Why There Are Most Mass Limits for Compact Objects

On this article, we are going to have a look at why there are most mass limits for objects which are supported towards gravity by degeneracy strain as an alternative of kinetic strain. We’ll have a look at the 2 recognized circumstances of this, white dwarfs and neutron stars; however it must be famous that comparable arguments will apply to any postulated object that meets the overall definition given above. For instance, the identical arguments would apply to “quark stars” or “quark-gluon plasma objects”, and so forth.

The Chandrasekhar Restrict

First, we’ll have a look at the utmost mass restrict for white dwarfs, the Chandrasekhar restrict. (Be aware that the primary derivation we are going to give under, utilizing the TOV equation, is a simplified model of the argument given in Shapiro & Teukolsky, who use numerical integration of the Lane-Emden equation. For this text we will likely be glad with a heuristic argument utilizing averages and received’t have to go to that excessive.)

We begin with the overall relativistic equation for hydrostatic equilibrium for a static, spherically symmetric object (we is not going to contemplate the rotation of neutron stars right here; that complicates the maths and modifications the numerical worth of the utmost mass restrict, however it doesn’t take away it). That is the Tolman-Oppenheimer-Volkoff equation, which we are going to write in a kind considerably completely different from the one through which it normally seems. Be aware that we’re utilizing models through which ##G = c = 1##.

$$
frac{dp}{dr} = – rho frac{m}{r^2} left( 1 + frac{p}{rho} proper) left( 1 + frac{4 pi r^3 p}{m} proper) left( 1 – frac{2m}{r} proper)^{-1}
$$

This type of the equation makes it simpler to see that what now we have right here is the Newtonian (non-relativistic) equation for hydrostatic equilibrium, with some relativistic correction components. For white dwarfs, nonetheless, it seems that we will ignore all of these correction components and simply have a look at the non-relativistic components for hydrostatic equilibrium. It is because the radius of white dwarfs is way bigger than their mass in geometric models, so ##r >> 2m## and the final issue on the RHS above may be taken to be ##1##, and their strain is all the time too small to make the correction phrases within the different two components important, so these components will also be taken to be ##1##.

We now make use of the truth that, for degenerate matter, now we have ##p = Ok rho^Gamma##, the place ##Ok## is a continuing that depends upon whether or not the degeneracy is non-relativistic or relativistic, so we’ll designate its two values as ##K_text{n}## and ##K_text{r}## (we are going to solely contemplate the 2 extremes and won’t have a look at the transition between them), and ##Gamma## is the “adiabatic index”, which is ##5/3## within the non-relativistic restrict and ##4/3## within the relativistic restrict. This offers ##dp / dr = Ok Gamma rho^{Gamma – 1} d rho / dr##. Lastly, we make use of the truth that, for a static, spherically symmetric object, ##dm / dr = 4 pi rho r^2##, to place issues when it comes to derivatives of ##m##.

We plug all this into the non-relativistic hydrostatic equilibrium equation to acquire:

$$
frac{d}{dr} left( frac{1}{4 pi r^2} frac{dm}{dr} proper) = – frac{1}{Ok Gamma} left( frac{1}{4 pi r^2} frac{dm}{dr} proper)^{2 – Gamma} frac{m}{r^2}
$$

Increasing and simplifying provides:

$$
frac{d^2 m}{dr^2} – frac{2}{r} frac{dm}{dr} + frac{left( 4 pi proper)^{Gamma – 1}}{Ok Gamma} left( frac{1}{r^2} frac{dm}{dr} proper)^{2 – Gamma} m = 0
$$

Quite than attempt to remedy this nasty differential equation straight, we will likely be glad right here with making tough order of magnitude estimates. For this goal, we outline ##M## as the entire mass of the white dwarf and ##R## as its floor radius, and we approximate ##dm / dr## with its common, ##M / R##, and ##d^2 m / dr^2## with ##M / R^2##. Substituting these into the above equation provides, after simplifying:

$$
M^{2 – Gamma} = frac{Ok Gamma}{left( 4 pi proper)^{Gamma – 1}} R^{4 – 3 Gamma}
$$

Now we’re ready to take a look at our two regimes. Within the non-relativistic regime, ##Gamma = 5/3## and now we have:

$$
M^{1/3} = frac{5}{3} frac{K_text{n}}{left( 4 pi proper)^{2/3}} frac{1}{R}
$$

Inverting this tells us that, as ##M## will increase, ##R## decreases because the dice root of ##M##. In different phrases, because the white dwarf will get extra huge, it will get extra compact. And because it will get extra compact, its density and strain enhance and it turns into relativistic. So so as to assess whether or not there’s a most mass restrict, we have to have a look at the relativistic regime. Right here, ##Gamma = 4/3## and now we have:

$$
M^{2/3} = frac{4}{3} frac{K_text{r}}{left( 4 pi proper)^{1/3}}
$$

Be aware that now, ##R## doesn’t seem in any respect within the equation! It’s simply an equation for ##M## when it comes to recognized constants. In different phrases, within the ultra-relativistic restrict, ##M## approaches a continuing limiting worth and can’t exceed it. That worth is the Chandrasekhar restrict. (Be aware that, to get the precise numerical worth for the restrict that’s utilized by astrophysicists, which is 1.4 photo voltaic lots, the tough order of magnitude calculation now we have completed right here will not be sufficient, however we received’t go into additional particulars about how that worth is definitely calculated right here. Our goal right here is solely to see, heuristically, why there should be a mass restrict in any respect.)

