the Split large-scale gaseous structures. 16 As seen in Figure 1b (interactive), this argument about the nature of the Gould's Belt is confirmed here. The right side of the assumed Gould's Belt (the Sco-Cen association) consists of the entire rightward wall of the Local Bubble, while its left side consists of clouds in the Radcliffe Wave, well beyond the leftward wall of the Local Bubble. The Local Bubble lies at the closest distance between the Radcliffe Wave and the Split, with most of the dense gas at its surface co-spatial with these two kpc-scale features.

We use measurements of the 3D positions and motions of stellar clusters to reconstruct the star formation history near the Local Bubble. We rely on curated samples of young stars from the literature, as summarized in Extended Data Table 1. Our sample includes: clusters associated with star-forming regions on the surface of the bubble (Taurus, Ophiuchus, Lupus, Chamaeleon, and Corona Australis), older members of the Sco-Cen association (Upper Centaurus Lupus, Lower Centaurus Crux, and Upper Scorpius) up to a maximum age of 20 Myr; and clusters in known star-forming regions along the Radcliffe Wave and the Split but beyond the boundaries of the Local Bubble itself (Perseus, Serpens, Orion).

As described in the Methods section, we derive the "tracebacks" of stellar clusters associated with the Local Bubble and related structures. The current 3D space motions of the young stellar clusters are shown as cones in the interactive version of Figure 1, with the apex of the cone pointing in the direction of motion. Prior research has shown that the 3D space motions of the youngest clusters (≲ 3 Myr) can be considered probes of the 3D space motions of the parental gas clouds in which they were born. 17 Using the young stars' motions to trace cloud motion, we see that not only do all star-forming clouds presently observed within 200 pc lie on the surface of the Local Bubble, but they also show strong evidence of outward expansion, primarily perpendicular to the Bubble's surface. Tracebacks of the clusters' motions over the past 20 Myr point to the likely origin of the Local Bubble-presumably the region where the supernovae driving the bubble went off. The clear implication of the observed geometry and motions is that all the well-known star-forming regions within 200 pc of the Sun formed as gas has been swept up by the Local Bubble's expansion.

The interactive version of Figure 1 also includes a model for Gould's Belt 18 , which illustrates that much of the motion previously attributed to the expansion of the assumed Gould's Belt 19 is instead likely due to the expansion of the Local Bubble. Recent work using complementary catalogs of young stars bolster this interpretation, finding evidence that the Sco-Cen stellar association -a key anchor of the Gould Belt -has a arc-like morphology consistent with recent sequential star formation, which we now know to be triggered by the Local Bubble. 20 A full animation of the stellar tracebacks is provided in Figure 2 (interactive). In the static version, we show select snapshots at -16 Myr, -15 Myr, -14 Myr, -10 Myr, -6 Myr, -2 Myr, and the present. We observe multiple epochs of star formation, with each generation of stars consistent with being formed at the edge of the Local Bubble's expanding shell. We find that 15-16 Myr ago, the Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC) clusters in the Sco-Cen stellar association were born about 15 pc apart, and that the Bubble itself was likely created by supernovae whose surviving members belong to these clusters.

Based on the amount of momentum injection required by supernovae to sweep up the total mass of the shell (1.4 -0.62 +0.65 × 10 6 M☉) given its present-day expansion velocity (6.7 -0.4 +0.5 km/s), we estimate that 15 -7 +11 supernovae were required to form the Local Bubble (see Methods section). Through an analysis of their existing stellar membership and an adopted Initial Mass Function (IMF), previous studies agree that UCL and LCC have produced 14-20 supernovae over their lifetimes. 6,7,21 However, previous work 6,7 also claims that UCL and LCC formed outside the present-day boundary of the Local Bubble, only entering its interior in the past few megayears, inconsistent with an argument that they are the progenitor population. By adopting new Gaia EDR3 estimates of the clusters' 3D velocities, better orbit integration, and a more accurate value for the Sun's peculiar motion, we find that UCL and LCC indeed coincide with the center of the bubble at its birth, lying just interior to its inner surface in the present day, thereby resolving this discrepancy. We explain the inconsistency between the stellar tracebacks for UCL and LCC proposed in this work and those from prior work in more detail in the Methods section. 6,7 Under the assumption that each star-forming molecular cloud formed due to the shell's expansion -powered by UCL and LCC near its center -we fit for the temporal and radial evolution of the Local Bubble by building on recent analytic frameworks. 22 As described in the Methods section, our idealized, spherical shell expansion model fits for the age of the Local Bubble, the duration between supernova explosions powering its expansion, and the ambient density of the interstellar medium prior to the first explosion. We find that an age of 14.4 -0.8 +0.7

