eas is le .
su ey (E ans e al. 2009) as a ain , ed young s ella
objec . A 2MASS image o he a ea a ound he a -
ge is shown in Figu e 2. In ecen ALMA obse a ions
as pa o he ODISEA p og am (Cieza e al. 2019), i
was ound o ha e a sphe ical, obla e en elope wi h a
diame e o 200-300 AU, which shows he kinema ics o
an expanding shell (Ruíz-Rod íguez e al. 2022). The
shell has been de ec ed in 12CO, bu no in he mm con-
inuum. A e co ec ing o ex inc ion, he objec has
Va iabili y in SSTc2d J163134.1-240100 3
an in a ed spec al ene gy dis ibu ion consis en wi h
a diskless pho osphe e. This is e y unusual – so a
e y ew young b own dwa s ha e been de ec ed in CO
wi h ALMA, and he ones ha a e, usually ha e an ob-
ious disk (Ricci e al. 2014). On he o he hand, nu-
me ous young b own dwa s ha e been de ec ed in he
submm/mm con inuum (Tes i e al. 2016; Sanchis e al.
2020). We a e hus con on ed wi h an en i ely new phe-
nomenon ha equi es new ideas, con inued explo a ion,
and ollow-up obse a ions.
Ruíz-Rod íguez e al. (2022) de e mine he na u e o
he sou ce based on KMOS spec a, and a e ca e ul o
ule ou al e na i e explana ions, such as an AGB s a
in he backg ound o a collapsing molecula co e. P ope
mo ions con i m ha he objec is indeed a membe
o he young popula ion in Ophiuchus, wi h an age o
∼2My . Mos likely, i is a 0.05 M⊙b own dwa ,
wi h mid/la e M spec al ype, o e luminous, and seen
h ough signi ican ex inc ion o AV∼42 mag. The bes
explana ion o he gaseous expanding shell is s ong,
uncollima ed mass loss. The age o he shell has been
es ima ed kinema ically o be ∼104yea s, a small ac-
ion o he b own dwa ’s age, and i s gas mass (unde
plausible assump ions) is 1.5M⊕. The 4σuppe limi in
he con inuum obse a ions co esponds o a dus mass
o <1M⊕.
A his poin he o igin o he expanding shell is un-
known. Ruíz-Rod íguez e al. (2022) sugges ha he
mass loss may be he esul o a he mal pulse p oduced
by he onse o Deu e ium bu ning, analogous o he
he mal pulses ha d i e mass loss in AGB s a s Ke -
schbaum e al. (2017). Ano he op ion o a b own dwa
o ejec a gaseous shell, o iginally sugges ed by Scholz &
Eislö el (2005), is h ough non-s eady cen i ugal winds
caused by ex emely as o a ion close o b eakup speed
– analogous o as o a ing OB s a s (Po e 1996). This
would lead ini ially o a ing (a ’dec e ion disk’), expand-
ing and dispe sing o a quasi-sphe ical s uc u e, consis-
en wi h he obla e shape o he shell o SST1624.
As poin ed ou in Sec ion 1, bo h pulsa ion and as
o a ion can cause a pe iodic lux change de ec able by
pho ome ic moni o ing, wi h he plausible pe iods o
pulsa ions being sho e han hose o as o a ion
nea b eakup (see Figu e 1). Tes ing o a iabili y in
SST1624 is he e o e a p omising way o explo e he o i-
gin o i s shell. This is he pu pose o his pape .
Fo he eco d, Ruíz-Rod íguez e al. (2022) also dis-
cuss ano he op ion o explain he shell, he engul men
and consump ion o a plane a y companion. They deem
his scena io as mo e exo ic and less p obable han he
he mal pulse. As his idea would no p oduce a a i-
abili y signa u e, we will no discuss i u he he e.
