A Scientific exploration only possible with Rubin-LSST: Search for hidden baryons through interstellar scintillation

With a 0.06 image/s movie of LMC/SMC at Rubin observatory

SCOC Workshop

Marc Moniez, 7 may 2025

Search for missing H₂ turbulent galactic gas

through scintillation detection (the OSER project)

Light received by telescope varies with

- timescale ~10 min (due to the relative velocity of the gas)

- modulation of a few % (depending on distances / turbulence parameters / source extension)

Distance scales

4 distance scales characterize the speckle pattern

• Diffusion radius R_diff
• separation (on screen) such that: σ[φ(r+R_diff)-φ(r)] = 1 radian
• Characterizes the strength of turbulence: strong turbulence -> small R_diff)

​

• Refraction radius R_ref
• size of the region from which most of the scattered signal, seen by an observer, originates ~ z₀λ/R_diff
• Larger scale structures of the diffusive gas can play a role if focusing/defocusing configurations happen
• Projected source size R_S�speckle from a pointlike source�is convoluted by the source�projected profile. -> Critically impacts�the contrast of the illumination pattern

Illumination in K_s�by a K0V star@8kpc (m_V=20.4) through a cloud@160pc (B68) with turbulence parameter R_diff =150km

4

m = σ_I/I = modulation index

Simulation of a realistic speckle from a stellar source crossing a (very) turbulent gaseous structure

Observable 1: t_ref (characteristic time)

V_T = relative velocity of the cloud w/r to the line of sight

-> V_T is also the apparent velocity of the illumination pattern entering the telescope

Observable 2: m (modulation index)

Scintillation@λ = 1 μm

of Sun@10kpc (V~20) through a cloud@160pc

with R_diff=1000km

Essentially depends on R_S and R_ref -> not on the details of the power spectrum of the fluctuations

Signature of scintillation

• Stochastic light-curve (not random)
• Autocorrelation (power spectrum)
• Characteristic time t_ref (few minutes)
• Modulation index m can be > a few %
• decreases with increasing star radius
• depends on cloud structure
• Signatures of a propagation effect
• Chromaticity (optical wavelengths)
• Long time-scale variations (few min.) ~ achromatic λ^−1/5
• Short time-scale variations (sub-min.) varies with λ^6/5
• Correlation between light-curves measured by 2 telescopes decreases with their mutual distance

Results from IR-NTT observations toward B68

-> A star scintillating through dusty gas?

1000 x 10s exposures in K_s

Night 1

Night 2

Scintillation with Vera Rubin telescope�(15s exposures)

For a given value of the turbulence parameter R_diff ->

modulation m = σ_I /<I> decreases with the sources’ magnitude since the apparent stellar radius increases (here MS star)

Typical light-curve with m ~ 1% (simulation)

Search for hidden baryons�with a 0.06 image/s movie of LMC at Rubin-LSST

• To discover scintillation effects, we need:
• Large diameter telescope(s) (>4m) because faint stars+short exposures
• Wide field camera (visible) because small optical depth
• fast readout -> to get highly sampled light-curves

---> Vera Rubin telescope is the unique option

• Extremely sensitive technique to gas clumpuscules with column density fluctuations down to 10^-7 --- i.e. ~1 mm of H₂ under normal (P, T) conditions --- per few 1000km
• A few hours' filming over 2 nights (for redundancy) at the LMC in G passband will provide decisive sensitivity to the hidden gas of the galactic halo, the last unknown baryonic component�� -> Early science possible during commissioning or micro-survey

Biblio : A&A 412, 105-120 (2003); A&A 525, A108 (2011); A&A 552, A93 (2013)

Marc Moniez, SCOC workshop, 7 may 2025: moniez@ijclab.in2p3.fr

>> Other communities should be interested in this movie (transits, flares, very short timescale variabilities...)

Complements

Atmosphere, atmosphere?

• Blurs PSF, but doesn’t affect the intensity collected by a large telescope
• ~ 5cm size speckle due to turbulent layers at ~ 10km
• Visible just before total solar eclipses: « shadow bands »

Fresnel diffraction on stars is a classic observation

• In radioastronomy:

classic technique for studying the interstellar medium

• In optics:

diffraction during asteroid/lunar occultations, clearly distinct from atmospheric effects

R_diff varies with 1/R_ref

Simulation of a turbulent cloud

Point source

Extended source (0.85 R_sun)

Monochromatic

λ=2.18

Polychromatic

(Ks passband)

m = σ_I/I modulation index

Illumination on Earth

- in Ks passband�- by a K0V star@1.2Kpc� (m_V=16.3)

- through a cloud@160pc

- with R_diff =100km

m=100%

m=55%

m

zoom

Simulation of a realistic scintillation speckle

Simulated light curve detected by a telescope

t

m=3.3%

m=3.3%

Monochromatic

λ=2.18

Polychromatic

(Ks passband)

Scintillation with Vera Rubin telescope�(15s exposures)

• Source@7Kpc (gal. plane) in V
• Screen@160pc : B68- visible gas

For given R_diff modulation m = σ_I /<I> decreases with the sources’ magnitude since the apparent stellar radius increases (here MS star)

Gas structures in Galactic plane

Gas structures in Galactic halo

LSST precision

for T_exp = 15s

Rdiff = 1000 km

Rdiff = 4000 km

Rdiff = 16,000 km

m = σ_I /<I>

V

Maximum fraction of LMC/SMC scintillating stars : 1%

For a given detection threshold of modulation

τ(m > m_threshold) < 10^-2 x f(m_threshold)

Modulation

index

fraction of gas turbulent enough to produce m > m_threshold

Series of light curves seen by telescopes 2000 km far from each other

• Decorrelation increases with the distance
• But statistical behaviour constant (power spectrum)

-> Strong signature of propagation effect (not intrinsic variability) with only 2 telescopes far apart

Illumination from a 0.5xR_sun star at 1Kpc

through a diffusor at 160pc with R_diff = 1000km

Impact of the size of the source