Myr, a time between supernova explosions of 1.1 -0.4 +0. 6 Myr, and an ambient density of 2.7 -1.0 +1.6 cm -3 provides the best-fit to the dynamical tracebacks. This best-fit model for the Local Bubble's expansion is also shown in the static and interactive versions of Figure 2.

Following the presumed birth of the Local Bubble 14 Myr ago, we observe four subsequent epochs of star The circumstances that led to the birth of the progenitor populations UCL and LCC are more difficult to constrain. Given the close proximity of both the Radcliffe Wave and the Split to the Local Bubble, the origin of UCL and LCC could be related to one of these kpc-scale gaseous structures, or to a past interaction between the two. While current kinematic data are limited, the 3D tracebacks of young stars in two constituent clouds along the Radcliffe Wave (Orion) and the Split (Serpens), but beyond the Local Bubble's influence suggest that the Radcliffe Wave and the Split could have converged 20 Myr ago at the location where UCL and LCC were born.

However, future follow-up work on the 3D motions of these two linear features would be needed to shed light on the true architecture of interstellar gas on kiloparsec scales at the time of UCL's and LCC's formation.

Regardless of UCL and LCC's potential origins, we find 6D (3D position and 3D velocity) observational support for the theory of supernova-driven star formation in the interstellar medium [23][24][25][26] a long-invoked theoretical pathway for molecular cloud formation seen in numerical simulations. 27 The abundance of new stellar radial velocity data expected in Gaia DR3 should not only allow for more refined estimates of the Local Bubble's evolution, but also enable similar studies farther afield, providing further observational constraints on supernova-driven star formation across our Galactic neighborhood. We also overlay the morphology of the 3D dust (gray blobby shapes 9 ) and the models for two Galactic scale features -the Radcliffe Wave (red) 16 and the Split (blue) 10 . The Per-Tau Superbubble 15 (green sphere) is also overlaid. The interactive version offers views from any direction (not just top-down), provides floating labels for star-forming regions, and includes additional layers (some not shown in this snapshot) which can be toggled on/off.

In Extended Data also lie inside the bubble, their ages (> 25 Myr) are larger than the bubble's estimated age and are thus excluded from our analysis 33 . Finally, we also include clusters beyond the Local Bubble but associated with star-forming regions along the Radcliffe Wave and the Split, including the Perseus 35 , Orion 17 , and Serpens 36 Molecular Clouds.

We rely on existing studies (see Extended Data Table 1) to determine stellar membership of each cluster, but we only include stars which are detected in the Gaia EDR3 catalog. We uniformly associate stellar members of each cluster with Gaia EDR3 using a crossmatch radius of 1" to obtain its sky coordinates, parallax, and proper motions. If radial velocities for the stars are provided along with the target selection in their original publication, we adopt those existing radial velocities in our analysis, some of which are obtained with high-quality ground based near-infrared spectroscopy. Otherwise, if no radial velocity data are provided, we utilize the Gaia radial velocities and restrict our analysis to only those stars which also have a Gaia radial velocity detection. We largely rely on the sample selection outlined in each cluster's original publication to filter outliers, many of which are defined using Gaia DR2 data. However, as an additional constraint, we require that all stars must have a parallax over error greater than two and small renormalized unit weight error (RUWE < 1.4). 37 We also require the radial velocity error to be < 5 km/s, a relatively generous cut chosen because our algorithm incorporates the errors on the individual stellar measurements when determining the mean cluster motion. After applying all these cuts, we perform a sigma-clipping procedure using the astropy 38 package, removing extreme outliers whose radial velocities are inconsistent with the rest of the cluster population at the 3𝜎 level. Finally, we require that each cluster have at least three stellar members. The mean stellar membership is much higher than this, averaging 37 stars per cluster.