3. OBSERVATIONS AND DATA REDUCTION
3.1. K-band moni o ing
We ha e moni o ed SST1624 o e wo nigh s, May 26 h
and May 27 h 2023, wi h ESO/NTT and he in a ed
came a SOFI, unde p oposal ID 111.24GN. O e all, his
obse ing un comp ised h ee nigh s, bu he hi d one
was no usable due o clouds. In each o he wo nigh s,
we s ayed on he a ge o 5-6 h, using he KS il e , DIT
o 20 s, NDIT o 5 o 10, and employing andom di he s.
In bo h nigh s, he condi ions we e s able, wi h clea
skies, low humidi y (8-26%), mode a e wind speed (5-
11 ms−1, well below he poin ing limi o 14 ms−1 o his
elescope), and empe a u e be ween 11 and 16 deg ees
Celsius, all acco ding o he ESO Me eo moni o . The
seeing in ou da a is be ween 1 and 2 a csec, wi hou any
apid changes.
The da a educ ion includes c oss- alk co ec ion, la
ielding, sky sub ac ion, and bad-pixel co ec ion. In-
di idual exposu es we e combined, whe e necessa y, o
achie e a uni o m o al exposu e ime pe ame o 200 s.
On each image, we pe o med ape u e pho ome y using
pho u ils (B adley e al. 2023) using a cons an ape u e
o 10 pixels, which ypically co esponds o 2-3 FWHMs
o he seeing-limi ed PSF. This includes backg ound sub-
ac ion, whe e he backg ound was measu ed in an an-
nulus a ound he sou ce. We op ed no o adap he
ape u e o he changing FWHM, o main ain consis-
ency ac oss he ligh cu e. The ape u e adius was
chosen o op imise he signal- o-noise in he ligh cu e,
a e expe imen ing wi h a wide ange o adii.
The aw ligh cu e o SST1624 and all o he s a s in
he ield show subs an ial a ia ions due o he change in
ai mass o e he cou se o he nigh . We chose ou s a s
in he same ield ha a e 2-3 mag b igh e han ou a ge
(ma ked yellow in Figu e 2), a e aged hei ligh cu es,
and sub ac ed his a e age om he a ge ligh cu e
(in uni s o magni udes) o co ec o a mosphe ic e -
ec s. The inal ligh cu e o SST1624 o hose wo
nigh s is shown in Figu e 3. Judged by he noise in he
compa ison s a ’s ligh cu es, he ypical e o o indi-
idual pho ome y poin s o he a ge is 0.01-0.02 mag
(o 1-2%).
We made su e ha he ligh cu e does no depend sig-
ni ican ly on he choice o compa ison s a s, by checking
he esul wi h only a subse o hem. We also checked
he compa ison s a s agains each o he , and do no ind
any signs o a iabili y in hem. We no e ha he same
calib a ion p ocess also co ec s o o he en i onmen al
in luences on he pho ome y, like changes in seeing. Us-
ing he compa ison s a s o calib a ion, we de e mine
ha SST1624 has a K-band magni ude o 14.08 ±0.05 in
he 2MASS sys em, consis en wi h i s 2MASS alue o
14.06.
3.2. A chi al WISE ligh cu es
We also ob ain he WISE ligh cu e a 3.6 (W1)
and 4.5 µm (W2) o SST1624 om he ’ALLWISE
Mul iepoch Pho ome y Table’ (Cu i e al. 2021) and
he ’NEOWISE-R Single Exposu e (L1b) Sou ce Ta-
ble’ (Mainze e al. 2014). The ALLWISE able con-
ains 23 epochs o his sou ce wi hin 200 d, s a ing a
MJD55254. Wi h one excep ion, all o hese measu e-
men s a e concen a ed in wo 24 h spans, sepa a ed by
4 Scholz e al.
0.05 0.10 0.15 0.20 0.25
MJD - 60091.0
-0.10
-0.05
0.00
0.05
0.10
Rela i e K-band magni ude
1.00 1.05 1.10 1.15 1.20 1.25
MJD - 60091.0
Fig. 3.— K-band ligh cu e o SSTc2d J163134.1-240100, measu ed o e wo nigh s in May 2023. The magni udes a e measu ed ela i e
o he a e age o ou ield s a s. The blue dashed lines ma k he a e age o a gi en nigh . The ypical e o o indi idual da apoin s is
0.01-0.02 mag.
