COS-Weak: Probing the CGM using analogs of weak Mg II absorbers at z <
  0.3
  We present a sample of 34 weak metal line absorbers at $z< 0.3$ complied via
the simultaneous detections ($3\sigma$) of the SiII$\lambda1260$ and
CII$\lambda1334$ absorption lines, with $W_{r}$(SiII)$<0.2$ \AA\ and
$W_{r}$(CII)$<0.3$ \AA, in archival HST/COS spectra. Our sample increases the
number of known low-$z$ "weak absorbers" by a factor of $>5$. The column
densities of HI and low-ionization metal lines obtained from Voigt profile
fitting are used to build simple photoionization models using CLOUDY. The
inferred densities and total hydrogen column densities are in the ranges of
$-3.3 < \log n_{\rm H}/{\rm cm^{-3}} < -2.4$ and $16.0 < \log N_{\rm H}/{\rm
cm^{-2}}<20.3$, respectively. The line of sight thicknesses of the absorbers
have a wide range of $\sim$1 pc$-$50 kpc with a median value of $\sim$500 pc.
The high-ionization OVI absorption, detected in 12/18 cases, always stems from
a different gas-phase. Most importantly, 85% (50%) of these absorbers show a
metallicity of $\rm [Si/H] > -1.0$ (0.0). The fraction of systems showing high
metallicity (i.e., $\rm [Si/H]>-1.0$) in our sample is significantly higher
than the HI-selected sample (Wotta et al. 2016) and the galaxy-selected sample
(Prochaska et al. 2017) of absorbers probing the circum-galactic medium (CGM)
at similar redshift. A search for galaxies has revealed a significant
galaxy-overdensity around these weak absorbers compared to random places with a
median impact parameter of 166 kpc to the nearest galaxy. Moreover, we find the
presence of multiple galaxies in $\sim80$% of the cases, suggesting group
environments. The observed $d\mathcal{N}/dz$ of $0.8\pm0.2$ indicates that such
metal-enriched, compact, dense structures are ubiquitous in the halos of
low-$z$ galaxies that are in groups. We suggest that these are transient
structures that are related to outflows and/or stripping of metal-rich gas from
galaxies.
