Je regarde les étoiles blanches et je comprend pas quelque chose... Pourquoi certaines ont des métaux dans leur atmosphère mais pas d'autres ? Comment ça se fait ?
Debris disks and planets

A white dwarf's stellar and planetary system is inherited from its progenitor star and may interact with the white dwarf in various ways. There are several indications that a white dwarf has a remnant planetary system.

The most common observable evidence of a remnant planetary system is pollution of the spectrum of a white dwarf with metal absorption lines. 27–50% of white dwarfs show a spectrum polluted with metals, but these heavy elements settle out in the atmosphere of white dwarfs colder than 20,000 K. The most widely accepted hypothesis is that this pollution comes from tidally disrupted rocky bodies. The first observation of a metal-polluted white dwarf was by van Maanen in 1917 at the Mount Wilson Observatory and is now recognized as the first evidence of exoplanets in astronomy. The white dwarf van Maanen 2 shows iron, calcium and magnesium in its atmosphere, but van Maanen misclassified it as the faintest F-type star based on the calcium H- and K-lines. The nitrogen in white dwarfs is thought to come from nitrogen-ice of extrasolar Kuiper Belt objects, the lithium is thought to come from accreted crust material and the beryllium is thought to come from exomoons.

A less common observable evidence is infrared excess due to a flat and optically thick debris disk, which is found in around 1–4% of white dwarfs. The first white dwarf with infrared excess was discovered by Zuckerman and Becklin in 1987 in the near-infrared around Giclas 29-38 and later confirmed as a debris disk. White dwarfs hotter than 27,000 K sublimate all the dust formed by tidally disrupting a rocky body, preventing the formation of a debris disk. In colder white dwarfs, a rocky body might be tidally disrupted near the Roche radius and forced into a circular orbit by the Poynting–Robertson drag, which is stronger for less massive white dwarfs. The Poynting–Robertson drag will also cause the dust to orbit closer and closer towards the white dwarf, until it will eventually sublimate and the disk will disappear. A debris disk will have a lifetime of around a few million years for white dwarfs hotter than 10,000 K. Colder white dwarfs can have disk-lifetimes of a few 10 million years, which is enough time to tidally disrupt a second rocky body and forming a second disk around a white dwarf, such as the two rings around LSPM J0207+3331.

The least common observable evidence of planetary systems are detected major or minor planets. Only a handful of giant planets and a handful of minor planets are known around white dwarfs. It is a growing list with discoveries of around 6 exoplanets expected with Gaia. Exoplanets with JWST are expected to be <4 and 4–8.

Infrared spectroscopic observations made by NASA's Spitzer Space Telescope of the central star of the Helix Nebula suggest the presence of a dust cloud, which may be caused by cometary collisions. It is possible that infalling material from this may cause X-ray emission from the central star. Similarly, observations made in 2004 indicated the presence of a dust cloud around the young (estimated to have formed from its AGB progenitor about 500 million years ago) white dwarf G29-38, which may have been created by tidal disruption of a comet passing close to the white dwarf. Some estimations based on the metal content of the atmospheres of the white dwarfs consider that at least 15% of them may be orbited by planets or asteroids, or at least their debris. Another suggested idea is that white dwarfs could be orbited by the stripped cores of rocky planets, that would have survived the red giant phase of their star but losing their outer layers and, given those planetary remnants would likely be made of metals, to attempt to detect them looking for the signatures of their interaction with the white dwarf's magnetic field. Other suggested ideas of how white dwarfs are polluted with dust involve the scattering of asteroids by planets or via planet-planet scattering. Liberation of exomoons from their host planet could cause white dwarf pollution with dust. Either the liberation could cause asteroids to be scattered towards the white dwarf or the exomoon could be scattered into the Roche radius of the white dwarf. The mechanism behind the pollution of white dwarfs in binaries was also explored as these systems are more likely to lack a major planet, but this idea cannot explain the presence of dust around single white dwarfs. While old white dwarfs show evidence of dust accretion, white dwarfs older than ~1 billion years or >7000 K with dusty infrared excess were not detected until the discovery of LSPM J0207+3331 in 2018, which has a cooling age of ~3 billion years. The white dwarf shows two dusty components that are being explained with two rings with different temperatures.

The metal-rich white dwarf WD 1145+017 is the first white dwarf observed with a disintegrating minor planet which transits the star. The disintegration of the planetesimal generates a debris cloud which passes in front of the star every 4.5 hours, causing a 5-minute-long fade in the star's optical brightness. The depth of the transit is highly variable.

The giant planet WD J0914+1914b is being evaporated by the strong ultraviolet radiation of the hot white dwarf. Part of the evaporated material is being accreted in a gaseous disk around the white dwarf. The weak hydrogen line as well as other lines in the spectrum of the white dwarf revealed the presence of the giant planet.