Let’s take a step again now and attempt to perceive what’s going on right here. A method of taking a look at it’s to ask the query: what’s the white dwarf’s power of gravity as a perform of density? By “power of gravity” right here we imply, heuristically, the inward pull that should be balanced by the outward power of strain so as to keep hydrostatic equilibrium. It seems that this will increase with density as ##rho^{4/3}##. So we must always anticipate that in any state of affairs through which ##Gamma to 4/3##, there will likely be a most mass restrict as a result of strain can not proceed to extend quicker than gravity. And we will see from the above {that a} relativistically degenerate electron fuel, as in a white dwarf, is one such state of affairs. (One other seems to be a supermassive star supported by radiation strain; because the mass will increase, the efficient ##Gamma## for radiation strain turns into relativistic and now we have the identical qualitative state of affairs as a white dwarf, although in fact with completely different constants so the precise numerical worth of the mass restrict is completely different.)

This argument concerning the power of gravity can actually be made mathematically. As Shapiro and Teukolsky word, the primary physicist to do that was Landau, in 1932, who got here up with an alternate method of understanding Chandrasekhar’s consequence, printed the yr earlier than, on the utmost mass of white dwarfs. Landau’s argument is easy: first, we discover an expression for the entire power ##E## (excluding relaxation mass power) of an object that’s supported by degeneracy strain; then we glance to see beneath what situations ##E## may have a minimal, which signifies a secure equilibrium.

The full power has two elements: the (optimistic) power of the fermions because of the degeneracy strain, and the (unfavourable) gravitational potential power because of the fermions all pulling on one another. The power as a consequence of degeneracy strain is the Fermi power ##E_F## per fermion, and we will use the Newtonian components for the gravitational potential power per fermion since we noticed above that the relativistic corrections to the TOV equation, that are of the identical order of magnitude because the relativistic corrections to the gravitational potential, are negligible. The full power per fermion, due to this fact, seems like this (I’m penning this in a barely completely different from that utilized in Shapiro & Teukolsky, for simpler comparability to the derivation given above):

$$
E = frac{hbar M^{1/3}}{mu_B^{1/3} R} – frac{M mu_B}{R}
$$

the place ##mu_B## is the baryon mass that’s related to the fermions offering the degeneracy strain; this would be the common of the proton and neutron mass in a typical white dwarf, since every electron is related to one proton and the proton-neutron ratio is roughly ##1##. (In a neutron star ##mu_B## would simply be the neutron mass.)

For there to be a secure equilibrium at a given worth of ##M##, there should be a minimal of ##E## at a finite worth of ##R##. It will happen if now we have ##dE / dR = 0## at a finite worth of ##R##. Since each phrases in ##E## scale as ##1 / R##, the expression for ##dE / dR## is straightforward:

$$
frac{dE}{dR} = – frac{1}{R^2} left( frac{hbar M^{1/3}}{mu_B^{1/3}} – M mu_B proper)
$$

Now we have a look at how ##dE / dR## varies with ##M##. If ##M## is small, the issue contained in the parentheses will likely be optimistic, so ##dE / dR## will likely be unfavourable and ##E## will lower with rising ##R##. That can make the starless relativistic and ultimately nonrelativistic. As soon as the star turns into nonrelativistic, the radial dependence of the Fermi power will change; it’ll scale as ##1 / R^2## as an alternative of ##1 / R##. Which means the gravitational potential power will, at some worth of ##R##, turn into bigger than the Fermi power, and that may trigger the signal of ##dE / dR## to flip from unfavourable to optimistic for the reason that gravitational potential power will increase with rising ##R## (to a limiting worth of ##0## as ##R to infty##). The finite worth of ##R## the place the signal flip happens will likely be a minimal of ##E## and due to this fact a secure equilibrium.

Nevertheless, if ##M## is massive, the issue contained in the parentheses will likely be unfavourable, so ##dE / dR## will likely be optimistic. In that case, ##E## may be decreased with out sure by lowering ##R##; each phrases scale the identical method with ##R## and lowering ##R## makes the star extra relativistic so the radial dependence of the Fermi power is not going to change. Which means there isn’t any secure equilibrium; the star will collapse.

