Importance of source structure on complex organics emission. I.
  Observations of CH$_3$OH from low-mass to high-mass protostars
  Complex organic molecules (COMs) are often observed toward embedded Class 0
and I protostars. However, not all Class 0 and I protostars exhibit COMs
emission. In this work, variations in methanol (CH$_3$OH) emission are studied
to test if absence of CH$_3$OH emission can be linked to source properties.
Combining both new and archival observations with ALMA and sources from the
literature, a sample of 184 low-mass and high-mass protostars is investigated.
The warm (T > 100 K) gaseous CH$_3$OH mass, $M_{\rm CH_3OH}$, is determined for
each source using primarily optically thin isotopologues. On average, Class I
protostellar systems seem to have less warm $M_{\rm CH_3OH}$ ($<10^{-10}$
M$_\odot$) than younger Class 0 sources ($\sim10^{-7}$ M$_\odot$). High-mass
sources in our sample show higher warm $M_{\rm CH_3OH}$ up to $10^{-7}-10^{-3}$
M$_\odot$. To take into account the effect of the source's overall mass on
$M_{\rm CH_3OH}$, a normalized CH$_3$OH mass is defined as $M_{\rm
CH_3OH}/M_{\rm dust,0}$, where $M_{\rm dust,0}$ is the cold + warm dust mass
within a fixed radius. Excluding upper limits, a simple power-law fit to the
normalized warm CH$_3$OH masses results in $M_{\rm CH_3OH}/M_{\rm
dust,0}\propto L_{\rm bol}^{0.70\pm0.05}$. This is in good agreement with a
simple hot core toy model which predicts that the normalized $M_{\rm CH_3OH}$
increases with $L_{\rm bol}^{0.75}$ due to the snowline moving outward. Sources
for which the size of the disk is equivalent or smaller than the estimated 100
K radius agree well with the best-fit power-law model, whereas sources with
significantly larger disks show up to two orders of magnitude lower normalized
warm CH$_3$OH masses. Based on the latter results, we suggest that source
structure such as a disk can result in colder gas and thus fewer COMs in the
gas phase. Additionally, optically thick dust can hide the emission of COMs.