4.6 4.8 5.0 5.2 5.4 5.6
MJD - 55250.0
11.6
11.8
12.0
12.2
12.4
ALLWISE Magni ude
W1
W2+1.0
186.2 186.4 186.6 186.8 187.0 187.2 187.4
MJD - 55250.0
Fig. 4.— ALLWISE ligh cu e o SSTc2d J163134.1-240100, comp ising 22 o he 23 da apoin s in he a chi e. The W2 magni udes
a e o se by 1.0 mag o cla i y.
Va iabili y in SSTc2d J163134.1-240100 5
0 500 1000 1500 2000 2500 3000 3500
MJD - 56700.0
11.6
11.8
12.0
12.2
12.4
12.6
12.8
NEOWISE Magni ude
W1
W2+1.0
Fig. 5.— NEOWISE-R ligh cu e o SSTc2d J163134.1-240100,
comp ising 237 da apoin s. A ew ou lie s a e no shown he e.
The W2 magni udes a e o se by 1.0 mag o cla i y. The ypical
pho ome ic e o is 0.03 mag.
∼180 d, and shown in Figu e 4.
NEOWISE-R p o ides 237 good de ec ions (as o Ap il
2024), o e a du a ion o ∼3500 d, s a ing a MJD
56700.0, wi h 20 day-long spans co e ed by abou 10 da -
apoin s each. Again he measu emen s a e a ailable o
he wo bands W1 and W2. We no e ha his ligh cu e
ends a MJD 60167 which co esponds o Augus 2023,
a ew mon hs a e ou dedica ed K-band moni o ing.
The e a e, howe e , no da apoin s coinciding wi h ou
obse a ions. The ull ligh cu e is shown in Figu e 5.
Bo h he ALLWISE and NEOWISE a chi es p o ide
e o ba s o indi idual pho ome y poin s. Those e o s
a e de e mined om an assessmen o he epea abili y o
he measu emen s as a unc ion o magni ude and galac-
ic la i ude (W igh e al. 2010). The nea es sou ce is a
b igh s a 36 a csec o he sou heas . This sou ce is he
eason he WISE a chi e epo s a isk o con amina ion.
The pho ome ic quali y in he ALLWISE pho ome y is
obus wi h a signal- o-noise a io >10 (i.e. he ph_qual
lag is ’AAAA’).
4. LIGHTCURVE ANALYSIS
4.1. The Max-F es
Fo he analysis p esen ed in his pape , we de ised a
simple, obus es o check o he p esence o a pe iod in
noisy da a by compa ing he a iance in he ligh cu e
be o e and a e sub ac ing a pe iodici y, a p ocedu e
he ea e called Max-F. In sho , he ou ine es s o
wha ex en he noise in he ligh cu e could be caused
by a pe iodici y wi h speci ic p ope ies.
We de ine a ange o es pe iods o a gi en da ase .
Fo each es pe iod we calcula e he phases o he
da apoin s (be ween 0.0 and 1.0). Then we sub ac
sinecu es o he gi en es pe iod, a ying he ampli-
ude and ze opoin o he phase. The la e is changed
om 0.0 o 1.0 wi h a s epsize o 0.01. We eco d he
s anda d de ia ion o he ligh cu e o each sub ac ed
sinecu e, σP,i, and he a iance a io F=σ2/σ2
P,i. He e
σis he s anda d de ia ion o he o iginal ligh cu e. We
ind he maximum alue o F o each pe iod, and plo
ha as a unc ion o pe iod.