The boundary between these two regimes will happen on the worth of ##M## at which the issue contained in the parentheses above is zero, and that would be the most attainable mass, which will likely be given by:

$$
M^{2/3} = frac{hbar}{mu_B^{4/3}}
$$

Evaluating this with the heuristic components above provides not less than a tough order of magnitude estimate for the fixed ##K_r##. Be aware, nonetheless, that this components will likely be formally the identical for a white dwarf and a neutron star; actually, it will likely be the identical for any object that’s supported by degeneracy strain, since we made no assumptions that have been particular to a specific sort of object. The one distinction between various kinds of objects will likely be a distinct worth of ##mu_B## based mostly on chemical composition. This components itself is heuristic, and there become different numerical components concerned; nonetheless, it’ll certainly end up that the suggestion implied by the above components, that the utmost mass of a neutron star will not be that completely different from the utmost mass of a white dwarf, is principally right.

After all, as we famous earlier, to really calculate the generally recognized numerical worth of the Chandrasekhar restrict for white dwarfs, the above formulation aren’t sufficient; we must do extra sophisticated numerical calculations. Chandrasekhar did these calculations when he initially printed his derivation of the restrict that got here to be named after him, in 1934; and subsequent calculations haven’t made any important modifications to the worth he obtained. Nevertheless, the numerical worth does rely considerably on the chemical composition of the white dwarf. When it comes to the primary components above, the chemical composition can have an effect on the worth of ##K_r##; when it comes to the second, it might have an effect on the worth of ##mu_B## based on the fraction of baryons which are protons. Chandrasekhar’s worth assumed that the chemical composition of the white dwarf was principally hydrogen and helium, and that’s the foundation for the generally used worth of 1.4 photo voltaic lots for his restrict. Nevertheless, in a while within the Fifties, when Harrison, Wakano, and Wheeler have been deriving a common equation of state for chilly matter, they used a distinct chemical composition for white dwarfs, one which was considerably richer in neutrons, and obtained a worth of 1.2 photo voltaic lots. So when taking a look at values within the literature for white dwarf most mass limits, one has to make sure to test the chemical composition that’s being assumed.

The Tolman-Oppenheimer-Volkoff Restrict

In 1938, Tolman, Oppenheimer, and Volkoff investigated the query of most mass limits for neutron stars. It was in the middle of these investigations that they derived the relativistic equation for hydrostatic equilibrium that we noticed within the earlier article, and which is known as after them. They went via a derivation much like the one Chandrasekhar had completed for white dwarfs and got here up with an analogous consequence: there’s a most mass restrict for neutron stars. When it comes to the above formulation, the one change can be a distinct worth of ##K_r## within the first components, or ##mu_B## within the second, to account for the change in the kind of fermions, from electrons to neutrons, and the truth that the identical fermions now account for each the mass and the degeneracy strain (whereas in a white dwarf, the electrons account for the degeneracy strain whereas the baryons account for the mass).

The attention-grabbing half was that the numerical worth of the mass restrict that they obtained for neutron stars was 0.7 photo voltaic lots–i.e., smaller than the white dwarf restrict that Chandrasekhar had calculated! The rationale for this, when it comes to the formulation we checked out within the final article, is straightforward: along with the modifications talked about above, Oppenheimer and Volkoff didn’t assume that the relativistic correction components within the TOV equation have been negligible, as we did within the earlier article. They included these components, and for neutron stars within the relativistic restrict, they don’t seem to be all negligible; the tip result’s to extend the RHS of the TOV equation by a numerical issue that finally ends up showing within the denominator of our formulation for the utmost mass and thus reduces the anticipated most mass by about half.

On the time, this was not essentially a serious problem, since no neutron stars had been noticed; however now we all know of many neutron stars which are considerably extra huge, so we all know one thing should be unsuitable with the unique TOV calculation. However even on the time, Tolman, Oppenheimer, and Volkoff had good motive to not take that quantity at face worth. Why? As a result of, despite the fact that not lots was recognized concerning the robust nuclear power on the time, it was evident that, at quick sufficient distances, smaller than the scale of an atomic nucleus, that power should turn into strongly repulsive; in any other case, atomic nuclei wouldn’t be secure on the measurement ranges they have been recognized to have.

This issues as a result of the derivations we went via above made an necessary assumption that we didn’t point out earlier than: that the fermions in query didn’t work together with one another in any respect, besides via the Pauli exclusion precept. If we add an interplay that’s repulsive at quick ranges, that modifications issues. Within the first derivation within the earlier article, the impact is to extend ##Gamma##, the adiabatic index, above the conventional worth it will have as a consequence of degeneracy and the Pauli exclusion precept alone. Within the second derivation, the impact is so as to add one other optimistic time period within the power because of the repulsive interplay.