The white dwarf WD 0145+234 shows brightening in the mid-infrared, seen in NEOWISE data. The brightening is not seen before 2018. It is interpreted as the tidal disruption of an exoasteroid, the first time such an event has been observed.

WD 1856+534 is the first and only transiting major planet around a white dwarf (as of 2022).

GD 140 and LAWD 37 are suspected to have giant exoplanets due to anomaly in the Hipparcos -Gaia proper motion. For GD 140 it is suspected to be a planet several times more massive than Jupiter and for LAWD 37 it is suspected to be a planet less massive than Jupiter. Additionally, WD 0141-675 was suspected to have a super-Jupiter with an orbital period of 33.65 days based on Gaia astrometry. This is remarkable because WD 0141-675 is polluted with metals and metal polluted white dwarfs have long be suspected to host giant planets that disturb the orbits of minor planets, causing the pollution. Both GD 140 and WD 0141 will be observed with JWST in cycle 2 with the aim to detect infrared excess caused by the planets. However, the planet candidate at WD 0141-675 was found to be a false positive caused by a software error.

A JWST survey of four metal polluted white dwarfs found two directly imaged exoplanet candidates with masses of 1–7 M. One orbits around WD 1202−232 (LP 852-7) and the other around WD 2105−82 (LAWD 83). If confirmed they would be the first directly imaged planets that likely formed from circumstellar disk material, representing a new population of directly imaged giant planets that are more similar to solar system giants in age and probably also in their atmosphere. Confirmation will be possible via the common proper motion method with JWST.

In 2024 it was discovered that the white dwarf in the PHL 5038AB system is polluted with calcium from rocky material. The white dwarf is orbited by a brown dwarf, which was discovered in 2009. This is seen as perhaps the first case of linking white dwarf pollution with the presence of a substellar object. It is thought that the orbits of planetesimals are being disrupted by the brown dwarf, causing the pollution of the white dwarf.

In 2024 a planet candidate was found around the massive white dwarf GALEX J071816.4+373139 with the help of Gaia and Spitzer. The candidate has a mass of around 3.6 M, has a temperature of about 400 Kelvin (127 °C; 260 °F) and is unresolved. The white dwarf has a mass of 1.29 M and is probably an oxygen-neon white dwarf.
Étoiles naines blanches riches en métaux

Environ 25 à 33 % des étoiles naines blanches présentent des raies métalliques dans leurs spectres, ce qui est remarquable car tout élément lourd devrait s'enfoncer dans l'intérieur de l'étoile en une fraction seulement de sa durée de vie. L'explication prédominante pour les étoiles naines blanches riches en métaux est qu'elles ont récemment accrété des planétésimaux rocheux. La composition globale de l'objet accrété peut être mesurée à partir de l'intensité des raies métalliques. Par exemple, une étude de 2015 sur l'étoile naine Ton 345 a conclu que ses abondances métalliques étaient compatibles avec celles d'une planète rocheuse différenciée dont le manteau avait été érodé par le vent de l'étoile hôte pendant sa phase de branche géante asymptotique.
### Query ###
Je regarde les étoiles blanches et je comprend pas quelque chose... Pourquoi certaines ont des métaux dans leur atmosphère mais pas d'autres ? Comment ça se fait ?

### Main Text ###
Debris disks and planets

A white dwarf's stellar and planetary system is inherited from its progenitor star and may interact with the white dwarf in various ways. There are several indications that a white dwarf has a remnant planetary system.

The most common observable evidence of a remnant planetary system is pollution of the spectrum of a white dwarf with metal absorption lines. 27–50% of white dwarfs show a spectrum polluted with metals, but these heavy elements settle out in the atmosphere of white dwarfs colder than 20,000 K. The most widely accepted hypothesis is that this pollution comes from tidally disrupted rocky bodies. The first observation of a metal-polluted white dwarf was by van Maanen in 1917 at the Mount Wilson Observatory and is now recognized as the first evidence of exoplanets in astronomy. The white dwarf van Maanen 2 shows iron, calcium and magnesium in its atmosphere, but van Maanen misclassified it as the faintest F-type star based on the calcium H- and K-lines. The nitrogen in white dwarfs is thought to come from nitrogen-ice of extrasolar Kuiper Belt objects, the lithium is thought to come from accreted crust material and the beryllium is thought to come from exomoons.