In order to transform the sky coordinates (right ascension 𝛼 and declination δ), parallax (π), proper motions (μRA,μDEC), and heliocentric radial velocities (vhelio) of members to an average 3D space position and 3D velocity of the cluster in a Heliocentric Galactic Cartesian reference frame (x, y, z, U, V, W), we utilize the extreme deconvolution algorithm. 39 The Extreme Deconvolution algorithm infers an n-dimensional distribution function using a Gaussian Mixture Model (GMM) given the presence of noisy, incomplete, and heterogeneous samples of the underlying population. We apply the algorithm to infer the average 6D phase information of each cluster,

given the observed values and estimated error covariances of its stellar members. For each star we use the astropy 38 functionality within galpy 40 to compute the star's Heliocentric Galactic Cartesian coordinates (x, y, z)

and associated space motions (U, V, W) given the observed Gaia quantities (𝛼, δ, π, μRA, μDEC, vhelio). We assume U points towards the Galactic center, V points toward the direction of Galactic rotation, and W points toward the North Galactic Pole. In order to accurately estimate errors on (x, y, z, U, V, W) for each star, we randomly sample one hundred times from a multi-dimensional Gaussian in (𝛼, δ, π, μRA, μDEC. vhelio) space assuming Gaussian uncertainties on all parameters as reported in the Gaia EDR3 catalog. Transforming each sample to a Heliocentric Galactic Cartesian coordinate frame, we then calculate the covariance of the set of samples for each star. Feeding the set of (x, y, z, U, V, W) values for the individual stellar cluster members and their associated sample covariances into the Extreme Deconvolution algorithm, we obtain the mean and variance of a single 6D Gaussian defining the average 3D position and 3D space motion of each cluster, as shown in Extended Data Table 1. We adopt a peculiar solar motion of (U☉, V☉, W☉) = (10.0, 15.4, 7.8) km/s 41 and correct the (U, V, W) values of each cluster for this solar motion to obtain its current 3D space velocity with respect to the Local Standard of Rest (LSR) frame (ULSR, VLSR, WLSR). We use the mean cluster motion to "traceback" its trajectory in the Galaxy. To "traceback" a cluster means to compute the 3D position and 3D motion that the cluster would have had at different times in the past, given estimates of its present 3D position and 3D motion. In practice, the full bound orbit of the cluster can be computed, and the small section of the orbit constituting its trajectory in the recent past is extracted.

We perform the dynamical traceback of each cluster using the galpy package 40 , which supports orbit integrations in a Milky Way-like potential, consisting of a bulge, disk, and dark matter halo. We sample the orbit from -20

Myr to the present day. Alongside each cluster, we also trace the Sun's orbit backward in time over the past 20

Myr and correct each cluster's orbit for the Sun's peculiar motion to ensure all orbits remain in the Local Standard of Rest frame. We emphasize that galpy only accounts for the gravitational potential of the Galaxy and does not consider the gravitational effects of the parental gas clouds in which many of these clusters are embedded. However, given the large extent of the Local Bubble relative to any individual star-forming region, and the fact that the Galactic potential should dominate over the gravity of any local gas, galpy still serves as a useful probe of the dynamics of the Local Bubble. This argument is bolstered by recent results from numerical simulations, which indicate that stellar orbits can be recovered with high fidelity up to 20 Myr in the past, even without explicitly modeling non-axisymmetric components of the potential. 42 The dynamical tracebacks for all clusters in Extended Data Table 1 are publicly available at the Harvard Dataverse (https://doi.org/10.7910/DVN/E8PQOD).