On the face of it, this would appear to point that the 2 derivations will now give us completely different solutions! Rising ##Gamma## ought to imply that the primary derivation now seems extra like its nonrelativistic kind, which does not result in a most mass. Nevertheless, including a optimistic power time period within the second derivation doesn’t change the general logic resulting in a most mass so long as that power scales as ##1 / R##, which we might anticipate it to do. The impact will simply be to extend the numerical worth of the utmost mass that we calculate.

The decision of this obvious contradiction between the 2 derivations is that, within the neutron star case, the “essential” worth of ##Gamma##, at which the star turns into unstable, is not ##4/3##, because it was for white dwarfs; that’s solely a limiting worth within the absence of different interactions. Within the presence of different interactions, the essential worth of ##Gamma## will increase, to the purpose the place even the bigger precise worth of ##Gamma## because of the repulsive interactions continues to be lower than the essential worth of ##Gamma## within the relativistic restrict. And meaning the identical logic as earlier than nonetheless goes via within the first derivation for neutron stars: within the relativistic restrict, ##Gamma## reaches a essential worth at which the mass turns into unbiased of radius and there’s a most mass.

Why should the worth of ##Gamma## within the first derivation for neutron stars all the time find yourself lower than the essential worth? The reply to this comes from taking a look at a restrict that relativity imposes on the equation of state of any sort of matter: that the velocity of sound within the matter can’t exceed the velocity of sunshine. The velocity of sound is given by ##v_s^2 = dp / drho##, and we will see that, if ##p = Ok rho^Gamma##, the restrict ##dp / drho le 1## will power ##Gamma## to lower because the star turns into increasingly more huge and increasingly more compressed and ##rho## due to this fact will increase. So there isn’t any method for the mass to extend indefinitely.

As we famous above, our conclusions right here, whereas they need to be common and apply to any equation of state, solely give a tough order of magnitude estimates of numerical values. Physicists have completed extra detailed calculations utilizing numerous equations of state for neutron star matter and have confirmed the existence of most mass limits for all of them, with values starting from about 1.5 to about 2.7 photo voltaic lots. Analyses of the conduct of the essential worth of ##Gamma## have additionally been completed utilizing numerous fashions; in not less than one case, the idealized case of a neutron star with uniform density, the calculations may be completed analytically, with out requiring numerical simulation, since closed kind equations for this case are recognized. For this case, the essential level at which the limiting worth of ##Gamma## imposed by the situation that the velocity of sound can’t exceed the velocity of sunshine is the same as the essential worth of ##Gamma## is on the level ##p = rho / 3##, which agrees with the prediction from an ideal fluid mannequin within the ultrarelativistic restrict (for instance, this is identical worth that applies to a “fuel” of photons). It’s noteworthy that, for this case, the worth of ##Gamma## equivalent to this restrict may be very massive, about ##3.5##. This confirms that even a particularly stiff equation of state will not be ample to withstand compression indefinitely within the relativistic restrict.

Fashionable observations have discovered that the overwhelming majority of neutron stars we observe are pulsars, quickly rotating, and fast rotation invalidates the calculations now we have been making right here since we assumed a static, spherically symmetric object. We might intuitively anticipate that rotation would compensate considerably for elevated gravity and due to this fact would possibly enhance the utmost mass restrict, and certainly it seems to; now we have noticed pulsars at shut to three photo voltaic lots, and no fashionable calculations for non-rotating neutron stars have indicated a restrict that enormous. Calculations for rotating neutron stars are extra sophisticated, however don’t change the fundamental conclusion: there’s nonetheless a most mass restrict, and it’s nonetheless essentially because of the similar mechanism as above: that relativity locations final limits on the power of degenerate matter to withstand compression. That could be a key motive why astrophysicists are extremely assured that darkish objects which are not directly detected by their gravitational results, and whose lots are estimated to be a lot bigger than the utmost mass restrict for neutron stars, are black holes.

A Last Be aware

As was talked about above, there are different, extra speculative configurations of degenerate matter proposed within the literature, resembling “quark stars”, however all of them are topic to the identical common mechanism now we have seen right here for max mass limits. As in comparison with neutron stars, these speculative configurations are simply modifications in chemical composition, which might modify the equations by numerical components of order unity however can’t change the fundamental conduct. So, though such speculative objects, in the event that they end up to exist, may need most mass limits considerably completely different from neutron stars, the variations will nonetheless be of order unity and won’t have an effect on the fundamental conclusion acknowledged above, that once we detect the oblique gravitational results of darkish objects with lots a lot better than the neutron star mass restrict, these objects must be assumed to be black holes.

References:

Shapiro & Teukolsky, 1983, Sections 3.3, 3.4, 9.2, 9.3, 9.5, 9.6