This p ocedu e gi es us a measu e o he maximum
ac ion o he noise ha could be explained by he p es-
ence o a sinusoidal pe iod. The ou ine cons i u es in
essence an F- es o he equali y o a iances, a s an-
da d s a is ical es . A simila es – bu only o e -
i y a p e iously adop ed pe iod – was used o example
by Scholz & Eislö el (2004) in he ligh cu e analysis o
young e y low mass sou ces. Fo no mally dis ibu ed
da ase s, he quan i y Fshould ollow an F-dis ibu ion
(hence he name).
4.2. K-band ligh cu e
The ligh cu e we ob ained om he SOFI obse a-
ions appea s un ema kable. By eye, i shows noise
a ound a la mean, in each o he wo consecu i e nigh s,
as can be app ecia ed om Figu e 3. The e is no ob ious
s uc u e, in pa icula no ob ious pe iodic a iabili y.
The s anda d de ia ion is 1.5 and 1.6%, o nigh 1 and
nigh 2 espec i ely.
To check o g adual ends, we i he da a in indi id-
ual nigh s wi h a s aigh line. In he i s nigh , he bes
i ing slope is -0.006 mag/day, which is consis en wi h
ze o. In he second nigh , he i gi es 0.026 mag/day o
0.010 mag/day when igno ing he e y i s ou lying da -
apoin . Again his would be consis en wi h ze o o e a
ime window o 6 h. The e o in he slope o bo h nigh s
amoun s o 0.03 mag/day, conside ably la ge han he
slope i sel , as expec ed o a null esul .
The e is, howe e , a signi ican o se be ween he mean
magni ude in he wo nigh s; he objec dimmed by 3%
o e he cou se o 1 d, a 10 σde ec ion gi en he e o
o he mean in he ligh cu es in indi idual nigh s. This
shi is no seen in indi idual compa ison s a s; he di -
e ence be ween he nigh ly a e ages o he ou com-
pa ison s a s is 0.55%, 0.12%, 0.60%, and 0.07%.
Gi en ha he obse ing se up was iden ical in bo h
nigh s, and he ange o ai masses was he same, oo,
his dimming is likely caused by in insic a iabili y in
ou a ge . We no e ha his o se , in combina ion wi h
he linea slopes measu ed o indi idual nigh s, could
be consis en wi h a sinusoidal a ia ion – in ha case
he objec would be nea maximum in he i s nigh ; he
pe iod would be on he o de o days.
We ca ied ou he Max-F es desc ibed in Sec ion
4.1, o a pe iod ange om 0.1 h o 6 h in s eps o 0.05 h.
The ampli udes we e a ied be ween 0.001 and 0.03 mag,
in s eps o 0.001 mag. The esul s a e shown in Figu e
6. Fo he i s nigh , we ind ha he maximum alue
o Fis 1.16, o a pe iod o 3.0 h. Fo he second nigh ,
he maximum Fis 1.12, o pe iods sho e han 1 h; o
longe pe iods he maximum is 1.04. Fo compa ison,
o N∼80, a F- alue o >1.45 would mean ha he
wo a iances be o e and a e sub ac ing he sinewa e
a e signi ican ly di e en wi h a alse ala m p obabili y <

6 Scholz e al.
0123456
Pe iod (h)
1.0
1.05
1.10
1.15
1.20
F
0123456
Pe iod (h)
Fig. 6.— Resul s o he Max-F es o ou K-band ligh cu e, o nigh 1 on he le , and nigh 2 on he igh . The igu e show he
alue o Fas a unc ion o es pe iod, as desc ibed in Sec ion 4.1 and 4.2.
5%. Taken oge he , hese esul s a e consis en wi h he
a iances being equal be o e and a e pe iod sub ac ion.
I also means ha a mos a ew pe cen o he a iance
can be explained by a po en ial pe iod.