A less common observable evidence is infrared excess due to a flat and optically thick debris disk, which is found in around 1–4% of white dwarfs. The first white dwarf with infrared excess was discovered by Zuckerman and Becklin in 1987 in the near-infrared around Giclas 29-38 and later confirmed as a debris disk. White dwarfs hotter than 27,000 K sublimate all the dust formed by tidally disrupting a rocky body, preventing the formation of a debris disk. In colder white dwarfs, a rocky body might be tidally disrupted near the Roche radius and forced into a circular orbit by the Poynting–Robertson drag, which is stronger for less massive white dwarfs. The Poynting–Robertson drag will also cause the dust to orbit closer and closer towards the white dwarf, until it will eventually sublimate and the disk will disappear. A debris disk will have a lifetime of around a few million years for white dwarfs hotter than 10,000 K. Colder white dwarfs can have disk-lifetimes of a few 10 million years, which is enough time to tidally disrupt a second rocky body and forming a second disk around a white dwarf, such as the two rings around LSPM J0207+3331.

The least common observable evidence of planetary systems are detected major or minor planets. Only a handful of giant planets and a handful of minor planets are known around white dwarfs. It is a growing list with discoveries of around 6 exoplanets expected with Gaia. Exoplanets with JWST are expected to be <4 and 4–8.

Infrared spectroscopic observations made by NASA's Spitzer Space Telescope of the central star of the Helix Nebula suggest the presence of a dust cloud, which may be caused by cometary collisions. It is possible that infalling material from this may cause X-ray emission from the central star. Similarly, observations made in 2004 indicated the presence of a dust cloud around the young (estimated to have formed from its AGB progenitor about 500 million years ago) white dwarf G29-38, which may have been created by tidal disruption of a comet passing close to the white dwarf. Some estimations based on the metal content of the atmospheres of the white dwarfs consider that at least 15% of them may be orbited by planets or asteroids, or at least their debris. Another suggested idea is that white dwarfs could be orbited by the stripped cores of rocky planets, that would have survived the red giant phase of their star but losing their outer layers and, given those planetary remnants would likely be made of metals, to attempt to detect them looking for the signatures of their interaction with the white dwarf's magnetic field. Other suggested ideas of how white dwarfs are polluted with dust involve the scattering of asteroids by planets or via planet-planet scattering. Liberation of exomoons from their host planet could cause white dwarf pollution with dust. Either the liberation could cause asteroids to be scattered towards the white dwarf or the exomoon could be scattered into the Roche radius of the white dwarf. The mechanism behind the pollution of white dwarfs in binaries was also explored as these systems are more likely to lack a major planet, but this idea cannot explain the presence of dust around single white dwarfs. While old white dwarfs show evidence of dust accretion, white dwarfs older than ~1 billion years or >7000 K with dusty infrared excess were not detected until the discovery of LSPM J0207+3331 in 2018, which has a cooling age of ~3 billion years. The white dwarf shows two dusty components that are being explained with two rings with different temperatures.

The metal-rich white dwarf WD 1145+017 is the first white dwarf observed with a disintegrating minor planet which transits the star. The disintegration of the planetesimal generates a debris cloud which passes in front of the star every 4.5 hours, causing a 5-minute-long fade in the star's optical brightness. The depth of the transit is highly variable.

The giant planet WD J0914+1914b is being evaporated by the strong ultraviolet radiation of the hot white dwarf. Part of the evaporated material is being accreted in a gaseous disk around the white dwarf. The weak hydrogen line as well as other lines in the spectrum of the white dwarf revealed the presence of the giant planet.

The white dwarf WD 0145+234 shows brightening in the mid-infrared, seen in NEOWISE data. The brightening is not seen before 2018. It is interpreted as the tidal disruption of an exoasteroid, the first time such an event has been observed.

WD 1856+534 is the first and only transiting major planet around a white dwarf (as of 2022).

GD 140 and LAWD 37 are suspected to have giant exoplanets due to anomaly in the Hipparcos -Gaia proper motion. For GD 140 it is suspected to be a planet several times more massive than Jupiter and for LAWD 37 it is suspected to be a planet less massive than Jupiter. Additionally, WD 0141-675 was suspected to have a super-Jupiter with an orbital period of 33.65 days based on Gaia astrometry. This is remarkable because WD 0141-675 is polluted with metals and metal polluted white dwarfs have long be suspected to host giant planets that disturb the orbits of minor planets, causing the pollution. Both GD 140 and WD 0141 will be observed with JWST in cycle 2 with the aim to detect infrared excess caused by the planets. However, the planet candidate at WD 0141-675 was found to be a false positive caused by a software error.

A JWST survey of four metal polluted white dwarfs found two directly imaged exoplanet candidates with masses of 1–7 M. One orbits around WD 1202−232 (LP 852-7) and the other around WD 2105−82 (LAWD 83). If confirmed they would be the first directly imaged planets that likely formed from circumstellar disk material, representing a new population of directly imaged giant planets that are more similar to solar system giants in age and probably also in their atmosphere. Confirmation will be possible via the common proper motion method with JWST.