In this section, we derive an analytic model for the radius and expansion velocity of the Local Bubble as a function of time, using constraints provided by the dynamical traceback data summarized in Extended Data Tables 1 and2 (see the Data Availability section to access the full traceback data on each cluster). The results of this section underpin our model for the temporal evolution of the Local Bubble shown in the static and interactive versions of Figure 2.

To model the expansion of the Local Bubble, we utilize recent literature that leverages 1D spherically-symmetric hydrodynamic simulations using the Athena++ code to study the dynamical evolution of superbubbles driven by clustered supernovae in a uniform medium. 22 Specific treatment is given to the effects of cooling at the shell/bubble interface. Building on this recent literature, we adopt an analytic model 22 describing the radius, R, of the superbubble's expanding shell as a function of time t since its birth, parameterized by the ambient density n0 of the interstellar medium, the cooling efficiency at the shell's surface 𝛳, the time separation between supernovae explosions 𝞓tSNe within the cluster powering its formation,, and the energy input per supernova explosion ESN, as follows:

At any time t the (x, y, z) coordinates of the surface of this expanding spherical shell, centered on (xcen, ycen, zcen), corresponding to the epicenter of the supernova explosions, can be parameterized as:

The 3D positions of each cluster at birth (derived from the dynamical tracebacks given the cluster's age) provide a constraint on the bubble's radius as a function of time. So, under the assumption that the formation of the young stars listed in Extended Data Table 2 was triggered by the shell's expansion, we can infer the parameters governing the Local Bubble's evolution using this analytic framework. We emphasize that this theoretical formalism is an approximation of the bubble's true morphology. We do not actually expect the bubble to expand spherically because the interstellar medium is highly turbulent with significant density fluctuations. Indeed the Local Bubble today is observed to have a complex, non-spherical morphology.

Extended Data Table 2 lists the 3D (x, y, z) birth positions for the subset of young clusters utilized to model the bubble expansion in the Local Standard of Rest Frame, given their stellar tracebacks and estimated ages.

Since several of the parameters governing the superbubble's evolution are degenerate, we make a number of simplifying assumptions. Prior work 6,7,21 estimates that 14-20 supernovae in Upper Centaurus Lupus (UCL)

and Lower Centaurus Crux (LCC) stellar groups over the past ~13 Myr have together created the Local Bubble.

The formalism of the analytic superbubble model assumes that all supernovae are driven from a single location. Rather than model each individual supernova explosion given the tracebacks of UCL and LCC, we assume that the epicenter of the explosion (xcen, ycen, zcen) lay equidistant between UCL and LCC at the time of the first explosion, texp. This approximation is justified as UCL and LCC lay roughly co-spatial at early times, when the most powerful supernovae driving the superbubble's expansion would have gone off. We leave the time of the first supernova explosion, texp , as a free parameter in our model. The subsequent evolution of the Local Bubble is governed by texp and Equation (1). We assume a fixed energy input per supernova of 10 51 erg.

We also assume a fixed cooling efficiency 𝛳 of 0.7. However, we test a variety of cooling efficiencies, ranging from 0.4 -0.9, and find that fixing the cooling efficiency to a value of 0.7 does not affect our estimate for the time of the first explosion, and only has a modest effect on the ambient density and duration between supernovae (with the variation falling within our reported uncertainties on these parameters, as we will later show in Extended Data Figure 1).

Having fixed the cooling efficiency and energy input per supernova , the free parameters of our model include the ambient density, n0 , the time between supernova explosions, 𝞓tSNe , and the time of the first supernova explosion texp . We infer the values of n0, 𝞓tSNe , and texp in a Bayesian framework. We assume that the density of the shell has a Gaussian profile with an uncertainty (or thickness) of ΔR, which corresponds to a log-likelihood of the following form:

Here, the R(ti , n0 , 𝞓tSNe) term is the radius of the Local Bubble's expanding shell governed by Equations ( 1)

and ( 2) , evaluated at time ti , corresponding to the difference between the time at which the ith cluster was born and the time of the first explosion (ti = tbirth,i -texp). The Local Bubble's shell with radius R(ti) is centered on (xcen , ycen , zcen), derived from the mean 3D position of UCL and LCC in the Local Standard of Rest at time texp, when the first supernova went off in UCL or LCC. The r(ti) term is the radius of a sphere with the same center as R(ti), such that the ith cluster born at traceback time ti lies on its surface, with coordinates of:

(𝑥(𝑡 𝑖 ) -𝑥 cen ) 2 + (𝑦(𝑡 𝑖 ) -𝑦 cen ) 2 + (𝑧(𝑡 𝑖 ) -𝑧 cen ) 2 = 𝑟(𝑡 𝑖 ) 2 (4)

Finally, the ΔR term in the log-likelihood given in Equation ( 3) should be interpreted as an error term, encompassing uncertainties in both the ages of the clusters and on their mean (ULSR, VLSR, WLSR) motions. We infer ΔR as an additional free parameter in our model. The total log-likelihood of all n clusters is the sum of their individual log-likelihoods, evaluated at the respective time of their births, which will be optimized when the difference between R(ti) and r(ti) is minimized.

Using the log-likelihood in Equation ( 3), we sample for the values of texp , n0 , 𝞓tSNe, and ΔR using the nested sampling code dynesty. 43 For texp, we adopt a truncated normal prior with a mean of -13 Myr and a standard deviation of 1 Myr over the range -16 Myr to -10 Myr (consistent with previous evolutionary synthesis models of UCL and LCC). 6,21 For n0 , we adopt a truncated log-normal prior with a mean of 2 cm -3 and a standard deviation of a factor of two over the range 0.1 to 10 cm -3 , consistent with the density range explored in the Athena++ simulations underpinning the expansion model. 22 For 𝞓tSNe , we adopt a truncated log-normal prior with a mean of 0.8 Myr and a standard deviation of a factor of two, over the range 0.05 Myr to 3 Myr (consistent with previous estimates of approximately 16 supernovae occurring in UCL and LCC over the past 13 Myr) 7 . Finally, guided by the typical errors on the 3D motions (a few km/s) and the ages (a few Myr), we adopt a truncated normal prior on ΔR, with a mean of 15 pc and a standard deviation of 5 pc, over the range 0 to 30 pc. We run with the default parameters of dynesty's dynamic nested sampler.

The results of our sampling procedure are summarized in Extended Data Figure 1. We find a median value and 1σ errors (computed using the 16th, 50th, and 84th percentiles of the samples) of 𝞓tSNe = 1.06 -0.39 +0.63 Myr , n0 = 2.71 -1.02 +1.57 cm -3 , texp = -14.39 -0.74 +0.78 Myr, and ΔR = 23.31 -2.29 +2.54 pc. The best-fit model corresponds to an epicenter of the bubble of (xcen , ycen , zcen) = (39, 7, -18) pc in the LSR frame 14.4 Myr ago. Adopting these median parameters, a model for the evolution of the Local Bubble is overlaid in Figure 2. We compute the radius of the Local Bubble's shell and its expansion velocity over its lifetime, as plotted in Extended Data Figure 2 (panels a. and b.). Based on our model, and leveraging the full set of posterior samples, we estimate a present-day expansion speed of 6.7 -0.4 +0.5 km/s and a radius of 165 ± 6 pc. This present-day expansion speed is consistent with the current range of 3D velocity magnitudes of stars at the surface of its shell (≈ 5 -9 km/s), assuming that the rest velocity of the shell lies within a few km/s of the Local Standard of Rest (as we expect it to, since the LSR is currently inside the Bubble).

However, as seen in Extended Data Figure 1, there is a strong covariance between 𝞓tSNe and n0 such that an increase in the density of the ambient medium can be compensated for by a decrease in the time between supernova explosions, and vice versa. Given the limitations of our modeling, we intend the superbubble evolution shown in Figure 2 to only serve as a possible, idealized, example of how the Local Bubble could have reached its present-day morphology.