To es whe he o no an ac ual pe iod wi h an ampli-
ude >0.01 mag (1%) can be eco e ed wi h his me hod,
we injec ed a i icial pe iods in o ou ligh cu e. We
chose a ange o pe iods om 1 o 5 h, and a cons an
ampli ude o 1%. Fo hose pa ame e s, he maximum
Fwould be ∼1.3, depending sligh ly on pe iod, wi h
a clea ly de ined maximum a he injec ed pe iod alue.
The injec ed pe iod is hen also isible by eye in he
ligh cu e. This demons a es ha pe iods wi h hese
cha ac e is ics a e easily eco e ed by ou Max-F es .
We can he e o e ule ou he p esence o pe iods o 1-5 h
wi h ampli udes la ge han 1% in he K-band ligh cu e.
4.3. WISE ligh cu es
In he ALLWISE and NEOWISE-R ligh cu es,
SST1624 shows clea a ia ions, domina ed by modula-
ions on imescales o days, wi h ypical peak- o-peak
ampli udes o 0.1-0.3 mag (see Figu es 4 and 5). These
a ia ions a e signi ican ly la ge han he ypical e o ,
which is 0.03 mag o a single epoch. The e o e i is
plausible o assume he sou ce is a iable in he WISE
ligh cu es. Taken he wo da ase s oge he , he a i-
abili y is sus ained o e mo e han 13 yea s. O e long
imescales, he b igh ness o his objec appea s o be
mos ly s able wi hin he ma gins o he in a-nigh a i-
a ions.
The NEOWISE-R da ase allows a mo e de ailed anal-
ysis. I consis s o 20 se s o poin s, each measu ed wi hin
a couple o days, and sepa a ed by abou 180 d (see Fig-
u e 5). Gi en his sampling, he NEOWISE-R ligh cu e
is no sensi i e o he sho imescales co e ed wi h ou
K-band moni o ing, bu can be use ul o pick up a i-
abili y on imescales anging om days o yea s.
We ca ied ou he same Max-F es we desc ibed in
Sec ion 4.1, o sea ch o a possible pe iodici y. Fo pe i-
ods anging om 1 o 10 d, and o he ull NEOWISE-R
ligh cu es, he maximum Fis 1.08 in W1 ( o a pe iod
o 6 d) and 1.12 in W2 ( o a pe iod o 8 d). We no iced
ha he maximum Finc eases signi ican ly i we un he
es only on he i s hal o he da ase . Fo band W1,
we ob ain peaks a pe iods o 3.6 and 6.1 and >9.0d,
wi h F > 1.25. Fo W2, he highes peak is o P= 6.0d,
wi h F∼1.25. The esul s o he i s hal o he da ase
a e shown in Figu e 8. As a eminde , his means ha
he a iance is educed o 80% (1/1.25) when sub ac -
ing his speci ic pe iodici y. Howe e , o his alue o
F he e is s ill a subs an ial p obabili y (11%) ha he
wo a iances be o e and a e sub ac ing he pe iod a e
equal (assuming no mally dis ibu ed samples).
In Figu e 7 we show he i s hal o he NEOWISE-R
da apoin s plo ed in phase o P= 6.0d. As can be
app ecia ed om his igu e, he pe iodici y looks plau-
sible by eye. I is he e o e concei able ha pa s o all
o he a iabili y seen in WISE da a is caused by a lux
modula ion wi h a pe iod o abou 6 d and an ampli ude
o ±0.05 mag. We show he Max-F esul s in Figu e 8,
wi h he pu a i e pe iod o 6.0 d ma ked.
Va iabili y in SSTc2d J163134.1-240100 7
0.0 0.2 0.4 0.6 0.8 1.0
Phase
-0.2
-0.1
0.0
0.1
0.2
W1 ela i e magni ude
0.0 0.2 0.4 0.6 0.8 1.0
Phase
-0.2
-0.1
0.0
0.1
0.2
W2 ela i e magni ude
Fig. 7.— The i s hal o he NEOWISE-R da ase , plo ed in phase o a pe iod o P= 6.0d. O e plo ed is he sinecu e wi h an
ampli ude o 0.05 mag o guide he eye. The ypical pho ome ic e o is 0.03 mag.