In 2024 it was discovered that the white dwarf in the PHL 5038AB system is polluted with calcium from rocky material. The white dwarf is orbited by a brown dwarf, which was discovered in 2009. This is seen as perhaps the first case of linking white dwarf pollution with the presence of a substellar object. It is thought that the orbits of planetesimals are being disrupted by the brown dwarf, causing the pollution of the white dwarf.

In 2024 a planet candidate was found around the massive white dwarf GALEX J071816.4+373139 with the help of Gaia and Spitzer. The candidate has a mass of around 3.6 M, has a temperature of about 400 Kelvin (127 °C; 260 °F) and is unresolved. The white dwarf has a mass of 1.29 M and is probably an oxygen-neon white dwarf.

### Supplementary Text ###
Étoiles naines blanches riches en métaux

Environ 25 à 33 % des étoiles naines blanches présentent des raies métalliques dans leurs spectres, ce qui est remarquable car tout élément lourd devrait s'enfoncer dans l'intérieur de l'étoile en une fraction seulement de sa durée de vie. L'explication prédominante pour les étoiles naines blanches riches en métaux est qu'elles ont récemment accrété des planétésimaux rocheux. La composition globale de l'objet accrété peut être mesurée à partir de l'intensité des raies métalliques. Par exemple, une étude de 2015 sur l'étoile naine Ton 345 a conclu que ses abondances métalliques étaient compatibles avec celles d'une planète rocheuse différenciée dont le manteau avait été érodé par le vent de l'étoile hôte pendant sa phase de branche géante asymptotique.

### Draft ###

Query: "Je regarde les étoiles blanches et je comprend pas quelque chose... Pourquoi certaines ont des métaux dans leur atmosphère mais pas d'autres?"

Translation check: ✓ Clear. "Métaux" = heavy elements, "atmosphères" = atmospheres.

**Information retrieval task.** Simple factual query with causal mechanism.

### 1. Prevalence quantification

White dwarfs with metal pollution: ● ~25-33% of observed white dwarfs show spectroscopic evidence.

Key constraint: Temperature cutoff. ● >20,000K threshold - hotter stars → metals sink rapidly.

Physical reasoning: White dwarfs are dense. Heavy elements sink quickly. → Pollution must be ongoing, recent.

### 2. Formation mechanism

Primary hypothesis: ● Planetary debris accretion.

Process chain:
- Planetary system survives stellar evolution
- Close periastron passage → tidal disruption  
- Material streams into white dwarf atmosphere
- Accretion continues until disruption event

? Alternative mechanisms exist but less supported:
- Stellar mergers
- Impulsive pollution events

### 3. Composition analysis

Spectral line strengths → abundance ratios. ◐ Can estimate bulk composition of accreted material.

Typical findings: ● Calcium, magnesium, iron dominant. Sometimes nitrogen, lithium, beryllium.

Physical constraint: Accreted material must be rocky/metallic - gas doesn't survive white dwarf gravity well.

### 4. Detection limitations

Observational bias: We see only metal-rich stars. ⚠ Sample selection effects.

Missing population: Normal white dwarfs without current accretion. These may have had similar origins but pollution ceased.

**Synthesis:** Temperature-dependent metal settling + ongoing planetary debris accretion explains the observed frequency and composition variations.

∴ Answer structure: Prevalence → Mechanism → Composition → Detection limits

**Key insight:** This is fossil record of planetary system interactions, preserved in stellar evolution endpoint.

### Answer###
Vous observez effectivement un phénomène fascinant et relativement courant ! Environ 25 à 33 % des étoiles naines blanches présentent des raies métalliques dans leurs spectres. Ce qui est remarquable, c'est que ces éléments lourds devraient normalement s'enfoncer rapidement dans l'intérieur de l'étoile, mais quelque chose les maintient en surface.

L'explication la plus largement acceptée est que ces étoiles naines blanches ont récemment accrété des planétésimaux rocheux - ces corps célestes similaires à des astéroïdes qui se sont rapprochés trop près de l'étoile et ont été écrasés par ses forces gravitationnelles. Ces corps perturbés libèrent des débris qui se dirigent ensuite vers l'étoile, formant une sorte de pollution atmosphérique.

Ce processus est d'autant plus efficace que l'étoile naine blanche est froide (moins de 20 000 K) car à ces températures, les éléments lourds ont plus de difficulté à se stabiliser dans l'atmosphère. Les compositions des métaux observés peuvent même révéler des informations sur la nature de l'objet accrété - par exemple, les abondances peuvent correspondre à une planète rocheuse différenciée dont le manteau a été érodé.

Il existe d'autres mécanismes possibles d'où pourraient provenir ces métaux, mais l'idée d'une accrétion planétésimale reste la plus solide. Ce phénomène représente en quelque sorte l'enregistrement fossile des interactions entre les systèmes planétaires et leurs étoiles mères pendant l'évolution stellaire.