In this section, we seek to shed additional light on the origin of the Local Bubble, by using a momentum analysis to test whether UCL and LCC harbored enough supernovae to excavate a cavity the size of the Local Bubble. To do so, we compute the momentum of the Local Bubble's shell from its current expansion velocity (calculated in the previous section and shown in Extended Data Figure 2) and estimates of its mass (obtained from 3D dust mapping). We can then further build on the analytic superbubble model constrained above to obtain the expected average momentum injection per supernova, p. The ratio of the total momentum of the shell to the average momentum injected per supernova provides a constraint on the number of supernovae required to power its expansion, which can then be compared with existing estimates for how many supernovae have gone off in UCL and LCC based on population synthesis modeling. 7,21 To obtain the momentum of the shell, we calculate its swept-up mass Mshell by integrating the 3D volume density derived from the 3D dust map 9 between a distance of [Rshell, Rshell + Rthickness] from the Sun and multiplying by the mean particle mass ⟨m⟩ = 1.4mH, where mH is the mass of a proton, and the factor of 1.4 corrects for the helium abundance. Rshell is the boundary (i.e. inner radius) of the Local Bubble (shown in Figure 1) and Rthickness is the shell's thickness, such that Rshell + Rthickness corresponds to the outer radius. The Local Bubble model we utilize estimates Rthickness to be between 50 -150 pc 13 , which is quite large, but encompasses the full depth of structure currently lying on the bubble' surface.

Adopting Rthickness = 100 pc with an estimated 1𝝈 uncertainty of 50 pc, we obtain Mshell = 1.4 -0.62 +0.65 × 10 6 M☉ for the swept-up mass. To estimate the current expansion velocity of the Bubble vexp, we leverage the posterior samples from our model describing the Local Bubble's evolution (fit to the dynamical tracebacks; see Extended Data (5)

Therefore, we again fix 𝛳 = 0.7, ESN = 10 51 erg, and leverage the samples of 𝞓tSNe, n0, and texp. We evaluate p̂ over the lifetime of each realization of the bubble and draw a random sample of p̂ from each distribution. We then take the 16th, 50th, and 84th percentiles of the random samples drawn from all realizations (see Extended Data is in good agreement with previous results 6,7,21 , which argue that UCL and LCC have produced 14-20 supernovae over their lifetimes based on an analysis of their current stellar membership and modeling of a Salpeter 44 Given uncertainties in the exact volume density of gas and dust inside the bubble traced by 3D dust -with some recent work suggesting that there is only a modest drop in volume density coincident with the cavity of hot ionized gas 45 -we note that these momentum calculations (and more broadly the analytic superbubble expansion model) are largely insensitive to the exact difference in density interior and exterior to the bubble's boundary. The analytic expansion model we adopt 22 includes the ambient density of the interstellar medium at the time of the first supernova explosion as a free parameter; however, this ambient density parameter is constrained using the dynamical traceback data (not the 3D dust maps) and does not require any explicit assumptions about the exact density interior and exterior to the bubble as the shell expands. Similarly, while the swept-up mass calculation does depend on 3D dust mapping 9 , the mass is only measured within the denser shell of neutral gas and dust (where the uncertainties on the underlying dust extinction are smaller) and has no dependence on the density in the hot inner cavity, which could be subject to larger uncertainties due to the very low levels of dust extinction.

Tracebacks of the average stellar cluster motions are expressed relative to the "Local Standard of Rest," which itself depends on the measured motion of the Sun. Even in the age of Gaia, the Sun's exact motion in the Galaxy is uncertain, especially along the "Y" direction (along Galactic rotation). In this paper, we revise estimates of the tracebacks of the UCL and LCC clusters from prior literature 6,7 , which argues that UCL and LCC were born outside of the present-day boundary of the Local Bubble even though their members were the likely progenitors of the Local Bubble. In this section, we revisit these extant stellar traceback calculations from prior work 6,7 finding that a revised calculation places UCL and LCC near the center of the Local Bubble when they formed.