To quan i y ypical imescales o a iabili y om he
NEOWISE-R ligh cu e, we calcula ed he di e ence be-
ween each pai o da apoin s ∆M, and plo his o e
hei ime di e ence ∆ (see Figu e 9). This analysis
was ca ied ou o he wo bands sepa a ely, bu hey
bo h gi e e y simila esul s. As can be seen in he ig-
u e, he o e all ange o ∆Mis no a s ong unc ion o
ime di e ence ∆ . The median o ∆M(shown as o ange
squa es) is 0.04-0.05 o he sho es ∆ (hou s o days),
sligh ly la ge han he pho ome ic e o o 0.03 mag.
The median o ∆Minc eases g adually o 0.07-0.10 up
o ∆ ∼1000 d, whe e i pla eaus. The 80% pe cen ile o
∆M(shown wi h o ange iangles poin ing downwa ds)
is by and la ge cons an a 0.1 mag o W1 and close o
0.2 mag o W2. O e all, he e is no s ong e idence o
long- e m a iabili y om he NEOWISE-R da ase .
5. SUMMARY AND DISCUSSION
SST1624 is a b own dwa wi h quasi-sphe ical mass
loss, a unique objec ha may gi e us insigh s in o im-
po an aspec s o he ea ly e olu ion o subs ella ob-
jec s. Two possible mechanisms o explain he mass loss
a e a he mal pulse om he onse o Deu e ium bu ning
o cen i ugal winds due o e y as o a ion. In bo h
cases, we may expec a iabili y in sho imescales, ei-
he due o pulsa ion o o a ion.
We p esen he e in a ed ligh cu es o SST1624 om
newly ob ained high-cadence K-band ESO/NTT obse -
a ions and om he publicly a ailable ALLWISE and
NEOWISE-R a chi es. O e all, he e is clea e idence
o a iabili y in SST1624, on imescales o >6h o days,
measu ed a 2-4.5 µm in all a ailable da ase s. On he
o he hand, we ule ou a iabili y, and speci ically he
p esence o a pho ome ic pe iod on imescales <6h,
o ampli udes >1%. We do iden i y a en a i e pho-
ome ic pe iod o 6 d (wi h a plausible ange o 3-7 d)
in he i s hal o he NEOWISE-R da ase , seen a
3.6 and 4.5 µm. The e may be addi ional a iabili y on
imescales o yea s.
We no e ha ou a ge was included in he a iabil-
i y sea ch by Pa k e al. (2021) using he i s 6.5 y o
NEOWISE da a – no a iabili y was ound. This s udy,
howe e , was only ocused on long- e m a ia ions o e
imescales longe han days. Also, he NEOWISE-R
da ase is now signi ican ly longe .
The a iabili y on day-long imescales, and he en a-
i e pe iod o 6 d, is bes explained by magne ic ac i i y,
i.e. spo s on he su ace co- o a ing wi h he objec . This
implies ha he o a ion pe iod o his objec is in he
o de o a ew days. Pe iod and ampli ude a e compa-
able o wha has been ound o young b own dwa s –
pe iods ypically ange om a ew hou s o se e al days,
ampli udes a e 2% o la ge (Scholz & Eislö el 2005;
Moo e e al. 2019). The WISE ligh cu e is e y clumpy,
wi h gaps o 6 mon hs be ween clumps o da a. O e such
long imescales, he spo dis ibu ion can change, lead-
ing o phaseshi s and ampli ude changes. Fo imescales
a ound 1 d, he numbe o da apoin s is e y low. These
wo cha ac e is ics in combina ion hampe any pe iod
sea ch and can easily explain why he pe iod is no ap-
pa en in he en i e da ase . To obus ly con i m he en-
8 Scholz e al.
0246810
Pe iod (d)
1.0
1.1
1.2
1.3
F
0246810
Pe iod (d)
Fig. 8.— Resul s o he Max-F es o he NEOWISE ligh cu e (W1 on he le , W2 on he igh ; in bo h cases only he i s hal o
he da apoin s was used). The igu e show he alue o Fas a unc ion o es pe iod, as desc ibed in Sec ion 4.1 and 4.3.
a i e pe iod o 6 d as he o a ion pe iod, a dedica ed
moni o ing campaign o e a leas one week, i possible
wi hou day ime gaps, would be needed.
The only o he concei able explana ion o he ob-
se ed a iabili y is he p esence o some esidual ci cum-
sub-s ella dus close o he objec ha causes a iabili y
h ough obscu a ion (Scholz e al. 2009) – howe e , he
lack o an in a ed excess and he lack o a mm con inuum
de ec ion ende s his in e p e a ion unlikely.
Coming back o he opic o he quasi-sphe ical mass
loss, he unique ea u e o his objec : Wi h ou dedi-
ca ed K-band moni o ing, we se ou o ind pe iods on
imescales o hou s, o dis inguish be ween pulsa ions (1-
4 h) and o a ion nea b eakup (>5h). Since we did
no ind pe iods on hese imescales, he a ailable da a
does no gi e us a de ini i e answe on he cause o he
mass loss. Ha ing said ha , he ac ha he e is clea
a iabili y o e imescales o days, bes explained by o-
a ional modula ions, would imply ha he objec is in
ac no o a ing close o b eakup. Ro a ion pe iods o
se e al days a e a he high end o he pe iod dis ibu-
ion o young b own dwa s, see Figu e 13 in Moo e e
al. (2019). This ende s he scena io whe e he mass loss
is due o he onse o Deu e ium bu ning, a ou ed and
wo ked ou by Ruíz-Rod íguez e al. (2022), mo e likely.
The absence o a pulsa ion pe iod in he ligh cu es could
mean ha he objec has s opped pulsa ing since he
ejec ion o he shell, o ha he pulsa ions happen a
ampli udes below 1%, as i is likely he case o pulsa -
ing M dwa s (Rod íguez e al. 2016; Rod íguez-López
2019).
This b own dwa dese es u he ime-se ies obse -
a ions o a) e i y he o a ion pe iod and b) sea ch
o pulsa ions wi h highe p ecision. As he i s b own
dwa wi h obse ed quasi-sphe ical mass loss, his sou ce
may p esen an oppo uni y o lea n mo e abou he hi h-
e o unexplo ed aspec s in he ea ly e olu ion o b own
dwa s. Deu e ium bu ning is a common ea u e in all
young b own dwa s, and i his leads o mass loss, ei-
he sus ained, o in bu s s, i would a ec he subs ella
mass unc ion and angula momen um e olu ion. Ex-
plo ing he p e alence and du a ion o subs ella mass
loss is an impo an ask o u u e wo k.
We hank he e e ee o a p omp and ho ough
epo on his pape . We hank he ESO eam a
he NTT o he suppo p io and du ing he obse -
ing un. This publica ion makes use o da a p od-
uc s om he Wide- ield In a ed Su ey Explo e ,
which is a join p ojec o he Uni e si y o Cali-
o nia, Los Angeles, and he Je P opulsion Labo a-
o y/Cali o nia Ins i u e o Technology, unded by he
Na ional Ae onau ics and Space Adminis a ion. AS ac-
knowledges suppo om he UKRI Science and Tech-
nology Facili ies Council h ough g an ST/Y001419/1/.
KM acknowledges suppo om he Fundação pa a
a Ciência e a Tecnologia (FCT) h ough he CEEC-
indi idual con ac 2022.03809.CEECIND and esea ch
g an s UIDB/04434/2020 and UIDP/04434/2020.
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