While previous studies 21 have proposed UCL and LCC as the likely progenitor population, the prior literature in question 6 is the first study to perform an unbiased stellar search of all nearby B stars to track down the remains of OB associations hosting supernovae capable of powering the Local Bubble. This prior literature 6 confirms that UCL and LCC are the only populations capable of having powered the Local Bubble, but, after tracing back the stellar members, find that both UCL and LCC formed > 100 pc outside the present-day boundary of the Local Bubble. Extended Data Figure 4a shows the extant stellar traceback results, where UCL and LCC only lay interior to the present-day boundary of Local Bubble's during the past few megayears. 6 The prior literature 6 noted this potential inconsistency (how does a supernova cause a bubble it does not lie within?), but argue that the location of the UCL and LCC clusters with respect to the center of the bubble is not crucial.

The prior literature 6 which places UCL and LCC outside the bubble use Hipparcos parallaxes and proper motions with radial velocities collected from the literature to derive the (U, V ,W) 3D space motion of each member. Using the (U, V ,W) value of each individual star, prior literature 6 calculates the mass-weighted mean (U, V ,W) velocity of all members of UCL and LCC, and then assign this group 3D U, V, W space velocity to each individual stellar member for the purposes of performing the tracebacks. The prior literature 6 also states that they correct the mean 3D group velocity for the Sun's peculiar motion, adopting a value of (U☉, V☉, W☉) = (10.0, 5.2, 7.2) km/s 46 , so that the traceback of each star is reported in the Local Standard of Rest frame.

As a first step, we attempt to reproduce the original results from prior literature 6 (see Extended Data Figure 4a)

using their own data. Specifically, we calculate the mass-weighted mean velocity of the UCL and LCC stars from their Table A1 6 and Equation 2 6 , finding (U, V, W = -7.1 -20.6, -5.8) km/s without correcting for the solar motion.

With a value for the Sun's peculiar motion of (10.0, 5.2, 7.2) km/s 46 used in the prior literature, these values translate to (ULSR, VLSR, WLSR = 2.9, -15.4, 1.4) km/s in the LSR frame. Extended Data Figure 4b shows dynamical tracebacks we calculate in galpy (see Methods) from the Table A1 data in prior literature and with the value for the Sun's peculiar motion of (10.0, 5.2, 7.2) km/s 46 . Using the default orbit integrator in galpy with its standard Milky Way potential 40 , we are unable to reproduce the results from prior literature, particularly the strong curvature in the tracebacks along the +X direction. The prior literature uses an epicyclic approximation for the stars' motions (see their Section 2) 6 , which may be responsible for part of the discrepancy. This systematic smearing out of the tracebacks toward +X in their Figure 2 6 manifests in the entire sample of B stars and is not isolated to UCL and LCC.

In examining the stellar motions of UCL and LCC, we find that the choice of the solar peculiar motion -necessary to convert to the LSR frame -has a non-negligible effect on where the birthplaces of UCL and LCC lie with respect to the center of the Local Bubble. There have been dozens of attempts to constrain the Sun's peculiar motion, but V☉ (motion in the direction of Galactic rotation, along Y) remains highly uncertain: current estimates range between V☉ = 4 and 16 km/s. 47 The value of V☉ = 5.2 km/s 46 adopted in prior literature 6,7 is one of the lowest values of V☉. As Extended Data Figure 4c shows, if we use the same Hipparcos data utilized in the prior literature but replace their value for V☉ (5.2 km/s 46 ) with a value of 15.4 km/s 41 we find that UCL and LCC are born near the bubble's center. Extended Data Figure 4d shows that updating the Hipparcos data with Gaia data and adopting the same peculiar motion used in this work 41 makes almost no difference to the tracebacks, suggesting that uncertainty in Sun's peculiar motion is the dominant source of uncertainty in determining the birth location of UCL and LCC. We adopt a value of 15.4 km/s 41 when calculating all the tracebacks in this paper, toward the upper end of estimates for V☉. Given the large uncertainty on V☉, we have tested the robustness of our results to the choice of solar peculiar motion and find that any motion for V☉ ≳ 10 km/s is entirely consistent with the physical scenario we propose here. This V☉ = 10 to 16 km/s range encompasses the vast majority of estimates for V☉ used in the field today. [47][48][49][50]